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Macromolecular crowding bridges in vivoand in vitrostudies by simulating cellular complexities such as high viscosity and limited space while maintaining the experimental feasibility. Over the last two decades, the impact of macromolecular crowding on protein stability and activity has been a significant topic of study and discussion, though still lacking a thorough mechanistic understanding. This article investigates the role of associated water dynamics on protein stability and activity within crowded environments, using bromelain and Ficoll-70 as the model systems. Traditional crowding theory primarily attributes protein stability to entropic effects (excluded volume) and enthalpic interactions. However, our recent findings suggest that water structure modulation plays a crucial role in a crowded environment. In this report, we strengthen the conclusion of our previous study, i.e., rigid-associated water stabilizes proteins via entropy and destabilizes them via enthalpy, while flexible water has the opposite effect. In the process, we addressed previous shortcomings with a systematic concentration-dependent study using a single-domain protein and component analysis of solvation dynamics. More importantly, we analyze bromelain’s hydrolytic activity using the Michaelis–Menten model to understand kinetic parameters like maximum velocity (Vmax) achieved by the system and the Michaelis–Menten coefficient (KM). Results indicate that microviscosity (not the bulk viscosity) controls the enzyme–substrate (ES) complex formation, where an increase in the microviscosity makes the ES complex formation less favorable. On the other hand, flexible associated water dynamics were found to favor the rate of product formation significantly from the ES complex, while rigid associated water hinders it. This study improves our understanding of protein stability and activity in crowded environments, highlighting the critical role of associated water dynamics.

The photochemistry and photophysics of thiocarbonyl compounds, analogues of carbonyl compounds with sulfur, have long been overshadowed by their counterparts. However, recent interest in visible light reactions has reignited attention toward these compounds due to their unique excited-state properties. This study delves into the ultrafast dynamics of 7-diethylaminothiocoumarin (TC1), a close analogue of the well-known probe molecule coumarin 1 (C1), to estimate intersystem crossing rates, understand the mechanisms of fluorescence and phosphorescence, and evaluate TC1’s potential as a solvation dynamics probe. Enclosing TC1 within an organic capsule indicates its potential applications, even in aqueous environments. Ultrafast studies reveal a dominant subpicosecond intersystem crossing process, indicating the importance of upper excited singlet and triplet states in the molecule’s photochemistry. The distinct fluorescence and phosphorescence origins, along with the presence of closely spaced singlet excited states, support the observed efficient intersystem crossing. The sulfur atom alters the excited-state behavior, shedding light on reactive triplet states and paving the way for further investigations.

Traditionally, deviation from Stokes–Einstein–Debye (SED) relation in terms of viscosity dependence of medium dynamics, i.e., τx∝(ηT)pwith p≠ 1, is taken as a signature of dynamic heterogeneity. However, it does not guarantee medium heterogeneity, as the decoupling may also originate from the deviation of the basic assumption of SED. Here, we developed a method to find a stronger relation between viscosity decoupling (p≠ 1) and dynamic heterogeneity in terms of rotational motion. Our approach exploited the fact that in heterogeneous media, a solvatochromic probe will be solvated to a different extent at different microdomains (subpopulations), and photoselection of these subpopulations can be achieved by excitation or emission wavelength-dependent measurements. We hypothesized that the dynamics of a homogeneous system might show viscosity decoupling, but the extent of decoupling at different excitations (or at different emissions) should not be different. On the other hand, in a heterogeneous medium, this extent of viscosity decoupling (p-value) should be different at different excitations (or at different emissions). As proof of concept, we investigated three versatile solvent media: squalane (viscous molecular liquid), 1-ethyle-3-methylimidazolium ethyl sulfate ionic liquid (IL), and [0.78 acetamide + 0.22 LiNO3] deep eutectic solvent (DES). We found that squalane is homogeneous, although it shows fractional viscosity dependence (p≠ 1). Interestingly, mild heterogeneity in IL and significant heterogeneity in the DES were observed. Overall, we conclude that the difference in the p-value as a function of excitation (or emission) wavelength-dependent might be a superior way for the detection of dynamic heterogeneity.

This Feature Article highlights the role of spatial confinement in controlling the fundamental behavior of molecules. Select examples illustrate the value of using space as a tool to control and understand excited-state dynamics through a combination of ultrafast spectroscopy and conventional steady-state methods. Molecules of interest were confined within a closed molecular capsule, derived from a cavitand known as octa acid (OA), whose internal void space is sufficient to accommodate molecules as long as tetracene and as wide as pyrene. The free space, i.e., the space that is left following the occupation of the guest within the host, is shown to play a significant role in altering the behavior of guest molecules in the excited state. The results reported here suggest that in addition to weak interactions that are commonly emphasized in supramolecular chemistry, the extent of empty space (i.e., the remaining void space within the capsule) is important in controlling the excited-state behavior of confined molecules on ultrafast time scales. For example, the role of free space in controlling the excited-state dynamics of guest molecules is highlighted by probing the cis–transisomerization of stilbenes and azobenzenes within the OA capsule. Isomerization of both types of molecule are slowed when they are confined within a small space, with encapsulated azobenzenes taking a different reaction pathway compared to that in solution upon excitation to S2. In addition to steric constraints, confinement of reactive molecules in a small space helps to override the need for diffusion to bring the reactants together, thus enabling the measurement of processes that occur faster than the time scale for diffusion. The advantages of reducing free space and confining reactive molecules are illustrated by recording unprecedented excimer emission from anthracene and by measuring ultrafast electron transfer rates across the organic molecular wall. By monitoring the translational motion of anthracene pairs in a restricted space, it has been possible to document the pathway undertaken by excited anthracene from inception to the formation of the excimer on the excited-state surface. Similarly, ultrafast electron transfer experiments pursued here have established that the process is not hindered by a molecular wall. Apparently, the electron can cross the OA capsule wall provided the donor and acceptor are in close proximity. Measurements on the ultrafast time scale provide crucial insights for each of the examples presented here, emphasizing the value of both “space” and “time” in controlling and understanding the dynamics of excited molecules.

