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Nature is a source of inspiration for the self-assembly of functional molecular nanoarchitectures based on intermolecular bonds. This work investigates the self-assembly of molecules (i) polyacryli...

A density functional theory (DFT) computational analysis, using the ωB97X-D functional, of a rapid amide cleavage in 2-carboxyphthalanilic acid (2CPA), where the amide group is flanked by two catalytic carboxyls, reveals key mechanistic information: (a) General base catalysis by a carboxylate coupled to general acid catalysis by a carboxyl is not operative. (b) Nucleophilic attack by a carboxylate on the amide carbonyl coupled to general acid catalysis at the amide oxygen can also be ruled out. (c) A mechanistic pathway that remains viable involves general acid proton delivery to the amide nitrogen by a carboxyl, while the other carboxylate engages in nucleophilic attack upon the amide carbonyl; a substantially unchanged amide carbonyl in the transition state; two concurrent bond-forming events; and a spatiotemporal-base rate acceleration. This mechanism is supported by molecular dynamic simulations which confirm a persistent key intramolecular hydrogen bonding. These theoretical conclusions, although not easily verified by experiment, are consistent with a bell-shaped pH/rate profile but are at odds with hydrolysis mechanisms in the classic literature.

We report the targeted decomposition of the organophosphate methyl paraoxon by means of its transesterification with 1-propanol catalyzed by magnesium oxide. Catalyst characterization by energy dispersive X-ray fluorescence (EDXRF), nitrogen adsorption/desorption measurements (BET and BJH methods), and temperature programmed desorption of CO2 (CO2-TPD) showed that the employed MgO presents properties favorable for the methyl paraoxon adsorption and transesterification to occur. A thorough kinetic investigation showed that rate enhancements up to 3 × 106-fold can be achieved in comparison with the spontaneous propanolysis of the substrate, and that the material can be used in additional cycles without loss of catalytic activity, with the catalyst recovery achieved through a simple washing procedure. Energies for adsorption of 1-propanol and methyl paraoxon onto a MgO model surface were obtained by density functional theory calculations, which showed that the latter displays a stronger affinity for the catalyst surface, and that the reaction should proceed with methyl paraoxon and 1-propanol molecules juxtapositioned at adjacent Mg2+ sites, with nucleophilic and electrophilic centers ca. 2.4 A away from each other. Additionally, MgO also promoted rate enhancements up to 5 × 104-fold in the propanolysis of a further range of representative phosphate triesters, and in most of the cases the final transesterified products are trialkyl phosphates structurally related to a family of flame-retardants. The results thus provide insights into the development of novel systems for the targeted conversion of organophosphorus compounds into value-added products by employing simple, highly efficient, and low-cost metal oxide catalysts.

The study of host-guest complexation between reactive 2-carboxyphthalanilic acid (CPA) and two cationic pillararenes has been carried out. Host-guest complexation with significant kinetic effects was observed only with the smaller cavity size pillararene (P5A). Kinetics in the pH range 1.50-6.40, ESI-MS

Chlorophylls (Chls) are natural pigments essential to photosynthesis that have been shown to act as photosensitizers for Photodynamic Therapy (PDT). Many studies have used Chls A and its colloidal formulations in the PDT field but little is known about the potential of Chls B for this application. Here we have detailed the interaction, spectroscopic, and photophysical properties of Chlorophyll b (Mg-Chl b), Pheophytin b (Pheo b), and its formulations with nonionic surfactants aiming photodynamic applications. Both pigments were formulated in some suitably nanostructured drug delivery systems based on aqueous solutions of Pluronics triblock copolymers and Tweens surfactants. High binding constants have evidenced that Chls B interact strongly with these micelles, and the results are correlated with logP, zeta potential, and hydrodynamic diameter. Spectroscopic studies showed high molar absorption coefficients in the therapeutic window (600–800 nm), significant fluorescence quantum yields, and solubilization of Chls B as monomers, especially for Pluronics systems. Pheo b is completely photostable in all investigated micellar systems, whereas Mg-Chl b is photobleached with a rate constant correlated with the interaction and relative location in these micelles. High singlet oxygen quantum yields identify these chlorophylls B as possible Type II photosensitizers for PDT. Mg-Chl b in particular is almost twice as effective as Mg-Chl a, which makes it one of the most potent generators of singlet oxygen in nature.

