Forward Electron Transfer Research Articles

Complexation Enhanced Excited‐State Deactivation by Lithium Ion Coordination to a Borondipyrromethene (Bodipy) Donor–Bridge–Acceptor Dyad

AbstractA donor–acceptor dyad was prepared comprised of a borondipyrromethene (Bodipy) chromophore as the acceptor and a dimethylamino moiety as the donor. The two groups are separated by a 2,2′‐biphenol moiety. The excited state of the Bodipy is efficiently quenched by electron transfer involving donation from the dimethyl amino group. The rate constant for forward electron transfer in DMF was measured to be ca. 2 × 1010 s–1. The charge recombination process is ultrafast. Titration of aliquots of LiClO4 to a DMF solution of the dyad resulted in alterations to the absorption spectrum associated with the 2,2′‐biphenol unit. Changes were modelled as Li+ ions bound to the oxygen atoms of the 2,2′‐biphenol to produce 1:1 (Li+/ligand) and 2:1 (Li+/ligand) complexes. The rate constant for excited state quenching is enhanced upon lithium ion binding.

Photoinduced Electron Transfer Dynamics of Cyclometalated Ruthenium (II)–Naphthalenediimide Dyad at NiO Photocathode

Both forward and backward electron transfer kinetics at the sensitizer/NiO interface is critical for p-type dye-sensitized photocathodic device. In this article, we report the photoinduced electron transfer kinetics of a Ru(II) chromophore–acceptor dyad sensitized NiO photocathode. The dyad (O26) is based on a cyclometalated Ru(N∧C∧N)(N∧N∧N) (Ru[II]) chromophore and a naphthalenediimide (NDI) acceptor, where N∧C∧N represents 2,2′-(4,6-dimethyl-phenylene)-bispyridine and N∧N∧N represents 2,2′,6′,6″-terpyridine ligand. When the dyad is dissolved in a CH3CN solution, electron transfer to form the Ru(III)–NDI– occurs with a rate constant kf = 1.1 × 1010 s–1 (τf = 91 ps), and electron–hole pair recombines to regenerate ground state with a rate constant kb = 4.1 × 109 s–1 (τb = 241 ps). When the dyad is adsorbed on a NiO film by covalent attachment through the carboxylic acid group, hole injection takes place first within our instrument response time (∼180 fs) followed by the subsequent electron shift onto the ...

Dynamics of intermolecular electron transfer from amines to the excited states of 9-fluorenone

Abstract Dynamics of photoinduced electron transfer (PET) reactions between the singlet (S 1 ) and the triplet (T 1 ) excited states of 9-fluorenone or simply fluorenone and a few aromatic and aliphatic amines have been investigated under both diffusive and non-diffusive conditions. Formation of the fluorenone anion radical confirms the electron transfer (ET) from the amines to the excited states of fluorenone. Rate constants for both the forward ET process, k CS , and the charge recombination (CR) process, k CR , have been determined in acetonitrile and benzene solutions. Sub-picosecond time-resolved transient absorption study reveals that quenching of the S 1 state in acetonitrile is biexponential. Lifetime of one of these two components is independent of quencher concentration and its values determined for aniline (7.1 ps), dimethylaniline (8.5 ps) and diethylaniline (6.8 ps) are very similar. It represents the non-diffusive component of the PET reaction. But the lifetime of the other component decreases with increasing quencher concentration and the rate of this diffusive component of the PET reaction nearly agrees with the value determined using steady state fluorescence quenching method. To determine the intrinsic values of the rates of the PET reactions involving the S 1 state of fluorenone, the PET dynamics have been studied in these three neat donor solvents. The forward ET process is biexponential and the lifetimes of these two components are very similar in these solvents and vary in the range 0.3–0.5 ps and 6–8 ps. Nonexponential dynamics of the PET reactions conducted in neat donor–solvents have been discussed using a simple solvent reorientational model.

Effect of anion coordination on electron transfer in double-linked zinc phthalocyanine–fullerene dyad

Evidence of the Marcus inverted region behavior for the forward electron transfer reaction in a double-linked zinc phthalocyanine–fullerene dyad was found. The charge separation and recombination processes of the dyad were studied in an ionic environment. The chloride ion coordination to the central zinc of the phthalocyanine macrocycle decreases the energy of the charge separated state and increases the energy of the phthalocyanine first singlet excited state, and thus increases the driving force for the charge separation and decreases it for the recombination. The charge separation is slowed down whereas the charge recombination is accelerated upon the chloride coordination.