Due to its numerous applications, triplet formation and resulting phosphorescence remain a frontier area of research for over eight decades. Facile intersystem crossing (ISC) is the primary requirement for triplet formation and observation of phosphorescence. The incorporation of a heavy atom in molecules is one of the common approaches employed to facilitate ISC. A detailed study of the excited state dynamics that governs ISC is necessary to understand the mechanism of heavy atom effect (HAE). Incorporation of iodine at the 3 position of coumarin-1 reduces fluorescence quantum yield (ϕf) drastically as expected, whereas bromine substitution at the same position increased the ϕf. Such a contrasting effect of the two heavy atoms suggests that there are other features yet to be discovered to fully understand the HAE. Detailed steady state and femtosecond transient absorption studies along with theoretical calculations suggest that the C3-X (X = Br, I) bond vibration plays an important role in the ISC process. The study reveals that while in the case of the iodo-derivative there is no energy barrier in the singlet triplet crossing path, there is a barrier in the case of the bromo-derivative, which slows the ISC process. Such an unexpected phenomenon is not limited to halocoumarins as this rationalizes the photobehavior of 1-bromo-/iodo-substituted naphthalenes as well.

Excited anthracene is well-known to photodimerize and not to exhibit excimer emission in isotropic organic solvents. Anthracene (AN) forms two types of supramolecular host–guest complexes (2:1 and 2:2, H:G) with the synthetic host octa acid in aqueous medium. Excitation of the 2:2 complex results in intense excimer emission, as reported previously, while the 2:1 complex, as expected, yields only monomer emission. This study includes confirming of host–guest complexation by NMR, probing the host–guest structure by molecular dynamics simulation, following the dynamics AN molecules in the excited state by ultrafast time-resolved experiments, and mapping of the excited surface through quantum chemical calculations (QM/MM-TDDFT method). Importantly, time-resolved emission experiments revealed the excimer emission maximum to be time dependent. This observation is unique and is not in line with the textbook examples of time-independent monomer–excimer emission maxima of aromatics in solution. The presence of at least one intermediate between the monomer and the excimer is inferred from time-resolved area normalized emission spectra. Potential energy curves calculated for the ground and excited states of two adjacent anthracene molecules via the QM/MM-TDDFT method support the model proposed on the basis of time-resolved experiments. The results presented here on the excited-state behavior of a well-investigated aromatic molecule, namely the parent anthracene, establish that the behavior of a molecule drastically changes under confinement. The results presented here have implications on the behavior of molecules in biological systems.

Deep eutectic solvents (DESs) are new-generation media that can be fine-tuned to have desired properties circumventing economic and environmental issues. Typically, these are ionic, and only recently, nonionic DESs, having interesting properties, are being explored. In this report, we examined the structure and dynamics of a nonionic lauric acid/menthol (LA/Men) DES through steady-state emission, solvation dynamics, time-resolved fluorescence anisotropy, and translational diffusion dynamics. The zero shift in the emission spectra of coumarin 153 (a solvatochromic dye) as a function of the excitation wavelength suggests that LA/Men DES is spatially homogenous. Decoupling (p= 0.63) of the average solvation time, ⟨τs⟩, from medium viscosity suggests the presence of mild dynamic heterogeneity in the system. Rotational time, ⟨τr⟩, which reflects the nature of the first solvation shell, shows little decoupling (p= 0.81), suggesting it to be fairly dynamically homogeneous at a shorter length scale. An Arrhenius-type analysis also proves that rotation is mainly controlled by medium viscosity. Translational diffusion time, ⟨τD⟩, which provides information at a larger length scale, is strongly decoupled from medium viscosity (p= 0.29). This indicates that at a larger length scale, the DES is quite dynamically heterogeneous. The slow component of solvation time, which is believed to originate at a larger length scale, correlates well with the translational diffusion timescale having similar activation energies. This suggests that their origin is same. Expectedly, for the long component of solvation time, the decoupling is quite strong (p= 0.30). Overall, our result demonstrates the structure and dynamics of the nonionic LA/Men DES, and the existence of length scale-dependent heterogeneity has been proposed.

One of the main differences in the intercellular environment compared to the laboratory condition is the presence of macromolecular crowders of various compositions, sizes, and shapes. In this article, we have contemplated a systematic shape dependency of macromolecular crowders on the thermodynamics and microsecond conformational fluctuation dynamics of protein unfolding by taking human serum albumin (HSA) as the model protein and similar-sized crowders, namely, dextran-40, ficoll-70, and PEG-35 as macromolecular crowders of different shapes, to mimic the cell environment. We observed that dextran-40 and ficoll-70 counteract the thermal denaturation and PEG-35 assists it. A complete thermodynamic analysis suggests that the stabilization by dextran-40 and ficoll-70 occurs mainly through stabilizing entropic effect, which is somewhat counteracted by the destabilizing enthalpic effect, in line with what is expected from the traditional interpretation of excluded volume and soft interaction. Surprisingly, the destabilizing effect of PEG-35 is not through unfavorable interaction but through a destabilizing entropic effect, which is opposite to the excluded volume prediction. Our speculation is that the modulation of the associated water structure due to crowder-induced distortion plays a crucial role in modulating the entropic component. Moreover, while a two-state model can approximate the overall thermal denaturation of HSA in the absence and presence of various crowders, the thermal denaturation profile of domain III of HSA involves a distinct intermediate state. The active-site dynamics of HSA are altered significantly in the presence of all the three differently shaped crowders used in the study. Rod-shaped dextran-40 and spherical ficoll-70 hinder the microsecond conformational dynamics of domain III of HSA in all states except the intermediate state. Mesh-like PEG-35 in addition to hindering the microsecond conformational dynamics shifts the intermediate state from 40 to 30 °C. Overall, our results provide new insight into deciphering the mechanism of crowder-induced changes in protein. Through our interpretation, we not only explain the unfavorable entropic contribution but also provide a physical basis to explain the entropy–enthalpy compensation.

Photoinduced electron transfer across an organic capsular wall between excited donors and ground-state acceptors is established to occur with rate constants varying in the range 0.32–4.0 × 1011s–1in aqueous buffer solution. The donor is encapsulated within an anionic supramolecular capsular host, and the cationic acceptor remains closer to the donor separated by the organic frame through Coulombic attraction. Such an arrangement results in electron transfer proceeding without diffusion. Free energy of the reaction (ΔG°) and the rate of electron transfer show Marcus relation with inversion. From the plot, λ and Velwere estimated to be 1.918 and 0.0058 eV, respectively. Given that the donor remains within the nonpolar solvent-free confined space, and there is not much change in the environment around the acceptor, the observed λ is believed to be because of “internal” reorganization rather than “solvent” reorganization. A similarity exists between the capsular assembly investigated here and glass and crystals at low temperature where the medium is rigid. The estimated electronic coupling (Vel) implies the existence of interaction between the donor and the acceptor through the capsular wall. Existence of such an interaction is also suggested by 1H NMR spectra. Results of this study suggest that molecules present within a confined space could be activated from outside. This provides an opportunity to probe the reactivity and dynamics of radical ions within an organic capsule.

