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Tardigrades are microscopic animals that survive desiccation by inducing biostasis. To survive drying tardigrades rely on intrinsically disordered CAHS proteins, which also function to prevent perturbations induced by drying in vitro and in heterologous systems. CAHS proteins have been shown to form gels both in vitro and in vivo, which has been speculated to be linked to their protective capacity. However, the sequence features and mechanisms underlying gel formation and the necessity of gelation for protection have not been demonstrated. Here we report a mechanism of fibrillization and gelation for CAHS D similar to that of intermediate filament assembly. We show that in vitro, gelation restricts molecular motion, immobilizing and protecting labile material from the harmful effects of drying. In vivo, we observe that CAHS D forms fibrillar networks during osmotic stress. Fibrillar networking of CAHS D improves survival of osmotically shocked cells. We observe two emergent properties associated with fibrillization; (i) prevention of cell volume change and (ii) reduction of metabolic activity during osmotic shock. We find that there is no significant correlation between maintenance of cell volume and survival, while there is a significant correlation between reduced metabolism and survival. Importantly, CAHS Ds fibrillar network formation is reversible and metabolic rates return to control levels after CAHS fibers are resolved. This work provides insights into how tardigrades induce reversible biostasis through the self-assembly of labile CAHS gels.

Optical stimulation in the red/near infrared range recently gained increasing interest, as a not-invasive tool to control cardiac cell activity and repair in disease conditions. Translation of this approach to therapy is hampered by scarce efficacy and selectivity. The use of smart biocompatible materials, capable to act as local, NIR-sensitive interfaces with cardiac cells, may represent a valuable solution, capable to overcome these limitations. In this work, a far red-responsive conjugated polymer, namely poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene-2,6-diyl]] (PCPDTBT) is proposed for the realization of photoactive interfaces with cardiomyocytes derived from pluripotent stem cells (hPSC-CMs). Optical excitation of the polymer turns into effective ionic and electrical modulation of hPSC-CMs, in particular by fastening Ca 2+ dynamics, inducing action potential shortening, accelerating the spontaneous beating frequency. The involvement in the phototransduction pathway of Sarco-Endoplasmic Reticulum Calcium ATPase (SERCA) and Na + /Ca 2+ exchanger (NCX) is proven by pharmacological assays and is correlated with physical/chemical processes occurring at the polymer surface upon photoexcitation. Very interestingly, an antiarrhythmogenic effect, unequivocally triggered by polymer photoexcitation, is also observed. Overall, red-light excitation of conjugated polymers may represent an unprecedented opportunity for fine control of hPSC-CMs functionality and can be considered as a perspective, noninvasive approach to treat arrhythmias., (© 2023 The Authors. Advanced Science published by Wiley-VCH GmbH.)

Nanostructuration is a promising tool for enhancing the performance of sensors based on electrochemical transduction. Nanostructured materials allow for increasing the surface area of the electrode and improving the limit of detection (LOD). In this regard, inverse opals possess ideal features to be used as substrates for developing sensors, thanks to their homogeneous, interconnected pore structure and the possibility to functionalize their surface. However, overcoming the insulating nature of conventional silica inverse opals fabricated via sol-gel processes is a key challenge for their application as electrode materials. In this work, colloidal assembly, atomic layer deposition and selective surface functionalization are combined to design conductive inverse opals as an electrode material for novel glucose sensing platforms. An insulating inverse opal scaffold is coated with uniform layers of conducting aluminum zinc oxide and platinum, and subsequently functionalized with glucose oxidase embedded in a polypyrrole layer. The final device can sense glucose at concentrations in the nanomolar range and is not affected by the presence of common interferents gluconolactone and pyruvate. This method may also be applied to different conductive materials and enzymes to generate a new class of highly efficient biosensors.

