Photochemical Electron Transfer Reactions Research Articles

Small Papillae Regulated by SPD25 are Critical for Balancing Photosynthetic CO2 Assimilation and Water Loss in Rice

BackgroundThe leaf epidermis plays an important role in the transmission of light and the regulation of water and gas exchange, which influences the photosynthesis of mesophyll cells. Small papillae (SP) are one of the important structural elements of the leaf epidermis. The mechanism of the effect that small papillae have on rice leaf photosynthetic performance remains unclear.ResultsIn this study, a small papilla deficient 25 (spd25) mutant was isolated from japonica rice Longjin1. Small papillae were absent on the adaxial and abaxial leaf surfaces of the spd25 mutant and the silicon and cuticular wax content in the spd25 mutant leaves decreased. Map-based cloning and functional analysis revealed that SPD25, encoding a guanine nucleotide exchange factor for Rop, is a novel allele of OsRopGEF10. The spd25 mutant showed an increased water loss rate and reduced relative water content. The lower stomatal conductance in the spd25 mutant prevented water loss but decreased the intercellular CO2 concentration and net assimilation rate. The fluorescence parameters showed that the inhibited CO2 assimilation reaction feedback regulated the photochemical electron-transfer reaction, but the performance of Photosystem II was stable. Further analysis indicated that the excess light energy absorbed by the spd25 mutant was dissipated in the form of non-photochemical quenching to avoid photodamage through the optical properties of small papillae.ConclusionsSPD25 regulates the development of small papillae on the surface of rice leaves, which play an important role in balancing photosynthetic gas exchange and water loss. This study deepens our understanding of the physiological mechanisms by which small papillae affect photosynthetic performance.

Visible light initiated release of calcium ions through photochemical electron transfer reactions.

Photolysis of anthraquinone or flavin photosensitizers in the presence of calcium EDTA complexes results in decomposition of the EDTA complex, releasing free Ca2+. In the case of the flavin sensitizers, it is shown that millimolar concentrations of Ca2+ can be released using visible light (>440 nm) and with quantum yields as high as 0.31. The utility of this system is further demonstrated by in situ photogelation of an alginate solution.

(Anthracen-9-yl)methyl sulfides as a mechanistic probe for chemical as well as photochemical electron-transfer reactions

We have examined the intermediacy of sulfur-centred radical cations in the one-electron oxidation of (anthracen-9-yl)methyl sulfides. Reaction of (anthracen-9-yl)methyl sulfides with ceric ammonium nitrate (CAN) gave predominantly nitration and oxidation products along with products arising through sulfur-centred radical cations in minor amounts. Upon irradiation, (anthracen-9-yl)methyl sulfides underwent efficient intramolecular single electron transfer leading to a variety of products arising through the intermediacy of sulfur-centred radical cations.

Molecular Density Functional Theory: Application to Solvation and Electron-Transfer Thermodynamics in Polar Solvents

A molecular density functional theory of solvation is presented. The solvation properties of an arbitrary solute in a given solvent, both described by a molecular force field, can be obtained by minimization of a position- and orientation-dependent free-energy density functional. In the homogeneous reference fluid approximation, the unknown excess term of the functional can be approximated by the angular-dependent direct correlation function of the pure solvent. This function can be extracted from a preliminary MD simulation of the pure solvent by computing the angular-dependent pair distribution function and solving subsequently the molecular Ornstein-Zernike equation. The corresponding functional can then be minimized on a three-dimensional cubic grid for positions and a Gauss-Legendre angular grid for orientations to provide the solvation free energy of embedded molecules at the same time as the solvent three-dimensional microscopic structure. This functional minimization procedure is much more efficient than direct molecular dynamics simulations combined with thermodynamic integration schemes. The approach is shown to be also pertinent to the molecular-level determination of electron-transfer properties such as reaction free energy and reorganization energy. It is illustrated for molecular solvation and photochemical electron-transfer reactions in acetonitrile, a prototypical polar aprotic solvent.