Fluorescence up-conversion (∼250 fs instrumental response) coupled with time correlated single photon counting measurements was performed to explore the complete Stokes shift dynamics of a dipolar solute probe, coumarin 153 (C153), in several ionic acetamide deep eutectic solvents (DESs) that contained lithium nitrate/bromide/perchlorate as electrolyte. Combined measurements near room temperature reflected a total dynamic Stokes shift of approximately 800–1100 cm–1and triexponential solvation response functions. Interestingly, the average rate of solvation became faster upon successive replacement of bromide by nitrate in these deep eutectics, and a subpicosecond time scale emerged in the measured solvation response when bromide was fully replaced by nitrate. Temperature dependent solute diffusion in these deep eutectics at the single molecule level, monitored by tracking the translational motion of rhodamine 6G (R6G) via fluorescence correlation spectroscopic (FCS) technique, revealed pronounced fractional viscosity dependence of the solute’s translational motion. Subsequently, this partial decoupling of solute translation was attributed to the microheterogeneous nature of these ionic DESs after examining the diffusion–viscosity relationship via the FCS measurements of R6G in several normal solvents at room temperature and in a liquid amide solvent at different temperatures.

The influence of mono- and bivalent anions and micelle/submicellar clusters of different ionic and nonionic surfactants on the hydrolysis of malachite green (MG+) was studied in the presence of low alkali concentrations. Large polarizable I–ion and bivalent SO42–ion show specific interaction toward MG+with the formation of dye–anion ion-pair resulting in a retardation of the reaction rate. Tensiometric studies for anionic surfactants sodium dodecyl sulfate (SDS) and dioctyl sodium sulfosuccinate (AOT) in the presence of dye indicate the formation of ion-pair micelles which subsequently break down to form normal micelles. A change of the absorbance value accompanied by a red shift in the electronic spectra with increase in surfactant concentration also evidenced the formation of ion-pair. The observed rate inhibition by the anionic surfactants and the interaction behavior/nature of binding between the dye and surfactant in the premicellar region have been analyzed in terms of different kinetic models. The Piszkiewicz cooperativity index (n) suggests 1:1 association between the anionic surfactant monomer and MG+in the submicellar cluster. A catalyzing effect for the cationic surfactant, cetyltrimethylammonium bromide (CTAB), was noticed in the post-CMC region leading toward a limiting reaction rate, and the Menger–Portonoy pseudophase model was employed to explain the behavior with evaluation of the binding parameter. The nonionic surfactant poly(oxyethylene) (20) sorbitan monolaurate (Tween 20) exerts inhibition effect on the hydrolysis reaction which may be explained qualitatively by the dearth of HO–ions in the vicinity of the micelle-bound dye and quantitatively in light of the pseudophase model. Different equilibrium constants and binding constant parameters suggest appreciable binding between the dye and surfactant monomer/micelles owing to electrostatic/hydrophobic interactions.

Polarimeters have broad applications in remote sensing, astronomy, and biomedical imaging to measure the emitted, reflected, or transmitted state of polarization. An intrinsic coincident (IC) full-Stokes polarimeter was previously demonstrated by our group, in a free space configuration, by using stain-aligned polymer-based organic photovoltaics. To minimize the model’s complexity, these were tilted to avoid crosstalk from back-reflections. We present a theoretical model of a monolithic IC polarimeter that considers the back-reflection’s influence for on-axis light. The model was validated using a monolithic four-detector polarimeter, which achieved an error of less than 3%. Additionally, an off-axis model was produced and validated for a simpler two detector polarimeter, demonstrating an error between the TM and TE polarized components of less than 3% for angles spanning an 18° incidence cone.

In the context of generating reactive organic radical cations within a confined capsule and exploring photoinduced electron transfer from encapsulated organic molecules to organic and inorganic acceptors through an organic molecular wall, we have investigated electron transfer from the upper excited state (S2) of azulene (Az) and guaiazulene (GAz) enclosed within an octa acid (OA) capsule to water-soluble 4,4′-dimethyl viologen2+(MV2+) and pyridinium+(Py+) salts or colloidal TiO2. S2fluorescence of OA encapsulated Az and GAz was quenched by electron acceptors such as MV2+and Py+salts. That electron transfer is responsible for S2fluorescence quenching was established by recording the transient absorption spectrum of the MV●+in the femtosecond time regime. Femtosecond time-resolved fluorescence experiments suggested that the time constant for the forward and reverse electron transfer from encapsulated Az and GAz to MV2+is 4 and 3.6 ps, and 55.7 and 36.9 ps, respectively. The observed S2fluorescence quenching by colloidal TiO2in aqueous buffer solution is attributed to electron transfer from encapsulated Az and GAz to TiO2. Lack of quenching by the wider band gap material ZrO2supported the above conclusion. FT-IR-ATR experiments confirmed that OA capsules containing Az and GAz can be adsorbed on TiO2films, and excitation of these resulted in S2fluorescence quenching. The observations presented here are important in the context of establishing the value of OA type cavitands where charge separation and donor shielding are critical.

Understanding the intricate factors governing intersystem crossing (ISC) in aromatic carbonyl compounds remains a long-standing interest among researchers. This study unveils the crucial roles of vibration in influencing the ISC of a typical aromatic carbonyl chromophore, benzanthrone, and how hydrogen bonding and solvent viscosity affect these vibrations and, thus, the associated ISC kinetics. We demonstrate that for benzanthrone, the ISC is exceedingly facile in an aprotic solvent, while in protic solvents, the ISC is significantly suppressed through the formation of the hydrogen-bonded state. Moreover, in a high-viscosity medium, ISC is further retarded due to restrictions of volume-changing motions, which may assist ISC. Theoretical calculations revealed that the C═O bond vibration and specific out-of-plane vibrations accompanying a volume change could be the probable coordinates for ISC. These findings provide valuable insights for tailoring the excited-state behavior of carbonyl-functionalized materials for diverse applications in photocatalysis, organic electronics, and biomedicine.