The design of soft and nanometer-scale photoelectrodes able to stimulate and promote the intracellular concentration of reactive oxygen species (ROS) is searched for redox medicine applications. In this work, we show semiconducting polymer porous thin films with an enhanced photoelectrochemical generation of ROS in human umbilical vein endothelial cells (HUVECs). To achieve that aim, we synthesized graft copolymers, made of poly(3-hexylthiophene) (P3HT) and degradable poly(lactic acid) (PLA) segments, P3HT- g -PLA. In a second step, the hydrolysis of sacrificial PLA leads to nanometer-scale porous P3HT thin films. The pore sizes in the nm regime (220-1200 nm) were controlled by the copolymer composition and the structural arrangement of the copolymers during the film formation, as determined by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The porous P3HT thin films showed enhanced photofaradaic behavior, generating a higher concentration of ROS in comparison to non-porous P3HT films, as determined by scanning electrochemical microscopy (SECM) measurements. The exogenous ROS production was able to modulate the intracellular ROS concentration in HUVECs at non-toxic levels, thus affecting the physiological functions of cells. Results presented in this work provide an important step forward in the development of new tools for precise, on-demand, and non-invasive modulation of intracellular ROS species and may be potentially extended to many other physiological or pathological cell models.

Mapping of the metabolic activity of tumor tissues represents a fundamental approach to better identify the tumor type, elucidate metastatic mechanisms and support the development of targeted cancer therapies. The spatially resolved quantification of Warburg effect key metabolites, such as glucose and lactate, is essential. Miniaturized electrochemical biosensors scanned over cancer cells and tumor tissue to visualize the metabolic characteristics of a tumor is attractive but very challenging due to the limited oxygen availability in the hypoxic environments of tumors that impedes the reliable applicability of glucose oxidase-based glucose micro-biosensors. Herein, the development and application of a new glucose micro-biosensor is presented that can be reliably operated under hypoxic conditions. The micro-biosensor is fabricated in a one-step synthesis by entrapping during the electrochemically driven growth of a polymeric matrix on a platinum microelectrode glucose oxidase and a catalytically active Prussian blue type aggregate and mediator. The as-obtained functionalization improves significantly the sensitivity of the developed micro-biosensor for glucose detection under hypoxic conditions compared to normoxic conditions. By using the micro-biosensor as non-invasive sensing probe in Scanning Electrochemical Microscopy (SECM), the glucose uptake by a breast metastatic adenocarcinoma cell line, with an epithelial morphology, is measured., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Current cancer research is limited by the availability of reliable in vivo and in vitro models that are able to reproduce the fundamental hallmarks of cancer. Animal experimentation is of paramount importance in the progress of research, but it is becoming more evident that it has several limitations due to the numerous differences between animal tissues and real, in vivo human tissues. 3D bioprinting techniques have become an attractive tool for many basic and applied research fields. Concerning cancer, this technology has enabled the development of three-dimensional in vitro tumor models that recreate the characteristics of real tissues and look extremely promising for studying cancer cell biology. As 3D bioprinting is a relatively recently developed technique, there is still a lack of characterization of the chemical cellular microenvironment of 3D bioprinted constructs. In this work, we fabricated a cervical tumor model obtained by 3D bioprinting of HeLa cells in an alginate-based matrix. Characterization of the spheroid population obtained as a function of culturing time was performed by phase-contrast and confocal fluorescence microscopies. Scanning electrochemical microscopy and platinum nanoelectrodes were employed to characterize oxygen concentrations-a fundamental characteristic of the cellular microenvironment-with a high spatial resolution within the 3D bioprinted cervical tumor model; we also demonstrated that the diffusion of a molecular model of drugs in the 3D bioprinted construct, in which the spheroids were embedded, could be measured quantitatively over time using scanning electrochemical microscopy.

The ability to exploit energy autonomously is one of the hallmarks of life. Mastering such processes in artificial nanosystems can open technological opportunities. In the last decades, light- and chemically driven autonomous systems have been developed in relation to conformational motion and self-assembly, mostly in relation to molecular motors. In contrast, despite electrical energy being an attractive energy source to power nanosystems, its autonomous harnessing has received little attention. Herein we consider an operation mode that allows the autonomous exploitation of electrical energy by a self-assembling system. Threading and dethreading motions of a pseudorotaxane take place autonomously in solution, powered by the current flowing between the electrodes of a scanning electrochemical microscope. The underlying autonomous energy ratchet mechanism drives the self-assembly steps away from equilibrium with a higher energy efficiency compared to other autonomous systems. The strategy is general and might be extended to other redox-driven systems., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Three recently synthesized neutral dinuclear carbonyl manganese complexes with the pyridazine bridging ligand, of general formula [Mn 2 (μ-ER) 2 (CO) 6 (μ-pydz)] (pydz = pyridazine; E = O or S; R = methyl or phenyl), have been investigated by cyclic voltammetry in dimethylformamide and acetonitrile both under an inert argon atmosphere and in the presence of carbon dioxide. This family of Mn(I) compounds behaves interestingly at negative potentials in the presence of CO 2 . Based on this behavior, which is herein discussed, a rather efficient catalytic mechanism for the CO 2 reduction reaction toward the generation of CO has been hypothesized.