Photoinduced intra- and intermolecular electron transfer in solutions and in solid organized molecular assemblies

The present paper highlights results of a systematic study of photoinduced electron transfer, where the fundamental aspects of the photochemistry occurring in solutions and in artificially or self-assembled molecular systems are combined and compared. In photochemical electron transfer (ET) reactions in solutions the electron donor, D, and acceptor, A, have to be or to diffuse to a short distance, which requires a high concentration of quencher molecules and/or long lifetimes of the excited donor or acceptor, which cannot always be arranged. The problem can partly be avoided by linking the donor and acceptor moieties covalently by a single bond, molecular chain or chains, or rigid bridge, forming D-A dyads. The covalent combination of porphyrin or phthalocyanine donors with an efficient electron acceptor, e.g. fullerene, has a two-fold effect on the electron transfer properties. Firstly, the electronic systems of the D-A pair result in a formation of an exciplex intermediate upon excitation both in solutions and in solid phases. The formation of the exciplex accelerates the ET rate, which was found to be as fast as >10(12) s(-1). Secondly, the total reorganization energy can be as small as 0.3 eV, even in polar solvents, which allows nanosecond lifetimes for the charge separated (CS) state. Molecular assemblies can form solid heterogeneous, but organized systems, e.g. molecular layers. This results in more complex charge separation and recombination dynamics. A distinct feature of the ET in organized assemblies is intermolecular interactions, which open a possibility for a charge migration both in the acceptor and in the donor layers, after the primary intramolecular exciplex formation and charge separation in the D-A dyad. The intramolecular ET is fast (35 ps) and efficient, but the formed interlayer CS states have lifetimes in microsecond or even second time domain. This is an important result considering possible applications.

Formation of cation–radical anion pairs derived from carboxybenzophenone–tetrabutylammonium salts. Pulse radiolysis studies

Pulse radiolysis of acetonitrile solutions of tetra-n-butyl ammonium salts of 2- and 4-carboxybenzophenones [BP-COO−···N+(C4H9)4] were performed in order to generate directly the reduced forms of the benzophenone moieties within pre-formed ion pairs. In earlier studies on photochemical electron transfer reactions, ion pairs containing a tetraalkyl ammonium cation and a benzophenone radical anion were formed in an electron transfer to the triplet BP from a quencher consisting of a tetraalkyl ammonium salt of (phenylthio)acetic acid. In the current work, the [BP•−COO−···N+(C4H9)4] ion pairs were formed by direct reduction of the salts without the complication of a third moiety, i.e., the (phenylthio)acetic anion. The spectra and kinetic parameters of the radiolytically-reduced salts were compared to the behavior of reduced forms of the 2- and 4-COOH substituted benzophenones. The results from the pulse radiolysis and photochemistry were compared and explained in terms of the different structures of the ion pairs.

Light-Harvesting Energy Transfer and Subsequent Electron Transfer of Cationic Porphyrin Complexes on Clay Surfaces

A novel energy-transfer system involving nonaggregated cationic porphyrins adsorbed on an anionic-type clay surface and the electron-transfer reaction that occurs after light harvesting are described. In the clay-porphyrin complexes, photochemical energy transfer from excited singlet zinc porphyrins to free-base porphyrins proceeds. The photochemical electron-transfer reaction from an electron donor in solution (hydroquinone) to the adsorbed porphyrin in the excited singlet state was also examined. Because the electron-transfer rate from the hydroquinone to the excited singlet free-base porphyrin is larger than that to the excited singlet zinc porphyrin, we conclude that the energy transfer accelerates the overall electron-transfer reaction.

Multifrequency Epr and Endor on Transient Radicals and Radical Pairs in Liquid Crystals and Proteins: Structure and Dynamics of Cofactors in Natural and Artifical Photosynthesis

Hotly debated issues in the area of biological electron transfer (ET) involve questions of how long-range ET processes proceed along pathways that incorporate non-covalently linked cofactors interacting with their protein microenvironment. To contribute answers to such questions, time-resolved EPR (TREPR) experiments at various microwave frequency/magnetic field settings have been performed. In this overview, representative examples of this work from our laboratory are given which comprise transient intermediates of light-induced ET in (i) bacterial photosynthetic reaction centers and (ii) biomimetic model complexes. In these complexes the donor and acceptor components are tethered together either covalently via cyclohexenyl bridges or non-covalently via Watson-Crick base-pairing. The studies are aiming at broadening our knowledge of structure-dynamics-function relationships associated with ET processes. It is demonstrated that high-field TREPR opens new perspectives in elucidating complex photochemical ET reactions with different paramagnetic states and species involved.