Byline: Pukhraj. Rishi, Ekta. Rishi, Anusha. Venkataraman, Lingam. Gopal, Tarun. Sharma, Muna. Bhende, Dhanashree. Ratra, Pratik. Sen, Parveen. Sen This retrospective, interventional case series analyses treatment outcomes in eyes with [...]

Byline: Pukhraj. Rishi, Ekta. Rishi, Mahesh. Uparkar, Tarun. Sharma, Lingam. Gopal, Pramod. Bhende, Muna. Bhende, Pratik. Sen, Parveen. Sen Aim: To describe the clinical features, treatment and outcome patterns in [...]

Byline: Pukhraj. Rishi, Pratik. Sen, Daraius. Shroff, Jay. Chhablani Dear Editor, We report the efficacy of combination therapy using photodynamic therapy (PDT) and intravitreal ranibizumab for choroidal neovascular membrane (CNVM) [...]

In the present paper, we have studied the temperature dependence of translational diffusion and solvation dynamics of a dissolved dipolar dye in the nonionic acetamide–urea deep eutectic solvent (DES), to characterize the viscosity coupling of the measured relaxation times and verify the dynamical heterogeneity aspect of this medium. Three different time-resolved experimental techniques have been employed for this purpose: fluorescence correlation spectroscopy, transient absorption (TA) spectroscopy, and optical Kerr effect (OKE) spectroscopy. The first method provides the proof that the translational diffusion time of a solute in acetamide–urea DES [fCH3CONH2+ (1 – f)CO(NH2)2, f= 0.6] exhibits a fractional viscosity dependence, with exponent 0.758, which, when compared with the viscosity–diffusion relationship for the same solute in common molecular solvents, suggests moderate deviation from the Stokes–Einstein relation. Stokes shift dynamics of a solvatochromic dye, 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran in this DES, followed via femtosecond TA measurements, have been found to be triexponential in nature and dominated by a ∼100 fs component. The other two components, which contribute to a total dynamic Stokes shift magnitude of ∼2500 cm–1, are characterized by time constants in the ∼5 and ∼50 ps regimes. Subsequent comparison with the femtosecond OKE measurements suggests that the relatively slower picosecond solvation components originate from the rapid reorientation of the solvent molecules, while the subpicosecond solvation response arises from the participation of the collective low-frequency solvent modes (such as intermolecular vibrations and librations). We find that the rotational diffusion lifetimes also exhibit fractional power dependence on medium viscosity and thus deviate from the Stokes–Einstein–Debye pprediction. All of these results therefore suggest that the nonionic acetamide–urea DES is a moderately heterogeneous medium.

Coumarins are well-known to exhibit environment-dependent excited-state behavior. We have exploited this feature to probe the accessibility of solvent water molecules to coumarins (guest) encapsulated within an organic capsule (host). Two sets of coumarins, one small that fits well within the capsule and the other larger that fits within an enlarged capsule, are used as guests. In our study, the two sets of coumarins serve different purposes: one is employed to explore electron transfer across the capsule and the other to release photoprotected acids into the aqueous environment. The capsule is made up of two molecules of octa acid (OA) and is soluble in an aqueous medium under slightly basic conditions. Molecular modeling studies revealed that while the OA capsule is fully closed with no access to water in the case of smaller coumarins, with the larger molecules, the capsule is not tight and the guest is in contact with water molecules, the number being dependent on the size of the coumarin. We have used the ultrafast time-dependent Stokes shift method to understand the solvent dynamics around the above guest molecules encapsulated within an OA capsule in an aqueous medium. Results depict that for the smaller sets of coumarins, water cannot access the guests within the OA cavity during their excited state lifetime. However, the case is completely different for the larger coumaryl esters. Distorted capsule structure exposes the guest to water, and a dynamics Stokes shift is observed. The average solvation time decreases with the increasing size of guests that clearly indicates accessibility of the encapsulated guests toward greater number of water molecules as the capsule structure distorts with increasing size of the guests. Results of the ultrafast solvation dynamics are consistent with that of molecular dynamics simulation.

Background: Human Serum Albumin (HSA) is the most abundant protein present in human blood plasma. It is a large multi-domain protein with 585 amino acid residues. Due to its importance in human body, studies on the interaction of HSA with different external agent is of vital interest. The denaturation and renaturation of HSA in presence of external agents are of particular interest as they affect the biological activity of the protein. Objective: The objective of this work is to study the domain-specific and overall structural and dynamical changes occurring to HSA in the presence of a denaturing agent, urea and a renaturing agent, sucrose. Methods: In order to carry out the domain-specific studies, HSA has been tagged using N-(7- dimethylamino-4-methylcoumarin-3-yl) iodoacetamide (DACIA) at Cys-34 of domain-I and pnitrophenyl coumarin ester (NPCE) at Tyr-411 site in domain-III, separately. Steady-state absorption, emission and solvation dynamic measurements have been carried out in order to monitor the domain-specific alteration of HSA caused by the external agents. The overall structural change of HSA have been monitored using circular dichroism spectroscopy. Results: The α-helicity of HSA was found to decrease from 65% to 11% in presence of urea and was found to further increase to 25% when sucrose is added, manifesting the denaturing and renaturing effects of urea and sucrose, respectively. The steady state studies show that domain-III is more labile towards denaturation as compared to domain-I. The presence of an intermediate state is observed during the denaturation process. The stabilization of this intermediate state in presence of sucrose is attributed as the reason for the stabilization of HSA by sucrose. From solvation dynamics studies, it could be seen that the solvation time of DACIA inside domain-I of HSA decreases and increases regularly with increasing concentrations of urea and sucrose, respectively, while in the case of NPCE-tagged domain-III, the effect of sucrose on solvation time is evident only at high concentrations of urea. Conclusion: The denaturing and renaturing effects of urea and sucrose could be clearly seen from the steady state studies and circular dichroism spectroscopy measurements. A regular change in solvation time could only be observed in the case of domain-I and not in domain-III. The results indicate that the renaturing effect of sucrose on domain-III is not very evident when protein is in its native state, but is evident in when the protein is denatured.