Enzymes are highly significant catalysts, essential to biological systems, and a source of inspiration for the design of artificial enzymes. Although many models have been developed describing enzymatic catalysis, a deeper understanding of these biocatalysts remains a major challenge. Herein we detail the formation, characterization, performance, and catalytic mechanisms of a series of bio-inspired supramolecular polymer/surfactant complexes acting as artificial enzymes. The supramolecular complexes were characterized and exhibited exceptional catalytic efficiency for the dephosphorylation of an activated phosphate diester, the reaction rate being highly responsive to: (a) pH, (b) surfactant concentration, and (c) the length of the hydrophobic chain of the surfactant. Under optimal conditions (at pH > 8 for the more hydrophobic systems and at pre-micellar concentrations), enzyme-like rate enhancements of up to 6.0 × 109-fold over the rate of the spontaneous hydrolysis reaction in water were verified. The catalytic performance is a consequence of synergy between the hydrophobicity of the aggregates and the catalytic functionalities of the polymer and the catalytic mechanism is modulated by the nature of the hydrophobic pockets of these catalysts, changing from a general base mechanism to a nucleophilic mechanism as the hydrophobicity increases. Taken as a whole, the present results provide fundamental insights, through an understandable model, which are highly relevant to the design of novel bioinspired enzyme surrogates with multifunctional potentialities for future practical applications.

In the present work, we study the transient laser-induced formation of 4-nitrophenolate (4-NPO-) in the ground electronic state and subsequent proton transfer reaction with acetic acid and water with numerical calculations and laser flash photolysis. We employ the Debye-Smoluchowski spherically-symmetric diffusion model of photoacid proton transfer to determine experimental conditions for studying thermally activated chemical reactions in the ground electronic state. Numerically calculated protonation and deprotonation probabilities for 4-NPO- and 4-nitrophenol (4-NPOH) in both ground and excited states showed the feasibility of efficiently producing the ground state anion in the photoacid cycle. We performed laser flash photolysis measurements of 4-NPOH to characterize the photo-initiated ground state protonation and deprotonation rate constants of 4-NPO-/4-NPOH as a function of acetic acid, pH, temperature and viscosity. Overall, the work presented here shows a simple way to study fast competing bimolecular proton transfer reactions in non-equilibrium conditions in the ground electronic state (GSPT).

A density functional theory (DFT) computational analysis, using the ωB97X-D functional, of a rapid amide cleavage in 2-carboxyphthalanilic acid (2CPA), where the amide group is flanked by two catalytic carboxyls, reveals key mechanistic information: (a) General base catalysis by a carboxylate coupled to general acid catalysis by a carboxyl is not operative. (b) Nucleophilic attack by a carboxylate on the amide carbonyl coupled to general acid catalysis at the amide oxygen can also be ruled out. (c) A mechanistic pathway that remains viable involves general acid proton delivery to the amide nitrogen by a carboxyl, while the other carboxylate engages in nucleophilic attack upon the amide carbonyl; a substantially unchanged amide carbonyl in the transition state; two concurrent bond-forming events; and a spatiotemporal-base rate acceleration. This mechanism is supported by molecular dynamic simulations which confirm a persistent key intramolecular hydrogen bonding. These theoretical conclusions, although not easily verified by experiment, are consistent with a bell-shaped pH/rate profile but are at odds with hydrolysis mechanisms in the classic literature.

Quenching of the fluorescence of pyrene by AuCl4− ion was investigated in aqueous micellar solutions of hexadecyltrimethylammonium chloride (CTAC) and 3-(N,N-dimethylmyristylammonio)propanesulfonate (SB3–14) in the presence of 0.010 mol L−1 HCl. Because AuCl4− is excluded from anionic sodium dodecyl sulfate (SDS) micelles due to electrostatic repulsion effects, no quenching of pyrene fluorescence by AuCl4− was observed in micellar SDS. Absorption spectral shifts show that AuCl4− binds strongly to both CTAC and SB3–14 micelles and time-resolved fluorescence results indicate that the quenching of the fluorescence of micelle-solubilized pyrene by AuCl4− is dynamic in nature. Fits of the pyrene fluorescence decays with the Tachiya model for micelle quenching in the limit of slow exchange of quenchers between the aqueous phase and micelles provided estimates of the micelle aggregation numbers that are slightly smaller than those determined with other quenchers, and intramicellar quenching rates 3–4 times higher than those observed with known collisional quenchers. The strong interaction of AuCl4− with SB3–14 and CTAC led to the stabilization of gold nanoparticles, highlighting the use of zwitterionic surfactants for the synthesis of stable and biocompatible gold nanoparticles.

Cucurbit[7]uril (CB7) catalyzes the acid hydrolysis of alkoxyphenyldioxolanes bearing both neutral and cationic alkoxy groups. The magnitude of the catalytic effect depends on the dioxolane structu...