Fullerenes are considered excellent photosensitizers, being highly suitable for photodynamic therapy (PDT). A lack of water solubility and low biocompatibility are, in many instances, still hampering the full exploitation of their potential in nanomedicine. Here, we used human serum albumin (HSA) to disperse fullerenes by binding up to five fullerene cages inside the hydrophobic cavities. Albumin was bioconjugated with folic acid to specifically address the folate receptors that are usually overexpressed in several solid tumors. Concurrently, tetramethylrhodamine isothiocyanate, TRITC, a tag for imaging, was conjugated to C 60 @HSA in order to build an effective phototheranostic platform. The in vitro experiments demonstrated that: (i) HSA disperses C 60 molecules in a physiological environment, (ii) HSA, upon C 60 binding, maintains its biological identity and biocompatibility, (iii) the C 60 @HSA complex shows a significant visible-light-induced production of reactive oxygen species, and (iv) folate bioconjugation improves both the internalization and the PDT-induced phototoxicity of the C 60 @HSA complex in HeLa cells.

The electronic, optical, and redox properties of thiophene-based materials have made them pivotal in nanoscience and nanotechnology. However, the exploitation of oligothiophenes in photodynamic therapy is hindered by their intrinsic hydrophobicity that lowers their biocompatibility and availability in water environments. Here, we developed human serum albumin (HSA)-oligothiophene bioconjugates that afford the use of insoluble oligothiophenes in physiological environments. UV-vis and electrophoresis proved the conjugation of the oligothiophene sensitizers to the protein. The bioconjugate is water-soluble and biocompatible, does not have any "dark toxicity", and preserves HSA in the physiological monomeric form, as confirmed by dynamic light scattering and circular dichroism measurements. In contrast, upon irradiation with ultralow light doses, the bioconjugate efficiently produces reactive oxygen species (ROS) and leads to the complete eradication of cancer cells. Real-time monitoring of the photokilling activity of the HSA-oligothiophene bioconjugate shows that living cells "explode" upon irradiation. Photodependent and dose-dependent apoptosis is one of the primary mechanisms of cell death activated by bioconjugate irradiation. The bioconjugate is a novel theranostic platform able to generate ROS intracellularly and provide imaging through the fluorescence of the oligothiophene. It is also a real-time self-reporting system able to monitor the apoptotic process. The induced phototoxicity is strongly confined to the irradiated region, showing localized killing of cancer cells by precise light activation of the bioconjugate., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

The kinetics of flash-induced re-reduction of the Photosystem II (PS II) primary electron donor P 680 was studied in solution and in trehalose glassy matrices at different relative humidity. In solution, and in the re-dissolved glass, kinetics were dominated by two fast components with lifetimes in the range of 2-7 μs, which accounted for >85% of the decay. These components were ascribed to the direct electron transfer from the redox-active tyrosine Y Z to P 680 + . The minor slower components were due to charge recombination between the primary plastoquinone acceptor Q A - and P 680 + . Incorporation of the PS II complex into the trehalose glassy matrix and its successive dehydration caused a progressive increase in the lifetime of all kinetic phases, accompanied by an increase of the amplitudes of the slower phases at the expense of the faster phases. At 63% relative humidity the fast components contribution dropped to ~50%. A further dehydration of the trehalose glass did not change the lifetimes and contribution of the kinetic components. This effect was ascribed to the decrease of conformational mobility of the protein domain between Y Z and P 680 , which resulted in the inhibition of Y Z  → P 680 + electron transfer in about half of the PS II population, wherein the recombination between Q A - and P 680 + occurred. The data indicate that PS II binds a larger number of water molecules as compared to PS I complexes. We conclude that our data disprove the "water replacement" hypothesis of trehalose matrix biopreservation., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Cells of the facultative photosynthetic bacterium Rhodobacter capsulatus exploit the simultaneous presence in the cultural medium of the toxic oxyanion tellurite (TeO 3 2- ) and the redox mediator lawsone (2-hydroxy-1,4-naphthoquinone) by reducing tellurite to metal Te 0 nanoprecipitates (TeNPs) outside the cells. Here we have studied the mechanism by which lawsone interacts with metabolically active cells and analysed both structure and composition of the TeNPs collected from the growth medium of phototrophycally grown R. capsulatus. High Resolution Transmission Electron Microscopy (HR-TEM) images and Energy-Dispersive X-ray (EDX) microanalysis of TeNPs showed a central core of polycrystalline tellurium interspersed in an organic matrix with a predominant protein-based composition. The main proteins from Te 0 nanostructures were identified by Liquid Chromatography tandem-Mass Spectrometry and were all correlated with the cell outer membrane composition. The interaction of reduced lawsone with tellurite and with the bacterial cells was probed by Cyclic Voltammetry and Scanning ElectroChemical Microscopy (SECM). We concluded that lawsone is required for the reduction of tellurite to metal Te 0 in a reaction mechanism dependent on reducing equivalents deriving from the cell photosynthetic metabolism. SECM experiments demonstrate that lawsone, by diffusing inside the bacterial cells, is effectively available at the membrane site of the photosynthetic electron transport chain., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020. Published by Elsevier B.V.)