In this contribution, the structural and dynamic changes occurring to papain in ethanol–water binary solvent mixtures have been investigated and compared with its denatured state. Steady-state fluorescence, solvation dynamics, time-resolved rotational anisotropy, circular dichroism (CD), and single molecular-level fluorescence correlation spectroscopic (FCS) studies were performed for this purpose. In ethanol–water mixtures with XEtOH= 0.6, N-(7-dimethylamino-4-methylcoumarin-3-yl)iodoacetamide (DACIA)-tagged papain was found to undergo a blue shift of 12 nm, while in the presence of 5 M GnHCl, a red shift of 5 nm was observed. Solvation dynamics of the system was also found to be different in the presence of these external agents. In ethanol–water mixtures, the average solvation time was found to increase almost 2-fold as compared to that in water, while in the presence of GnHCl, only a marginal increase could be observed. These changes of DACIA-tagged papain in ethanol–water mixtures are attributed to the aggregation of the protein in the presence of ethanol. The residual anisotropy was found to increase 14-fold, and the rotational time component corresponding to the rotation of the probe molecule was found to increase by 4-fold in the ethanol–water mixture which also gives a notion of the papain aggregation. Atomic force microscopy (AFM) confirms this aggregate formation, which is also quantified by the FCS study. The hydrodynamic radius of the protein aggregates in ethanol–water mixtures was calculated to be ∼155 Å as compared to the corresponding value of 18.4 Å in the case of native monomer papain. Also, it confirmed that the aggregate formation takes place even in the nanomolar concentration of papain. Analysis of circular dichroism spectra of papain showed that an increase in the β-sheet content of papain at the expense of α-helix and the random coil with an increase of the ethanol mole fraction may be responsible for this aggregation process.

Stretchable conductive polymer films are required to survive not only large tensile strain but also stay functional after the reduction in applied strain. In the deformation process, the elastomer substrate that is typically employed plays a critical role in response to the polymer film. In this study, we examine the role of a polydimethylsiloxane (PDMS) elastomer substrate on the ability to achieve stretchable PDPP-4T films. In particular, we consider the adhesion and near-surface modulus of the PDMS tuned through UV/ozone (UVO) treatment on the competition between film wrinkling and plastic deformation. We also consider the role of PDMS tension on the stability of films under cyclic strain. We find that increasing the near-surface modulus of the PDMS and maintaining the PDMS in tension throughout the cyclic strain process promote plastic deformation over film wrinkling. In addition, the UVO treatment increases film adhesion to the PDMS resulting in a significantly reduced film folding and delamination. For a 20 min UVO-treated PDMS, we show that a PDPP-4T film root-mean-square roughness is consistently below 3 nm for up to 100 strain cycles with a strain range of 40%. In addition, although the film is plastically deforming, the microstructural order is largely stable as probed by grazing incidence X-ray scattering and UV–visible spectroscopy. These results highlight the importance of neighboring elastomer characteristics on the ability to achieve stretchable polymer semiconductors.

The intercellular environment is known to be very different from the environment where most of the elementary biological processes are studied in the laboratory. As a result, there was a considerable effort on cell mimicking either by confinement or by introducing macromolecular crowding. In the present study, dextran of varying sizes has been used to crowd the environment of a protein, human serum albumin (HSA), and its structure, dynamics, and activity were studied as a function of crowder concentration. By employing bulk and single molecular level spectroscopic measurements, we elucidate the overall structure and local microsecond dynamics of HSA. Further, we have attempted to correlate these structural changes with its activity.

Polymer conductors that are solution-processable provide an opportunity to realize low-cost organic electronics. However, coating sequential layers can be hindered by poor surface wetting or dissolution of underlying layers. This has led to the use of transfer printing where solid film inks are transferred from a donor substrate to partially fabricated devices using a stamp. This approach typically requires favorable adhesion differences between the stamp, ink, and receiving substrate. Here, we present a shear-assisted organic printing (SHARP) technique that employs a shear load on a post-less polydimethylsiloxane (PDMS) elastomer stamp to print large-area polymer films that can overcome large unfavorable adhesion differences between the stamp and receiving substrate. We explore the limits of this process by transfer printing poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) films with varied formulation that tune the adhesive fracture energy. Using this platform, we show that the SHARP process is able to overcome a 10-fold unfavorable adhesion differential without the use of a patterned PDMS stamp, enabling large-area printing. The SHARP approach is then used to print PEDOT:PSS films in the fabrication of high-performance semitransparent organic solar cells.

Huntington’s disease (HD) is a genetic disorder caused by a CAG expansion mutation in Huntingtingene leading to polyglutamine (polyQ) expansion in the N-terminus side of Huntingtin (Httex1) protein. Neurodegeneration in HD is linked to aggregates formed by Httex1 bearing an expanded polyQ. Initiation and elongation steps of Httex1 aggregation are potential target steps for the discovery of therapeutic molecules for HD, which is currently untreatable. Here we report Httex1 aggregation inhibition by calmidazolium chloride (CLC) by acting on the initial aggregation event. Because it is hydrophobic, CLC was adsorbed to the vial surface and could not sustain an inhibition effect for a longer duration. The use of bovine serum albumin (BSA) prevented CLC adsorption by forming a BSA–CLC complex. This complex showed improved Httex1 aggregation inhibition by interacting with the aggregation initiator, the NT17part of Httex1. Furthermore, biocompatible CLC-loaded BSA nanoparticles were made which reduced the polyQ aggregates in HD-150Q cells.

The oxidation of l-serine by chloro and chlorohydroxo complexes of gold(III) was spectrophotometrically investigated in acidic buffer media in the absence and presence of the anionic surfactant sodium dodecyl sulfate (SDS). The oxidation rate decreases with increase in either [H+] or [Cl–]. Gold(III) complex species react with the zwitterionic form of serine to yield acetaldehyde (principal reaction product) through oxidative decarboxylation and subsequent deamination processes. A reaction pathway involving one electron transfer from serine to Au(III) followed by homolytic cleavage of α-C–C bond with the concomitant formation of iminic cation intermediate has been proposed where Au(III) is initially reduced to Au(II). The surfactant in the submicellar region exhibits a catalytic effect on the reaction rate at [SDS] ≤ 4 mM; however, in the postmicellar region an inhibitory effect was prominent at [SDS] ≥ 4 mM. The catalytic effect below the critical micelle concentration (cmc) may be attributable to the electrostatic attraction between serine and SDS that, in turn, enhances the nucleophilicity of the carboxylate ion of the amino acid. The inhibition effect beyond cmc has been explained by considering the distribution of the reactant species between the aqueous and the micellar pseudophases that restricts the close association of the reactant species. The thermodynamic parameters ΔH0and ΔS0associated with the binding between serine and SDS micelle were calculated to be −14.4 ± 2 kJ mol–1and −6.3 ± 0.5 J K–1mol–1, respectively. Water structure rearrangement and micelle–substrate binding play instrumental roles during the transfer of the reactant species from aqueous to micellar pseudophase.