Kinetics of reductive N-O bond fragmentation: the role of a conical intersection.

N-alkoxyheterocycles can act as powerful one-electron acceptors in photochemical electron-transfer reactions. One-electron reduction of these species results in formation of a radical that undergoes N-O bond fragmentation to form an alkoxy radical and a neutral heterocycle. The kinetics of this N-O bond fragmentation reaction have been determined for a series of radicals with varying substituents and extents of delocalization. Rate constants varying over 7 orders of magnitude are obtained. A reaction potential energy surface is described that involves avoidance of a conical intersection. A molecular basis for the variation of the reaction rate constant with radical structure is given in terms of the relationship between the energies of the important molecular orbitals and the reaction potential energy surface. Ab initio and density functional electronic structure calculations provide support for the proposed reaction energy surface.

Excited Doublet States of Electrochemically Generated Aromatic Imide and Diimide Radical Anions

The radical anions of aromatic diimides have been implicated recently in a wide variety of photochemical electron transfer reactions. Photoexcitation of these radical anions produces powerfully reducing species. Yet, the properties of the π* excited doublet states of these organic radical anions remain obscure. The radical anions of three aromatic imides with increasingly larger π systems, N-(2,5-di-tert-butylphenyl)phthalimide, 1, N-(2,5-di-tert-butylphenyl)-1,8-naphthalimide, 2, and N-(2,5-di-tert-butylphenyl)perylene-3,4-dicarboximide, 3, as well as the three corresponding aromatic diimides, N,N‘-bis(2,5-di-tert-butylphenyl)pyromellitimide, 4a, N,N‘-bis(2,5-di-tert-butylphenyl)-naphthalene-1,8:4,5-tetracarboxydiimide, 5a, and N,N‘-bis(2,5-di-tert-butylphenyl)perylene-3,4:9,10-tetracarboxydiimide, 6, were produced by electrochemical reduction of the neutral molecules in an optically transparent thin layer electrochemical cell. The radical anions of these imides and diimides all exhibit intense visible and weaker near-IR absorption bands corresponding to their D0 → Dn transitions. Excited states of the radical anions were generated by subpicosecond excitation into these absorption bands. Excitation of 1•- and 2•- resulted in decomposition of these radical anions, whereas excitation of 3•-−6•- yielded transient spectra of their D1 → Dn transitions and the lifetimes of D1. The lifetimes of the D1 excited states of the radical anions of 3•-−6•- are all less than 600 ps and increase as the D0−D1 energy gap increases. These results impose design constraints on the use of these excited radical anions as electron donors in electron-transfer systems targeted toward molecular electronics and solar energy conversion.

Generation of electrophilic phenyltellurium and phenylselenium species via photochemical electron transfer

Photochemical electron transfer (PET) reactions were carried out using PET sensitizer (TPT) and dichalcogenides (DPDT/DPDS). Sensitizer fluorescence quenching by dichalcogenides is found to be very efficient and it leads to the electron-transfer process. A phenylchalcogenide cation produced via dichalcogenide radical cation, was successfully utilized in cyclization reactions using variety of substrates. Thermal cyclization reaction supports the PET mechanism and also indicates the potentiality of the method in organic synthesis.