Protein reduction/oxidation processes trigger and finely regulate a myriad of physiological and pathological cellular functions. Many biochemical and biophysical stimuli have been recently explored to precisely and effectively modulate intracellular redox signaling, due to the considerable therapeutic potential. Here, we propose a first step toward an approach based on visible light excitation of a thiophene-based semiconducting polymer (P3HT), demonstrating the realization of a hybrid interface with the Cytochrome c protein (CytC), in an extracellular environment. By means of scanning electrochemical microscopy and spectro-electrochemistry measurements, we demonstrate that, upon optical stimulation, a functional interaction between P3HT and CytC is established. Polymer optical excitation locally triggers photoelectrochemical reactions, leading to modulation of CytC redox activity, either through an intermediate step, involving reactive oxygen species formation, or via a direct photoreduction process. Both processes are triggered by light, thus allowing excellent spatiotemporal resolution, paving the way to precise modulation of protein redox signaling., Competing Interests: Declaration of Interest There are no conflicts to declare., (Copyright © 2020 The Author(s). Published by Elsevier Inc. All rights reserved.)

Hybrid nanomaterials are a subject of extensive research in nanomedicine, and their clinical application is reasonably envisaged in the near future. However, the fate of nanomaterials in biological environments poses serious limitations to their application; therefore, schemes to monitor them and gain control on their toxicity could be of great help for the development of the field. Here, we propose a probe for PEGylated nanosurfaces based on hyaluronic acid (HA) functionalized with rhodamine B (RB). We show that the high-affinity interaction of this fluorogenic hyaluronan (HA-RB) with nanoparticles exposing PEGylated surfaces results in their sensing, labeling for super-resolution imaging, and synergistic cellular internalization. HA-RB forms nanogels that interact with high affinity-down to the picomolar range-with silica nanoparticles, selectively when their surface is covered by a soft and amphiphilic layer. This surface-driven interaction triggers the enhancement of the luminescence intensity of the dyes, otherwise self-quenched in HA-RB nanogels. The sensitive labeling of specific nanosurfaces also allowed us to obtain their super-resolution imaging via binding-activated localization microscopy (BALM). Finally, we show how this high-affinity interaction activates a synergistic cellular uptake of silica nanoparticles and HA-RB nanogels, followed by a differential fate of the two partner nanomaterials inside cells.

Protein aggregation is a complex physiological process, primarily determined by stress-related factors revealing the hidden aggregation propensity of proteins that otherwise are fully soluble. Here we report a mechanism by which glycolytic glyceraldehyde-3-phosphate dehydrogenase of Arabidopsis thaliana (AtGAPC1) is primed to form insoluble aggregates by the glutathionylation of its catalytic cysteine (Cys149). Following a lag phase, glutathionylated AtGAPC1 initiates a self-aggregation process resulting in the formation of branched chains of globular particles made of partially misfolded and totally inactive proteins. GSH molecules within AtGAPC1 active sites are suggested to provide the initial destabilizing signal. The following removal of glutathione by the formation of an intramolecular disulfide bond between Cys149 and Cys153 reinforces the aggregation process. Physiological reductases, thioredoxins and glutaredoxins, could not dissolve AtGAPC1 aggregates but could efficiently contrast their growth. Besides acting as a protective mechanism against overoxidation, S-glutathionylation of AtGAPC1 triggers an unexpected aggregation pathway with completely different and still unexplored physiological implications., Competing Interests: The authors declare no competing interest.