Introduction: Interaction of surfactants with proteins can decipher important information regarding the stability and behavior of proteins. For multi-domain proteins, these interactions vary domain wise and these details are crucial in understanding the contribution of different domains of the protein in its overall activity. Objective: The objective of the present work is to study the interaction of surfactants with domain III of Human Serum Albumin (HSA) and to compare the same with the global interaction. Methods: Interaction of the anionic Sodium Dodecyl Sulphate (SDS) and the Cationic Cetyltrimethylammonium Bromide (CTAB) surfactants with domain III of Human Serum Albumin (HSA) has been studied using 8-Anilino-1-Naphthalene-Sulphonate (ANS) as a fluorescent marker. Circular Dichroism (CD) spectroscopy has been used to study the protein-surfactant interaction for the overall protein. Results: SDS is found to interact sequentially with domain III of HSA having two detectable intermediate states in the binding process. In case of CTAB, we have observed only one intermediate state for its interaction with domain III. Although Quantum yield measurement can reflect the presence of such intermediate state, the overall conformational change of the HSA on addition of surfactants, studied by Circular Dichroism (CD) spectroscopy, and the ANS-Trp distance measurement by FRET could not resolve the presence of such intermediate states. The esterase activity of HSA in presence of different amount of surfactants is also in accordance with our above observation. Conclusion: The interaction of both the surfactants with HSA is found to be sequential in nature. The most important conclusion revealed from our study is that the nature of protein-surfactant interaction is not same throughout the entire protein. Our study reveals that different parts of the multi-domain HSA have different affinity to the surfactant molecules.

An intrinsic coincident full-Stokes polarimeter is demonstrated by using strain-aligned polymer-based organic photovoltaics (OPVs) that can preferentially absorb certain polarized states of incident light. The photovoltaic-based polarimeter is capable of measuring four Stokes parameters by cascading four semitransparent OPVs in series along the same optical axis. This in-line polarimeter concept potentially ensures high temporal and spatial resolution with higher radiometric efficiency as compared to the existing polarimeter architecture. Two wave plates were incorporated into the system to modulate the S_3 Stokes parameter so as to reduce the condition number of the measurement matrix and maximize the measured signal-to-noise ratio. Radiometric calibration was carried out to determine the measurement matrix. The polarimeter presented in this paper demonstrated an average RMS error of 0.84% for reconstructed Stokes vectors after normalized to S_0. A theoretical analysis of the minimum condition number of the four-cell OPV design showed that for individually optimized OPV cells, a condition number of 2.4 is possible.

Introduction: Papain is a cysteine protease enzyme present in papaya and known to help in digesting peptide. Thus the structure and function of the active site of papain is of interest. Objective: The objective of present study is to unveil the overall structural transformation and the local structural change around the active site of papain as a function of chemical denaturant. Methods: Papain has been tagged at Cys-25 with a thiol specific fluorescence probe N-(7- dimethylamino-4-methylcoumarin-3-yl) iodoacetamide (DACIA). Guanidine hydrochloride (GnHCl) has been used as the chemical denaturant. Steady state, time-resolved, and single molecular level fluorescence techniques was applied to map the change in the local environment. Results: It is found that papain undergoes a two-step denaturation in the presence of GnHCl. Fluorescence correlation spectroscopic (FCS) data indicate that the size (hydrodynamic diameter) of native papain is ~36.8 Å, which steadily increases to ~53 Å in the presence of 6M GnHCl. FCS study also reveals that the conformational fluctuation time of papain is 6.3 µs in its native state, which decreased to 2.7 µs in the presence of 0.75 M GnHCl. Upon further increase in GnHCl concentration the conformational fluctuation time increase monotonically till 6 M GnHCl, where the time constant is measured as 14 µs. On the other hand, the measurement of ellipticity, hence the helical structure, by circular dichroism spectroscopy is found to be incapable to capture such structural transformation. Conclusion: It is concluded that in the presence of small amount of GnHCl the active site of papain takes up a more compact structure (although the overall size increases) than in the native state, which has been designated as the intermediate state.

Over the past 20 years, the most studied and debated aspect of macromolecular crowding is how it affects protein stability. Traditionally, it is explained by a delicate balance between the stabilizing entropic effect and the stabilizing or destabilizing enthalpic effect. However, this traditional crowding theory cannot explain experimental observations like (i) negative entropic effect and (ii) entropy–enthalpy compensation. Herein, we provide experimental evidence that associated water dynamics plays a crucial role in controlling protein stability in the crowded milieu for the first time. We have correlated the modulation of associated water dynamics with the overall stability and its individual components. We showed that rigid associated water would stabilize the protein through entropy but destabilize it through enthalpy. In contrast, flexible associated water destabilizes the protein through entropy but stabilizes through enthalpy. Consideration of entropic and enthalpic modulation through crowder-induced distortion of associated water successfully explains the negative entropic part and entropy–enthalpy compensation. Furthermore, we argued that the relationship between the associated water structure and protein stability should be better understood by individual entropic and enthalpic components instead of the overall stability. Although a huge effort is necessary to generalize the mechanism, this report provides a unique way of understanding the relationship between protein stability and associated water dynamics, which might be a generic phenomenon and should trigger much research in this area.

In the present investigation, graphene (G) and graphene decorated with Ag (Ag-G), Au (Au-G) metal nanoparticles as well as AuAg alloy (AuAg-G) nanoparticles (NPs) have been successfully synthesized via solution chemical route. The specimens have been characterized by using XRD, Raman, UV–Vis spectroscopy and TEM. TEM study confirms the formation of few-layered graphene sheets decorated with metal and alloy NPs. The relative change in the positions of D and G bands in Raman spectra of Ag-G, Au-G and AuAg-G as compared to graphene reveals a significant electronic interaction between graphene and metal or alloy NPs. A better electronic interaction between AuAg alloy NPs and graphene as compared to that of pure metal NPs has been shown by an enhancement in the intensities of G and D bands in Raman spectrum of AuAg-G as compared to those of Ag-G and Au-G. UV–Vis spectroscopic analysis also indicates, a larger blue shift in the absorption peak of the graphene in AuAg-G than that of metal decorated graphene, suggesting a better interaction between graphene and alloy NPs. These results indicate that alloy NPs can tune the electronic structure of graphene better than metal NPs. Such changes in electronic interaction will lead to modification of the electronic behaviour of graphene, making these materials suitable for many electronic applications.