Zwitterionic surfactants have unique properties for applications in separation methods and catalysis. Their properties and efficiencies depend on two main factors: surfactant structure and preferential interactions of zwitterionic surfactant interfaces with anions. Structural changes are related to hydrocarbon chain length, distance between charges, and type and order of functional groups in the polar head. Interactions of anions with zwitterionic micelles follow the Hofmeister series and change the surface charge. The interactions between surfactants and molecules/ions allow the rational control of separation by chromatography and micellar capillary electrophoresis; cloud point extraction; and stabilization and catalytic activity of biomolecules and nanoparticles.

Designing artificial enzymes with tailored molecular interactions between the substrate and active site is of major intellectual and practical significance. We report the improved catalytic efficiency of a supramolecular polymer/surfactant complex comprised of PAIM–, a poly(acrylic acid) derivative with imidazole groups attached to the polymer by amide bonds, and the cationic surfactant cetyltrimethylammonium bromide (CTAB). Supramolecular complex formation, at concentrations below the respective CMC values, provides convenient hydrophobic pockets for the reactants close to the multiple catalytic centers, where imidazole and carboxylate groups act as nucleophiles for the degradation of a model phosphate triester, delivering the highly efficient performance of the supramolecular catalysts. Catalytic effects are on the order of thousands for nucleophilic catalysis and are higher by 2 orders of magnitude for the supramolecular polymer/surfactant complex at pH 9. The reported supramolecular catalytic complexe...

This essay focuses on the debate between Warshel et al. (proponents of preorganization) and Menger and Nome (proponents of spatiotemporal effects) over the source of fast enzyme catalysis. The Warshel model proposes that the main function of enzymes is to push the solvent coordinate toward the transition state. Other physical-organic factors (e.g., desolvation, entropic effects, ground state destabilization, etc.) do not, ostensibly, contribute substantially to the rate. Indeed, physical organic chemistry in its entirety was claimed to be "irrelevant to an enzyme's active site". Preorganization had been applied by Warshel to his "flagship" enzyme, ketosteroid isomerase, but we discuss troubling issues with their ensuing analysis. For example, the concepts of "general acid" and "general base", known to play a role in this enzyme's mechanism, are ignored in the text. In contrast, the spatiotemporal theory postulates that enzyme-like rates (i.e., accelerations >108) occur when two functionalities are held rigidly at contact distances less than ca. 3 A. Numerous diverse organic systems are shown to bear this out experimentally. Many of these are intramolecular systems where distances between functionalities are known. Among them are fast intramolecular systems where strain is actually generated during the reaction, thereby excluding steric compression as a source of the observed enzyme-like rates. Finally, the account ends with structural data from four active sites of enzymes, obtained by others, all showing contact distances between substrate analogues and enzyme. To our knowledge, contact distances less than the diameter of water are found universally among enzymes, and it is to this fact that we attribute their extremely fast rates given the assumption that enzymes, whatever their particular mechanism, obey elementary chemical principles.

Er2O3/Al2TiO5 and Al2TiO5 were evaluated as catalysts for the propanolysis of the organophosphate pesticide dimethyl 4-nitrophenyl phosphate (methyl paraoxon). The solid catalysts were characterized by energy-dispersive X-ray fluorescence (EDXRF), N2 physisorption (BET and BJH methods), and scanning and transmission electron microscopy (SEM and TEM). Physical–chemical characterizations revealed that the erbium content is 11.7% w/w in the novel solid and results in improved catalyst performance when compared with Al2TiO5, with textural properties favorable for methyl paraoxon diffusion in agglomerates composed of polydisperse spherical nanoparticles. Kinetic measurements at 80 °C show that the Er2O3/Al2TiO5 catalyst promotes a catalytic effect of at least 7 × 105-fold when compared with the first-order rate constant of the spontaneous propanolysis of methyl paraoxon.

Phosphoimidazole-containing compounds are versatile players in biological and chemical processes. We explore catalytic and mechanistic criteria for the efficient formation of cyclic aryl phosphoimidazoles in aqueous solution, viewed as a template reaction for the in situ synthesis of related compounds. To provide a detailed analysis for this reaction a series of o-(2′-imidazolyl)naphthyl (4-nitrophenyl) phosphate isomers were examined to provide a basis for analysis of both mechanism and the influence of structural factors affecting the nucleophilic attack of the imidazolyl group on the phosphorus center of the substrate. Formation of the cyclic aryl phosphoimidazoles was probed by NMR and ESI-MS techniques. Kinetic experiments show that cyclization is faster under alkaline conditions, with an effective molarity up to 2900 M for the imidazolyl group, ruling out competition from external nucleophiles. Heavy atom isotope effect and computational studies show that the reaction occurs through a SN2(P)-type me...