Levels of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in cells and cell redox balance are of great interest in live cells as they are correlated to several pathological and physiological conditions of living cells. ROS and RNS detection is limited due to their spatially restricted abundance: they are usually located in sub-cellular areas (e.g., in specific organelles) at low concentration. In this work, we will review and highlight the electrochemical approach to this bio-analytical issue. Combining electrochemical methods and miniaturization strategies, specific, highly sensitive, time, and spatially resolved measurements of cellular oxidative stress and redox balance analysis are possible. Graphical abstract In this work, we highlight and review the use of electrochemistry for the highly spatial and temporal resolved detection of ROS/RNS levels and of redox balance in living cells. These levels are central in several pathological and physiological conditions and the electrochemical approach is a vibrant bio-analytical trend in this field.

Using isotope labeled water (D 2 O and H 2 17 O) and pulsed W-band (94 GHz) high-field multiresonance EPR spectroscopies, such as ELDOR-detected NMR and ENDOR, the biologically important question of detection and quantification of local water in proteins is addressed. A bacterial reaction center (bRC) from Rhodobacter sphaeroides R26 embedded into a trehalose glass matrix is used as a model system. The bRC hosts the two native radical cofactor ions (primary electron donor) and (primary electron acceptor) as well as an artificial nitroxide spin label site-specifically attached to the surface of the H-protein domain. The three paramagnetic reporter groups have distinctly different local environments. They serve as local probes to detect water molecules via magnetic interactions (electron-nuclear hyperfine and quadrupole) with either deuterons or 17 O nuclei. bRCs were equilibrated in an atmosphere of different relative humidities allowing us to control precisely the hydration levels of the protein. We show that by using oxygen-17 labeled water quantitative conclusions can be made in contrast to using D 2 O which suffers from proton-deuterium exchange processes in the protein. From the experiments we also conclude that dry trehalose operates as an anhydrobiotic protein stabilizer in line with the "anchorage hypothesis" of bio-protection. It predicts selective changes in the first solvation shell of the protein upon trehalose-matrix dehydration with subsequent changes in the hydrogen-bonding network. Changes in hydrogen-bonding patterns usually have an impact on the global function of a biological system.

The ubiquinol:cytochrome (cyt) c oxidoreductase (or cyt bc 1 ) is an important membrane protein complex in photosynthetic and respiratory energy transduction. In bacteria such as Rhodobacter capsulatus it is constituted of three subunits: the iron-sulfur protein, cyt b and cyt c 1 , which form two catalytic domains, the Q o (hydroquinone (QH 2 ) oxidation) and Q i (quinone (Q) reduction) sites. At the Q o site, the pathways of bifurcated electron transfers emanating from QH 2 oxidation are known, but the associated proton release routes are not well defined. In energy transducing complexes, Zn 2+ binding amino acid residues often correlate with proton uptake or release pathways. Earlier, using combined EXAFS and structural studies, we identified Zn coordinating residues of mitochondrial and bacterial cyt bc 1 . In this work, using the genetically tractable bacterial cyt bc 1 , we substituted each of the proposed Zn binding residues with non-protonatable side chains. Among these mutants, only the His291Leu substitution destroyed almost completely the Q o site catalysis without perturbing significantly the redox properties of the cofactors or the assembly of the complex. In this mutant, which is unable to support photosynthetic growth, the bifurcated electron transfer reactions that result from QH 2 oxidation at the Q o site, as well as the associated proton(s) release, were dramatically impaired. Based on these findings, on the putative role of His291 in liganding Zn, and on its solvent exposed and highly conserved position, we propose that His291 of cyt b is critical for proton release associated to QH 2 oxidation at the Q o site of cyt bc 1 ., (Copyright © 2016 Elsevier B.V. All rights reserved.)