Rhythm is a dansyl: A novel dansyl–pyrene conjugate functions as a fluorescence signaling unit showing strong fluorescence quenching upon interaction with copper(II) metal ions. On the other hand, the two‐photon absorption (TPA) cross‐section increased from 374 to 3022 GM (at 770 nm) upon interaction with copper(II) ions, thus allowing their selective detection in aqueous solutions.

Although worm-like micelles were invented 35 years ago, its formation pathway remains unclear. Inspired by the fact that a single molecular level experiment could provide meaningful and additional information, especially in a heterogeneous subpopulation, herein, we present a single molecular level study on the formation of wormlike micelles by cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) in water. Our results indicated a coexistence of normal spherical micelles along with a big wormlike micelle in its formation path. More interestingly, we have two unique insights into the formation mechanism, which are inaccessible in ensemble averaged experiments: (i) at extremely low concentrations of the surfactant, [CTAB]/[NaSal] ∼ 0.06, the wormlike micelle attains the highest size; and (ii) the relative concentration of wormlike micelles is highest when [CTAB]/[NaSal] ∼ 2.

Quantum-confined two-dimensional (2D) Ruddlesden–Popper (RP) perovskite nanoplates (NPls) are drawing considerable attention in recent times. The effect of external perturbations like air, moisture, heat, polarity of the solvent, and, specifically, light irradiation has significant impact on the RP perovskite structure in different ways. Though some reports are available on the effect of light irradiation on RP NPl single crystals, films, and flakes, no comprehensive study is available for the RP NPl colloidal state. To extend the understanding, we studied the fate of UV irradiation on a colloidal, orange-emitting oleylammonium iodide-based RP NPl, (C18H35NH3)2(CH3NH3PbI3)2PbI4(n= 3). A constant UV exposure for 10 h transforms the RP NPl structure into a purely 3D methylammonium lead iodide (CH3NH3PbI3, MAPbI3) nanorod (NR) having a photoluminescence quantum yield of 65%. Our experimental results reveal that this structural transformation takes place by ligand desorption, followed by structural association in an oriented fashion. The obtained MAPbI3NR shows excellent optical and crystalline phase stability for more than 2 months.

Marcus theory is applied on ESPT reactions in the adiabatic limit, using cyano-naphthol derivatives. A systematic variation of the reaction exergonicity is achieved by changing the proton donor, solvent (acts as base) and the temperature. Marcus inverted regions are observed for the proton recombination steps when the solvent is changed and temperature variation was used to vary the reaction exergonicity.

The dependence of fluorescence and solvation dynamics of coumarin 480 (C480) in a dimyristoyl–phosphatidylcholine (DMPC) vesicle on excitation wavelength (λex) was studied with femtosecond fluorescence upconversion. The study revealed an ultrafast 1.5‐ps component of solvation that was not detected earlier. C480 exhibits pronounced red‐edge excitation shift (REES) by 10 nm in a DMPC vesicle. This is due to the microheterogeneity of the lipid vesicle. In lipids, the probe is distributed in different locations with varying static and dynamic electrostatic responses. Solvent relaxation becomes faster and the amount of dynamic Stokes shift decreases with increasing λex. For excitation at the red end (λex=430 nm), the solvation time was found to be 1.5 ps. However, for excitation at the blue end, (λex=390 nm), there are two substantially slower components of 250 and 2000 ps. It seems that for λex=390 nm, the major contribution to total emission is due to the probe (C480) molecules in the hydrophobic and restricted locations inside the lipid bilayer. Excitation at 430 nm preferentially selects the probe molecules in a highly mobile environment (water pool of the lipid).

The kinetics of the oxidation of propan-1-ol and propan-2-ol by KMnO4in HClO4medium has been studied in absence and presence of Tween-20. In the absence of Tween-20 the reaction is of first order with respect to each of permanganate and H+, but of complex order with respect to substrate. The active oxidant species HMnO4reacts with the alkanol molecule to form an intermediate complex, which decomposes in the rate-determining step to form the respective product and Mnv. In the presence of Tween-20, both the oxidant and the substrate are distributed between the aqueous phase and the micellar pseudo-phase and then react. Different kinetic and thermodynamic parameters have been evaluated. Compensation between water structure destruction and substrate-micelle interaction plays an important role in the presence of the surfactant.

Abstract The kinetics of oxidation of α,β-unsaturated compounds by platinum(IV) in the presence of alkali {[OH−]= (1–9) × 10−3 mol dm−3} have been investigated over the 303–318 K temperature range . The rate of the reaction is dependent on the first power of the concentrations of substrates, oxidant, and alkali. The rate constant increases with an increase in ionic strength and also with increasing dielectric constant of the medium. The oxidation rates follow the order: –CN > –CONH2 > –COO−. The values of the third order rate constant (k3) for the oxidation of acrylonitrile, acrylamide and acrylate are 1.24, 0.826 and 0.628 mol−2 dm6 s−1 respectively, at 303 K. The oxidations of the substrates by PtCl5(OH)2− take place by an inner-sphere mechanism. Platinum(IV) is reduced to platinum(II) by the substrates in a one-step two-electron transfer process to give reaction products. The major reaction product, HCHO, is identified from the reaction mixture using i.r. spectrometry, n.m.r. and C, H, N analysis. A tentative reaction mechanism, leading to the formation of products, has been suggested. The activation parameters of the reaction have been evaluated.

Abstract The reducing behaviour of methanol and ethanol towards permanganate in perchloric acid medium have been investigated in the absence and presence of the surfactant Tween-20. In the absence of surfactant the reaction is of first order with respect to both oxidant and H+ but of complex order in substrate. The alcohol molecule reacts with HMnO4 to form an intermediate complex which decomposes in the rate-determining step to give the product and Mnv. Effects of urea and acetonitrile on the reaction rate have also been studied. In the presence of Tween-20, the reaction appears to follow Berezin’s model where both the oxidant and the substrate are partitioned between the aqueous and the micellar phase and then react. Different thermodynamic and kinetic parameters have been evaluated. The reaction in the presence of the surfactant is entropy-controlled rather than enthalpy-controlled.

The oxidative behaviour of the 1,2-dicarbonyl compounds, viz., glyoxal, biacetyl and benzil, towards permanganate in ethanoic acid medium in the presence of HClO4has been investigated. The reaction is first order with respect to MnO−4, substrate, as well as H+. The rate decreases with an increase in ionic strength. Different thermodynamic parameters have been evaluated. The protonated dialdehyde or diketone reacts with permanganate ion to form an intermediate ester which decomposes in a slow step to produce the corresponding carboxylic acid viaC—C bond cleavage.