We report the dephosphorylation reactions of the organophosphates diethyl 2,4-dinitrophenyl phosphate (DEDNPP) and dimethyl 4-nitrophenyl phosphate (methyl paraoxon) by the oxime 2-(hydroxyimino)-N-phenyl-acetamide (Ox 1). Rate enhancements of 107-fold over the rate constant for the spontaneous hydrolysis are observed in aqueous medium in presence of the anionic form of the oxime. Ox 1 represents a new family of nucleophiles which could be used for the degradation of toxic organophosphates. Copyright © 2016 John Wiley & Sons, Ltd.

A kinetic study on dinitrophenylphosphate monoester hydrolysis in the presence of a cationic pillararene, P5A, has been carried out. Formation of the supramolecular complex between phosphate ester and P5A has been studied by NMR showing complexation-induced upfield proton shifts indicative of aromatic ring inclusion in the pillararene cavity. Molecular dynamic calculations allow structure characterization for the 1 : 1 and 1 : 2 complexes. As a result of the supramolecular interaction both the acidity of DNPP and its hydrolysis rate constants are increased. Catalysis results from combination of both electrostatic stabilization reducing the negative electron density on the PO3(=) oxygens and monoester dianion destabilization by the steric effects of close NMe3(+) groups hindering the hydrogen-bonding with water and destabilising the monoester dianion.

Binding constants, Log K ≈ 6.6 M-1, and NMR characterization of the complexes formed by sulfobetaines and cucurbit[7]uril (CB7) support the electrostatic interaction as major driving force. This very strong binding motif is cross-linked by additional CB7 molecules resulting in the formation of supramolecular nanoparticles (SNPs) with an average diameter of 172 nm and negative surface potential. The time course evolution of the particle size and the surface potential suggests the very fast formation of an amorphous aggregate that absorbs additional amount of sulfobetaine. These aggregates afford very stable (more than two weeks) nanoparticles in aqueous dispersion. The reversibility of the sulfobetaine/CB7 host:guest complexes allows SNPs disaggregation by adding a competitive guest as shown by treatment with tetraethylammonium chloride. The addition of this competitive cation triggers a SNPs to micelle transition. The potential application of these nanoparticles as drug delivery vehicles was investigated by using carboxyfluorescein. These experiments revealed that upon externally induced disruption of the SNPs (by tetraethylammonium chloride) the fluorescent dye was trapped into micellar aggregates that can be further disrupted by cyclodextrin additionhttp://www.usc.es/ Financial support from Ministerio de Economia y Competitividad of Spain (projects CTQ2014-55208-P, CTQ2017-84354-P, CTQ2014-59646-R and MAT2015-67458-P), Xunta de Galicia (GR 2007/085; IN607C 2016/03 and Centro singular de investigación de Galicia accreditation 2016-2019, ED431G/09) and the European Union (European Regional Development Fund – ERDF), is gratefully acknowledged. J.M. received a Ramón y Cajal (RYC-2013- 13784) and a starting grant from the ERC (DYNAP677786) SI

Historically, the chemistry of phosphate transfer-a class of reactions fundamental to the chemistry of Life-has been discussed almost exclusively in terms of the nucleophile and the leaving group. Reactivity always depends significantly on both factors; but recent results for reactions of phosphate triesters have shown that it can also depend strongly on the nature of the nonleaving or "spectator" groups. The extreme stabilities of fully ionised mono- and dialkyl phosphate esters can be seen as extensions of the same effect, with one or two triester OR groups replaced by O(-). Our chosen lead reaction is hydrolysis-phosphate transfer to water: because water is the medium in which biological chemistry takes place; because the half-life of a system in water is an accepted basic index of stability; and because the typical mechanisms of hydrolysis, with solvent H2O providing specific molecules to act as nucleophiles and as general acids or bases, are models for reactions involving better nucleophiles and stronger general species catalysts. Not least those available in enzyme active sites. Alkyl monoester dianions compete with alkyl diester monoanions for the slowest estimated rates of spontaneous hydrolysis. High stability at physiological pH is a vital factor in the biological roles of organic phosphates, but a significant limitation for experimental investigations. Almost all kinetic measurements of phosphate transfer reactions involving mono- and diesters have been followed by UV-visible spectroscopy using activated systems, conveniently compounds with good leaving groups. (A "good leaving group" OR* is electron-withdrawing, and can be displaced to generate an anion R*O(-) in water near pH 7.) Reactivities at normal temperatures of P-O-alkyl derivatives-better models for typical biological substrates-have typically had to be estimated: by extended extrapolation from linear free energy relationships, or from rate measurements at high temperatures. Calculation is free from these limitations, able to handle very slow reactions as readily as very fast ones, and capable of predicting rate constants with levels of accuracy acceptable to the experimentalist. We present an updated overview of phosphate transfer, with particular reference to the mechanisms of the reactions of alkyl derivatives and triesters. The intention is to present a holistic (not comprehensive!) overview of the reactivity of typical phosphate esters, in terms familiar to the working chemist, at a level sufficient to support informed predictions of reactivity for structures of interest.