Ionic liquids (ILs) are promising materials exploited as solvents and media in many innovative applications, some already used at the industrial scale. The chemical structure and physicochemical properties of ILs can differ significantly according to the specific applications for which they have been synthesized. As a consequence, their interaction with biological entities and toxicity can vary substantially. To select highly effective and minimally harmful ILs, these properties need to be investigated. Here we use the so called chromatophores--protein-phospholipid membrane vesicles obtained from the photosynthetic bacterium Rhodobacter sphaeroides--to assess the effects of imidazolinium and pyrrolidinium ILs, with chloride or dicyanamide as counter anions, on the ionic permeability of a native biological membrane. The extent and modalities by which these ILs affect the ionic conductivity can be studied in chromatophores by analyzing the electrochromic response of endogenous carotenoids, acting as an intramembrane voltmeter at the molecular level. We show that chromatophores represent an in vitro experimental model suitable to probe permeability changes induced in cell membranes by ILs differing in chemical nature, degree of oxygenation of the cationic moiety and counter anion., (Copyright © 2015 Elsevier B.V. All rights reserved.)

Conformational protein dynamics is known to be hampered in amorphous matrixes upon dehydration, both in the absence and in the presence of glass forming disaccharides, like trehalose, resulting in enhanced protein thermal stability. To shed light on such matrix effects, we have compared the retardation of protein dynamics in photosynthetic bacterial reaction centers (RC) dehydrated at controlled relative humidity in the absence (RC films) or in the presence of trehalose (RC-trehalose glasses). Small scale RC dynamics, associated with the relaxation from the dark-adapted to the light-adapted conformation, have been probed up to the second time scale by analyzing the kinetics of electron transfer from the photoreduced quinone acceptor (QA(-)) to the photoxidized primary donor (P(+)) as a function of the duration of photoexcitation from 7 ns (laser pulse) to 20 s. A more severe inhibition of dynamics is found in RC-trehalose glasses than in RC films: only in the latter system does a complete relaxation to the light-adapted conformation occur even at extreme dehydration, although strongly retarded. To gain insight into the large scale RC dynamics up to the time scale of days, the kinetics of thermal denaturation have been studied at 44 °C by spectral analysis of the Qx and Qy bands of the RC bacteriochlorin cofactors, as a function of the sugar/protein molar ratio, m, varied between 0 and 10(4). Upon increasing m, denaturation is slowed progressively, and above m ∼ 500 the RC is stable at least for several days. The stronger retardation of RC relaxation and dynamics induced by trehalose is discussed in the light of a recent molecular dynamics simulation study performed in matrixes of the model protein lysozyme with and without trehalose. We suggest that the efficiency of trehalose in retarding RC dynamics and preventing thermal denaturation stems mainly from its propensity to form and stabilize extended networks of hydrogen bonds involving sugar, residual water, and surface residues of the RC complex and from its ability of reducing the free volume fraction of protein alone matrixes.

The photosynthetic reaction center (RC) is a membrane pigment-protein complex that catalyzes the initial charge separation reactions of photosynthesis. Following photoexcitation, the RC undergoes conformational relaxations which stabilize the charge-separated state. Dehydration of the complex inhibits its conformational dynamics, providing a useful tool to gain insights into the relaxational processes. We analyzed the effects of dehydration on the electronic structure of the primary electron donor P, as probed by visible-NIR and light-induced FTIR difference spectroscopy, in RC films equilibrated at different relative humidities r. Previous FTIR and ENDOR spectroscopic studies revealed that P, an excitonically coupled dimer of bacteriochlorophylls, can be switched between two conformations, P866 and P850, which differ in the extent of delocalization of the unpaired electron between the two bacteriochlorophyll moieties (PL and PM) of the photo-oxidized radical P(+). We found that dehydration (at r = 11%) shifts the optical Qy band of P from 866 to 850-845 nm, a large part of the effect occurring already at r = 76%. Such a dehydration weakens light-induced difference FTIR marker bands, which probe the delocalization of charge distribution within the P(+) dimer (the electronic band of P(+) at 2700 cm(-1), and the associated phase-phonon vibrational modes at around 1300, 1480, and 1550 cm(-1)). From the analysis of the P(+) keto C[double bond, length as m-dash]O bands at 1703 and 1713-15 cm(-1), we inferred that dehydration induces a stronger localization of the unpaired electron on PL(+). The observed charge redistribution is discussed in relation to the dielectric relaxation of the photoexcited RC on a long (10(2) s) time scale.