Photochemical Electron-Transfer Reactions between Sulfides and Tetranitromethane. Oxidation vs Fragmentation of the Sulfide Radical-Cation Intermediate

Oxidation and/or fragmentation products are observed in the photochemical reaction of the alkyl phenyl sulfides 1a−d with tetranitromethane (TNM). The product distribution depends markedly on the substrate structure. Thus, methyl phenyl sulfide (1a) and benzyl phenyl sulfide (1b) give only the corresponding sulfoxides (oxidation). However, when the radical cation 1b•+ is generated by chemical oxidation with triarylaminium salts (Ar3N•+) in acetonitrile, in addition to oxidation fragmentation is also observed, and with an excess of Ar3N•+ oxidation is facilitated and no fragmentation is produced. For the photoreaction of diphenylmethyl phenyl sulfide (1c) with TNM, fragmentation is the main reaction, while for triphenylmethyl phenyl sulfide (1d) only this process is observed. The ease of C−S bond scission in these sulfur-centered radical cations 1•+ follows the ease of alkyl cation formation, i.e., Ph3C > Ph2CH > PhCH2 > CH3.

Photochemical Charge Transfer and Hydrogen Evolution Mediated by Oxide Semiconductor Particles in Zeolite-Based Molecular Assemblies

Two integrated systems for light-induced vectorial electron transfer are described. Both utilize photosensitized semiconductor particles grown in linear channel zeolites as components of the electron transfer chain. One system consists of internally platinized zeolites L and mordenite containing TiO2 particles and methylviologen ions, with a size-excluded photosensitizer, tris(2,2‘-bipyridyl-4,4‘-dicarboxylate)ruthenium (RuL32+), adsorbed on the external surface of the zeolite/TiO2 composite. In the other system, Nb2O5 replaces TiO2. The kinetics of photochemical electron transfer reactions and charge separation were studied by diffuse reflectance flash photolysis. Despite very efficient initial charge separation, the TiO2 system does not generate hydrogen photochemically in the presence of an electrochemically reversible, anionic electron donor, methoxyaniline N,N‘-bis(ethyl sulfonate). Only the Nb2O5-containing composites evolved hydrogen photochemically under these conditions. These results are interpreted in terms of semiconductor band energetics and the irreversibility of electron transfer from Nb2O5 to intrazeolitic platinum particles.

Magnetic Field Effects on the Photochemical Electron-Transfer Reactions of 10-Methylphenothiazine and Dicyanobenzene Derivatives in Nonviscous Homogeneous Solutions

Magnetic field effects on the photochemical electron-transfer reactions of 10-methylphenothiazine with 1,4-dicyanobenzene and tetrafluoro-1,4-dicyanobenzene are investigated by a nanosecond laser photolysis technique. For these reactions, the recombination of the geminate radical ion pairs as well as the yields and decay rates of the escaped radical ions showed clear magnetic field dependence in nonviscous homogeneous solutions. The mechanisms of the magnetic field effects are ascribed to the hyperfine coupling and Δg mechanisms at low and high magnetic fields, respectively. The smaller magnetic field effects on the reaction of the fluorinated derivative than those of 1,4-dicyanobenzene are ascribed to its smaller back-electron-transfer rate because its free-energy change is smaller than that of the nonsubstituted one.

Ultrafast photoinduced electron transfer reactions in supramolecular arrays: From charge separation and storage to molecular switches

Abstract

Photochemical electron transfer reactions in the acceptor complex of reaction centers of Rhodopseudomonas spheroides treated with sodium dodecyl sulfate

We have compared some photochemical properties of the reaction-center complex of Rhodopseudomonas spheroides (wild-type) treated with various amounts of either sodium dodecyl sulfate (SDS) or dodecyl dimethylamine N-oxide. In the presence of the latter, the native structure and activity of the reaction center are preserved even at high concentrations of detergent. In contrast, SDS denatures the protein. It does this by a cooperative process, as shown by the sigmoidal relationship between primary photochemical activity and SDS concentration. SDS binds to the reaction center at up to about 6 g SDS/g protein. The most notable modifications brought about by low SDS concentrations are: a slowing down of rereduction of P+ by Q-B from 1 s-1 to 0.25 s-1 and, to a lesser extent, of P+ by Q-A; the reoxidation of Q-A and Q-B by N,N,N',N'-tetramethyl-1,4-phenylenediamine is accelerated as an exponential function of SDS concentration. In this case Q-A become more accessible to external acceptors but not Q-B. It is thought that these changes are mainly due to progressive binding of SDS resulting in the unfolding of the protein, probably accompanied by the loss of the metal. Herbicides partly protect the reaction center against SDS denaturation, but their efficiency to block electron transfer between QA and QB is greatly reduced in the altered reaction centers present in the transition region of the denaturation curve. It is concluded that in the early region of the SDS denaturation curve, no dissociation of the reaction center in subunits occurs. In the transition region, it appears that the reaction center reaches the critical state in which mainly dissociation of polypeptide H from the complex of chains L and M and loss of QA take place.