The hydrolytic reactivity of simple phosphate diesters with very good leaving groups is known to be practically independent of the nonleaving group at 100 °C. Calculations on the (too slow to measure) hydrolysis at 25 °C of a series of p-nitrophenyl diesters ROPO2––OpNP with a wide range of nonleaving group OR indicate a small but significant effect at the lower temperature, making the R = methyl ester the most reactive. This is in the opposite sense to the much larger effect observed for the reactions of triesters and consistent with a reaction driven primarily by leaving group departure. The calculations use a continuum model with up to four discrete water molecules: two or three waters give the best results.

This paper presents the physicochemical properties of micellar aggregates formed from a series of zwitterionic surfactants of the type 3-(1-alkyl-3-imidazolio)propane-sulfonate (ImS3-n), with n = 10, 12, 14, and 16. The ImS3-n dipolar ionic surfactants represent a versatile class of dipolar ionic compounds, which form normal and reverse micelles. Furthermore, they are able to stabilize nanoparticles in water and in organic media. Aqueous solubility is too low at room temperature to allow characterization of micellar aggregates but increases with addition of salts, allowing determination of aggregation number and cmc. As expected, these parameters depend on the length of the alkyl chain, and cmc values follow Klevens equation. In the presence of NaClO4, all ImS3-n micelles become anionoid by incorporating ClO4(-) on the micellar interface. A special feature of these surfactants is the ability to form reverse micelles and solubilize copious amounts of saline solutions in chloroform. (1)H NMR and infrared spectroscopic evidence showed that the maximum water to surfactant molar ratio w0 achievable depends on the concentration and type of salt dissolved. Reverse micelles of the ImS3-n surfactants can be used to stabilize metallic nanoparticles, whose size may be tuned by the amount of water dissolved.

The aggregation of supramolecular amphiphiles formed from hexamethylated p-sulfonatocalix[6]arene (SC6HM) and dodecyltrimethylammonium bromide (C12TAB) was studied by capillary electrophoresis experiments and by kinetic probes. The hydrolysis of 4-methoxybenzenesulfonyl chloride (MBSC) was used to investigate the micropolarity of the micellar aggregates and their ability to solubilize and stabilize labile organic compounds against hydrolysis. Further insights were obtained using a more sophisticated kinetic probe: the basic hydrolysis of p-nitrophenylvalerate (NPV). This probe provides information on the ionic composition of the micellar interface and on the potential of the aggregates to be used as nanoreactors. The results obtained revealed that the charge of the micellar aggregates can be tuned from anionic to cationic through the adjustment of the C12TAB : SC6HM molar ratio and confirmed that these micelles have good solubilization properties. On the other hand, the kinetics of the p-nitrophenylvalerate basic hydrolysis suggest that, in the concentration range comprised between the first and second CMCs, Br(-) anions do not take part in the micellar structure.

The reactions of O,O-diethyl 2,4-dinitrophenylphosphate triester (1) with piperidine in ionic liquids and four conventional organic solvents (COS) were subjected to kinetic and product studies. Analytical techniques (UV-vis and NMR) identified two pathways: nucleophilic attack at the phosphoryl center and at the C-1 aromatic carbon. The nucleophilic rate constants (kTN) for these parallel reactions were separated into two terms: kPN and kArN for the corresponding electrophilic centers. Both the rate and the selectivities of the reactions are strongly dependent on the nature of the ionic liquid used, and a good correlation with the solvent acceptor capacity to form hydrogen bonds (β) was observed. Remarkably, an exclusive attack at the phosphoryl center was found using [Bmim]DCA, [Bmpyrr]DCA and [Bmpy]DCA as the reaction solvents. In contrast, with [Bmim]PF6 as the reaction solvent, attack at the C-1 aromatic was the main path (94%). These results suggest that ionic liquids can be considered to be designer solvents because by an appropriate choice of the anion it is possible to steer the selectivity of this reaction.