The oxidative behaviour and relative reactivities of three alkanols and 11 aryl alcohols towards gold(III) in sodium acetate–acetic acid buffer were studied. The reactions are first order with respect to [Au(III)] and in each [R1R2CHOH]. Both H+and Cl−ions retard the rate of reactions. The reactions obey the following rate expression: $- {\rm d[Au(III)]/ d}t = \left \{k_{1} + {(k_{2} - k_{1})K_{2} \over [\rm Cl^{-}]} + {(k_{3} - k_{1}) K_{2}K_{3} \over \rm [H^{+}][Cl^{-}]}\right \} \rm [R^{1}R^{2}CHOH][Au(III)]$AuCl4−, AuCl3(OH2) and AuCl3(OH)−are the reactive species of gold(III) and the reactivity follows the order AuCl3(OH)−> AuCl3(OH2) > AuCl4−. The activation parameters of the reactions were calculated. Alkanols react with gold(III) in the order ethanol > methanol > 2‐propanol. The unsubstituted benzyl alcohol reacts with gold(III) at a faster rate than benzhydrol. The pseudo‐first‐order rate constants in the oxidations of aryl alcohols follow the order NO2> H > Cl > OMe. The reactions occur via C—H bond cleavage and through the intermediate formation of free radicals. Copyright © 2001 John Wiley & Sons, Ltd.

The oxidative behaviour and relative reactivities of three alkanols and 11 aryl alcohols towards gold(III) in sodium acetate–acetic acid buffer were studied. The reactions are first order with respect to [Au(III)] and in each [R¹R²CHOH]. Both H⁺ and Cl⁻ ions retard the rate of reactions. The reactions obey the following rate expression: $- {\rm d[Au(III)]/ d}t = \left \{k_{1} + {(k_{2} - k_{1})K_{2} \over [\rm Cl^{-}]} + {(k_{3} - k_{1}) K_{2}K_{3} \over \rm [H^{+}][Cl^{-}]}\right \} \rm [R^{1}R^{2}CHOH][Au(III)]$ AuCl4⁻, AuCl3(OH2) and AuCl3(OH)⁻ are the reactive species of gold(III) and the reactivity follows the order AuCl3(OH)⁻ > AuCl3(OH2) > AuCl4⁻. The activation parameters of the reactions were calculated. Alkanols react with gold(III) in the order ethanol > methanol > 2-propanol. The unsubstituted benzyl alcohol reacts with gold(III) at a faster rate than benzhydrol. The pseudo-first-order rate constants in the oxidations of aryl alcohols follow the order NO2 > H > Cl > OMe. The reactions occur via C—H bond cleavage and through the intermediate formation of free radicals. Copyright © 2001 John Wiley & Sons, Ltd.

The kinetics of oxidation of some neutralized α-hydroxy compounds such as glycolic (GA), lactic (LA), α-hydroxyisobutyric(IB), mandelic (MA), atrolactic (AL), and benzilic (BA) acids by tetrachloroaurate(III) have been studied. The substrates are oxidized to give formaldehyde, acetaldehyde, acetone, benzaldehyde, acetophenone, and benzophenone for the respective reactions. The rate of the reaction increases with increasing [substrate] and pH but decreases with increase in [Cl⁻¹]. Temperature influence is quite marked in all these reactions. A mechanism involving the formation of an unstable complex, which decomposes to give the respective reaction products, is proposed. The reactivity of the α-hydroxy acids towards gold(III) are as follows: AL > MA > BA > IB > LA > GA. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 873–882, 1999

The kinetics of electron-transfer reactions between permanganate ion and ethyl and aryl methyl ketones have been studied in aqueous MeCO2H acid medium in the presence of HClO4 at different temperatures. For ethyl methyl ketone and XC6H4COMe (X = p-Cl, p-Br or p-NO2) the reaction obeys the rate law −d[MnO4−]/dt = (kKe[H+][MnO4−][RCO Me])/(1 + Ke[H+][RCOMe]).␣But the oxidations of XC6H4COMe (X = p-Me and p-OMe)␣follow the rate equation −d[MnO4−]/dt = k3[H+][MnO4−][RCOMe]. The reaction involves a fast pre-equilibrium with intermediate formation of a permanganate ester before the two-electron transfer, rate-determining, step. A number of thermodynamic parameters have been evaluated.

The kinetics of the reaction between glycolaldehyde (GA) and tetrachloroaurate(III) in acetic acid-sodium acetate buffer has been studied. The reaction is first-order with respect to [Au^III] as well as [GA]. Both H⁺ and Cl⁻ ions retard the rate of reaction. AuCl4⁻, AuCl3(OH2), and AuCl3(OH)⁻ are the reactive species of gold(III) with gradually increasing reactivity. A reaction mechanism involving two-electron transfer rate determining steps has been proposed. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 613–619, 1998

The kinetics of oxidation of methanol by bromate ion in hydrochloric acid medium has been investigated. A mechanism consistent with the experimental observations is suggested.

The kinetics of the reaction between nitrous acid and gold(III) in an HCl medium was studied. The reaction was first order with respect to [AuIII] and [HNO2]·H+ and Cl- ions inhibit the rate and alkali metal ions have specific effects on the rate. The reaction appears to involve different gold(III) species, viz. AuClinf4sup-, AuCl3(OH2) and AuCl3(OH)-, which undergo a two-equivalent reduction to gold(I) leading to the formation of NOinf2sup+which under-goes rapid hydrolysis to give nitric acid.

Summary The reaction between sulphurous acid and hexachloroiridate(IV) appears to take place through the formation of an intermediate complex followed by decomposition to give oxidation products. The rate is retarded as the hydrogen ion concentration increases. Thermodynamic parameters associated with the equilibrium step as well as with the slowest step have been calculated. The probable mechanism of the reaction is discussed.

The kinetics of the reaction between hydrogen peroxide and gold(III) in a hydrochloric acid medium has been studied. The reaction is first order with respect to [AuIII] as well as [H2O2]. The reaction rate has an inverse dependence on [H+] and [Cl−]. AuCl4−, AuCl3(OH2), and AuCl3(OH)−are the reactive species of Au(III). A rate expression consistent with the experimental findings has been proposed.