Photosensing Device Composed of Bilayer Membranes of Polymer-Pendant Ru(bpy)32+and Prussian Blue

Abstract A photosensing device was fabricated based on a photochemical electron transfer reaction. A graphite electrode was coated with bilayer membranes of polymer-pendant Ru(bpy)3 2+ and Prussian Blue (PB) to give a bilayer-coated device. It was irradiated with visible light in an aqueous electrolyte and the induced photocurrent was measured as a function of the applied potential. The excitation of PB was responsible for the anodic photocurrent, while the excitation of the Ru complex mainly induced the cathodic photocurrent by an electron-transfer mechanism.

Laser Induced CIDNP Study of Photochemical Electron Transfer Reaction between Covalently Linked Porphyrin–Tryptophan Compounds and 1,4-Benzoquinone

Abstract Argon ion laser excitation of covalently linked porphyrin–tryptophan compound in the presence of 1,4-benzoquinone in benzene-d6 resulted in strong 1H-NMR polarizations of tryptophan moiety. These CIDNP effects are reasonably explained by the mechanism involving the secondary electron transfer from tryptophan moiety to porphyrin cation radical as the key step.

Photochemical electron-transfer reactions of 1,1-diarylethylenes

The dimerizations, nucleophilic additions, and oxygenation reactions of 1,1-diarylethylene (1), induced by photoexcited cyanoanthracenes, have been investigated. 1,1-Diphenylethylene (1a) yields 2 + 2 and 2 + 4 cyclodimers and one dehydrodimer, whereas 1,1-dianisylethylene (1b) gives two dehydrodimers, and 1,1-bis(4-(dimethylamino)phenyl)ethylene (1c) forms no dimers. Formation of the dehydrodimers is accompanied by reduction of the cyanoanthracene sensitizer. For the dimerization of 1b, the quantum yields and product ratios can be altered by the addition of quinones. The hydroquinone then replaces the reduced sensitizer. The reaction constant for nucleophilic addition of methanol to 1a/sup .+/ is 1.6 x 10/sup 9/ M/sup -1/s/sup -1/; for addition to 1b/sup .+/ it is approx.10/sup 7/ M/sup -1/s/sup -1/. Methanol does not add efficiently to 1c/sup .+ / . Photooxygenation of the diarylethylene yields mainly the corresponding 3,3,6,6-tetraaryl-1,2-dioxane in a chain process. In the presence of methanol and oxygen, 1a yields mainly 1-hydroperoxy-2-methyoxy-1,1-diphenylethane.

Solvent reorganization in optical and thermal electron-transfer processes

Dielectric continuum models used to calculate the solvent reorganization energy in thermal and photochemical electron-transfer reactions are discussed. Expressions are derived for the reorganization energy for charge redistribution within a single ellipsoidal or spherical solvent cavity. The expressions for the ellipsoidal cavity model differ in important respects from those derived previously. The Marcus two-sphere and the ellipsoidal cavity model differ in important respects from those derived previously. The Marcus two-sphere and the ellipsoidal cavity models can satisfactorily predict the band maximum for the metal-to metal charge-transfer transition in a number of ion-paired and bridged binuclear systems, provided careful consideration is given to the cavity dimensions. In complexes containing 2,2'-bipyridine ligands the solvent can approach close to the charge centers by filling the space between the ligands: this results in a small effective radius and relatively large values for the solvent reorganization energy. On the other hand, in complexes containing ammine ligands the effective radius is increased due to the effects of dielectric saturation resulting from specific ligand interactions with the solvent. Despite the limitations of the two-sphere model the reorganization energies calculated are, in general, not very different from those obtained with the ellipsoidal cavity model.