Silica-borax pearl samples impregnated with 0.17 and 0.64% Cr3+ were characterized by specific surface area measurements, UV-Vis spectroscopy, energy-dispersive X-ray fluorescence and laser-scanning confocal microscopy. Pearl stability against oxidizing conditions was tested by adding samples to an aqueous hydrogen peroxide solution. The reaction was examined by UV-Vis spectroscopic measurements of the supernatant and laser-scanning confocal microscopy images of the substrate. Overall, hydrogen peroxide-induced Cr3+ to Cr6+ oxidation across the solid-liquid interface promoted solid matrix cleavage pearl degradation and concomitant formation of multiple scattering centers was observed. A dual-detection scheme was employed in the confocal microscopy measurements allowing us to separate scattering and absorptive contributions to the observed signals. The confocal microscopy images indicate that Cr3+ oxidation induced by hydrogen peroxide solutions occurs throughout the entire pearl sample and indicate that oxidation reactions induce leakage of chromate ion into aqueous solutions.

We report a study of the P–O bond cleavage of the anion of 2,4-dinitrophenyl ethyl phosphate (2,4-DNPEP), promoted by methylamine. The aminolysis reaction was investigated in the gas phase and an aqueous environment using the hybrid quantum mechanical/effective fragment potential (QM/EFP) approach. According to the B3LYP/6-31++G(d,p) results, in the gas phase the cleavage of the P–O bond of 2,4-DNPEP proceeds in one step through a concerted mechanism, with an activation free energy of 39 kcal/mol. Including solvent effects at the B3LYP/6-31++G(d,p)/EFP level changes the reaction to a two-step associative mechanism. The first step corresponds to the formation of the P–NHMe bond and the second step to concerted proton transfer and P–O(2,4-dinitrophenolate) cleavage. In the absence of any assistance of water molecules to promote the proton transfer, the second step is rate determining with an activation free energy for the breakdown of the intermediate of 21.8 kcal/mol at the MP2/cc-pVTZ level and a free energy of activation of 32.3 kcal/mol for the overall reaction. However, the proton transfer with the assistance of a water molecule may occur without significant energy barrier. The results show how solvent can play an important role in favoring an associative mechanism for the aminolysis of phosphate diesters, through the formation and stabilization of a pentacoordinate phosphorane intermediate.

Al3+-Mg2+ mixed oxides were prepared by coprecipitation and characterized with scanning electron microscopy (SEM), energy dispersive X-ray fluorescence (EDXRF), temperature programmed desorption of CO2 (CO2-TPD), and N2 adsorption/desorption isotherms (BET and BJH methods). By increasing the MgO concentration up to 31.8% (w/w), X-ray diffraction (XRD) measurements suggested an incipient magnesium aluminate spinel (MgAl2O4) phase. However, the spinel crystalline structure was obtained only after calcination at 950 °C. These materials were tested as catalysts in the propanolysis reaction of methyl paraoxon. This reaction in the presence of the more efficient incipient MgAl2O4 spinel is of the order of 2.5 × 105-fold faster than the spontaneous propanolysis reaction and results in the formation of a product that is structurally related to a family of flame retardants. The different products of propanolysis and hydrolysis were identified by electrospray ionization mass spectrometry (ESI(+)-MS), ESI(+)-MS/MS) ...

The microscopic pKa of the pyridine N of the phosphate monoester 2-pyridyl phosphate (MPP) is 2.76, significantly higher than that expected for the leaving group (RO) oxygen of a typical monoester ROPO3=. This favours the prototropic equilibrium considered to be the key step in the rapid hydrolysis of a typical monoester, and the observed rate of hydrolysis of the monoanion MPP− is shown to be close to that expected for the protomer MPP+=, with neutral 2-pyridone as the leaving group. Copyright © 2013 John Wiley & Sons, Ltd.

This work describes the characterization of the solid biomass, and its aqueous extracts, obtained from the cones of Araucaria angustifolia and Pinus elliottii, in order to understand the mechanism of Cr(III) biosorption in these biosorbents, as well as to characterize the residues of the process. Trivalent chromium was quantified, for the first time in a complex biomass system, by fluorescence suppression using naphthalene as probe, and the quantification could be used safely for both biosorbents Araucaria and Pinus. The results showed that Cr(III) can be effectively removed using Araucaria and Pinus wastes and their biosorption capacities are higher than those of other lignocellulosic materials under similar conditions. Carboxylic moieties present on the biosorbent surface seem to play a major role on Cr(III) binding.

Notwithstanding its half-life of 70 years at 25 °C, the spontaneous hydrolysis of the anion of di-2-pyridyl phosphate (DPP) is thousands of times faster (ca. 3000 at 100 °C, over 10000-fold at 25 °C) than expected for a diester with leaving groups of pK(a) 9.09. The kinetic parameters do not permit a conclusive choice between five possible mechanisms considered, but the combination of kinetics and calculational evidence supports a single-step, concerted, S(N)2(P) mechanism involving the attack of solvent water on phosphorus assisted by intramolecular catalysis by a (weakly basic) pyridine nitrogen acting as a general base. Catalysis is relatively efficient for this mechanism, with an estimated effective molarity (EM) of the general base of15 M, consistent with the absence of catalysis by typical buffers. Further new results confirm that varying the nonleaving group has minimal effect on the rate of spontaneous diester hydrolysis, in striking contrast to the major effect on the corresponding reaction of triesters: though protonation of one nitrogen of DPP(-) increases the rate of hydrolysis by 6 orders of magnitude, in line with expectation.

Zwitterionic surfactants are relatively overlooked and some of their properties resemble those of ionic and non-ionic surfactants, but others are unique. There is unimpeachable evidence that aqueous solutions of zwitterionic micelles interact specifically with anions, forming “anionoid” micelles, which concentrate cations in the surfaces. Thus, unlike ionic micelles, both the cation and anion of the electrolyte solution can be concentrated at zwitterionic interfaces. This unique effect, known as the “Chameleon Effect”, can be used to catalyze a variety of simple reactions, as the attraction of ions to the micelle brings the reactants together. Furthermore, zwitterionic surfactants stabilize metallic nanoparticles and the magnetically stirred two-phase system could be reused 3 more times in the hydrogenation of cyclohexene (Pd:cyclohexene ratio of 1:18300), with very little loss in activity, and an average turn-over frequency of 1000 h − 1 .

Palladium nanoparticles (NPs) stabilized by ImS3-14, a zwitterionic surfactant structurally related to ionic liquids, are revealed here to be good catalysts for the reduction of a large number of substituted aromatic nitro compounds. Our mass spectrometry results are consistent with the formation of amino products in a direct route, where the aromatic nitro compounds are initially reduced to nitroso compounds, which are then reduced to the hydroxylamine derivatives and finally to the anilines. Activation parameters showed that for most Pd catalysts reported in the literature, the mechanism seems to be similar, with lower enthalpy of activation (ΔH ‡ ) being compensated by more negative entropy of activation (ΔS ‡ ). As a result, the reaction is thermally compensated and the rate constants for most reactions rather similar. Furthermore, Pd NPs stabilized by ImS314 showed efficient catalytic activities for the reduction of aromatic nitro compounds, with high conversion and good selectivity even using very low loadings of metal.

2-Naphthol (2NOH) was used as a fluorescent probe in order to examine and quantify the changes in hydrogen ion concentration in micelles formed by the zwitterionic 3-(tetradecyldimethylammonium)-propanesulfonate (SB3-14) surfactant or by anionic sodium dodecyl sulfate (SDS). In the presence of SDS, 2NOH is incorporated into the anionic micelle and the neutral form of the probe becomes the dominant species. The results are consistent with a microenvironment probably with a higher acidity and/or lower polarity in the micellar surface. The addition of SB3-14 generates a plateau at pH 3 to 9 with a fluorescent component of low intensity, which indicates the partial formation of 2NO(-)*, promoted by proton transfer to water. Theoretical results provided information on the structural parameters, emission wavelength, and changes in ΔpK(a) values due to the solvent, which are consistent with a solubilization site similar to aqueous ethanol. Zwitterionic surfactants concentrate anions such as trifluoroacetate in zwitterionic micelles, and as a result, the micellar surface charge becomes negative and promotes hydrogen ion incorporation into the micellar surface. Effects observed on the proton transfer between 2NOH* and anions in zwitterionic micellar solutions are complex and, besides the well-known anion incorporation, include changes in the surface potential and acidity of the surface. Zwitterionic micelles are able to emulate the mostly zwitterionic nature of biological membranes, and in the complex nature of zwitterionic micelles, we found reasons for the selection of zwitterionic headgroups in surfactants in natural systems as major components of biological interfaces.

This work presents a detailed kinetic and mechanistic study of biologically interesting dephosphorylation reactions involving the exceptionally reactive nucleophilic group, hydroxamate. We compare results for hydroxamate groups anchored on the simple molecular backbone of benzohydroxamate (BHA) and on the more complex structure of the widely used drug, deferoxamine (DFO). BHA shows extraordinary reactivity toward the triester diethyl 2,4-dinitrophenyl phosphate (DEDNPP) and the diester ethyl 2,4-dinitrophenyl phosphate (EDNPP) but reacts very slowly with the monoester 2,4-dinitrophenyl phosphate (DNPP). Nucleophilic attack on phosphorus is confirmed by the detection of the phosphorylated intermediates formed. These undergo Lossen-type rearrangements, resulting in the decomposition of the nucleophile. DFO, which is used therapeutically for the treatment of acute iron intoxication, carries three hydroxamate groups and shows correspondingly high nucleophilic activity toward both triester DEDNPP and diester EDNPP. This result suggests a potential use for DFO in cases of acute poisoning with phosphorus pesticides.

In this study, peroxidase from alfalfa sprout extract (Medicago sativa) was immobilized on a gold electrode, using a solution of the zwitterionic surfactant 3-(1-tetradecyl-3-imidazolio)propanesulfonate (ImS3-14) containing well-dispersed gold nanoparticles. In the presence of hydrogen peroxide, the peroxidase enzyme catalyzes the oxidation of hydroquinone to o-quinone, which is electrochemically reduced at a potential of +0.09 V vs. Ag/AgCl. The hydroquinone biosensor exhibited excellent electrocatalytic activity under the optimized experimental conditions ([peroxidase] = 97 units mL−1, [H2O2] = 7.4 × 10−5 M, pH 7.0, frequency of 100 Hz, pulse amplitude of 60 mV and scan increment of 4.0 mV). Peak current linearity was observed in the range of 6.96 × 10−7 to 8.54 × 10−6 M, and the detection limit was estimated to be 1.88 × 10−7 M. This biosensor demonstrated good repeatability and reproducibility with relative standard deviations of 3.6% and 6.5%, respectively, and an acceptable stability (at least 100 determinations). The method has been applied to the determination of hydroquinone in skin-brightening creams and the recoveries were 98.3–104.5%.

The low oxidative stability of biodiesel derived from polyunsaturated oils is one of the great challenges to be overcome in biodiesel technology. Here, we use a high activity catalyst Pd/ImS3-12@Al 2 O 3 , prepared by adsorption of water-prepared palladium nanoparticles on aluminum oxide, for partial biodiesel hydrogenation. The catalyst shows higher hydrogenation activity than conventional Pd/C, leading in the absence of ionic liquid (IL) to fully hydrogenated product. In the presence of BMIM·NTf 2 (1-butyl-3-methylimdazolium bis(trifluoromethylsulfonyl)imide) IL the activity of the Pd/ImS3-12@Al 2 O 3 catalyst could be controlled and selective reduction to monoene product, with large preference for formation of the cis isomer, could be achieved at low temperature and pressure, with very low catalyst loading, no detectable metal leaching and good recyclability. The partially hydrogenated biodiesel is far more stable to oxidation than the crude, without compromising its cold-flow properties.

Acyl transfers are key reactions in biology and in the laboratory. In biological systems they are involved in energy transport, in the assembly of complex molecules and in the mechanisms of efficient action of many hydrolytic enzymes. We report a mechanistic and calculational study of the selective N-acylation reactions of amino acids by substituted aspirins, under mild conditions, in water at 25 °C. The acetylated amino-acid products of the reactions were identified by nuclear magnetic resonance, and the reaction steps were studied by density functional theory. Copyright © 2012 John Wiley & Sons, Ltd.

We report a detailed photophysical study of 1-dodecyl-4-[5-(4-dodecyloxyphenyl)-1,3,4-oxadiazole-2-yl]pyridinium bromide ( 454Do ), a cationic amphiphile that behaves as a fluorescent liquid crystal. Excitation and emission spectra of the probe in different environments result in significant changes in quantum yields which are correlated with changes in lifetimes and theoretical calculations.

Chemical composition and species distribution in clay surfaces are key factors which control sorption and desorption processes of Cr3+ between sediment and water. Adsorption of Cr3+ on silica gel has been studied by following the quenching of naphthalene fluorescence in aqueous sodium dodecyl sulfate (SDS) micelles, which concentrate naphthalene and Cr3+ in the micellar surface. Adsorption for pH ≤ 5 follows a Langmuir model, and experimental data fitting is successfully described by a model based on discrete sites on the surface with variable charge as a function of pH. Probably the mechanism of Cr3+ adsorption at the silica gel surface is predominantly electrostatic, with outer-sphere complexation and ion exchange with adsorbed H+ ions. Confocal laser-scanning fluorescence microscopy measurements reveal the Cr3+ spatial distribution in three dimensions. Specifically, both images and spectra measured with confocal microscopy indicate that Cr3+ is distributed across the entire particle, not just adsorptio...