Rate constants of primary reversible reactions of electron transfer do not depend on temperature. Numerical experiment

The process of irreversible photochemical charge separation in photosynthetic bacterial reaction centers is proposed to be characterized by the effective rate constant. A formula to compute this effective rate constant is derived. Similar rate constant was previously considered by R.A. Marcus (Marcus R.A. 1956. J. Chem. Phys.24, 966–978) in order to describe nonphotochemical intermolecular electron transfer. The effective rate constant of the irreversible charge separation in photosynthetic bacterial reaction centers is shown to depend on the temperature. In contrast, rate constants of the forward electron transfer from the excited singlet primary donor to the bacteriochlorophyll and from its ion-radical to the bacteriopheophytin acceptor do not depend on temperature.

Analysis of the Kinetics of P+HA– Recombination in Membrane-Embedded Wild-Type and Mutant Rhodobacter sphaeroides Reaction Centers between 298 and 77 K Indicates That the Adjacent Negatively Charged QA Ubiquinone Modulates the Free Energy of P+HA– and May Influence the Rate of the Protein Dielectric Response

Time-resolved spectroscopic studies of recombination of the P(+)HA(-) radical pair in photosynthetic reaction centers (RCs) from Rhodobacter sphaeroides give an opportunity to study protein dynamics triggered by light and occurring over the lifetime of P(+)HA(-). The state P(+)HA(-) is formed after the ultrafast light-induced electron transfer from the primary donor pair of bacteriochlorophylls (P) to the acceptor bacteriopheophytin (HA). In order to increase the lifetime of this state, and thus increase the temporal window for the examination of protein dynamics, it is possible to block forward electron transfer from HA(-) to the secondary electron acceptor QA. In this contribution, the dynamics of P(+)HA(-) recombination were compared at a range of temperatures from 77 K to room temperature, electron transfer from HA(-) to QA being blocked either by prereduction of QA or by genetic removal of QA. The observed P(+)HA(-) charge recombination was significantly slower in the QA-deficient RCs, and in both types of complexes, lowering the temperature from RT to 77 K led to a slowing of charge recombination. The effects are explained in the frame of a model in which charge recombination occurs via competing pathways, one of which is thermally activated and includes transient formation of a higher-energy state, P(+)BA(-). An internal electrostatic field supplied by the negative charge on QA increases the free energy levels of the state P(+)HA(-), thus decreasing its energetic distance to the state P(+)BA(-). In addition, the dielectric response of the protein environment to the appearance of the state P(+)HA(-) is accelerated from ∼50-100 ns in the QA-deficient mutant RCs to ∼1-16 ns in WT RCs with a negatively charged QA(-). In both cases, the temperature dependence of the protein dynamics is weak.

Selective Abolishment of Electron Transfer at A1 Site in Cyanobacterial Photosystem I with Minimal Structural Disturbance

Photosystem I/PSI is an integral membrane protein complex that harvests solar energy into reducing power. PSI electron transfer cofactors consist of 6 chlorophylls, 2 phylloquinones (PhQs) and 3 Fe4S4 clusters. The electron transfer in PSI of the cyanobacterium Synechocystis sp. PCC 6803 has been studied in two site-directed variants, M688HPsaA and M668HPsaB variants. In those two variants, the Met residue that acts as axial ligand to the primary electron acceptor A0 has been substituted with a His either on the PsaA(M688HPsaA) or the PsaB(M668HPsaB) subunits. Room temperature transient EPR study indicated that the Met to His mutation on the A-sided (M688HPsaA) variant slowed down the forward electron transfer from A1A to FX. This might resulted from the formation of a second hydrogen bond to the C1 carbonyl oxygen of PhQ in the A1A site. Flash-induced absorbance change at 480nm confirmed the slower forward electron transfer from A1A to FX in the M688HPsaA variant. Likewise, forward electron transfer from A1B to FX was also slowed down in the B-sided variant. This result supported the formation of a second hydrogen bond to the PhQ in the A1B site as well. Furthermore, we detected new components of charge recombination kinetics in these two variants. Each of the components corresponded to charge recombination between A1A to P700+ in M688HPsaA PSI and A1B to P700+ in M668HPsaB PSI, respectively. We propose that the Met to His mutation abolished the forward electron transfer from A1A to FX in the M688HPsaA variant and A1B to FX in the M668HPsaB variant. This is consistent with the establishment of a second hydrogen bond to the PhQ. Two models of the primary charge separation in PSI are discussed in the context of this finding.

Water-soluble sulfonated–graphene–platinum nanocomposites: facile photochemical preparation with enhanced catalytic activity for hydrogen photogeneration

By using sulfonated graphene as a support for the platinum (Pt) cocatalyst, a new water-soluble composite (GSO3Pt) characteristic of small-sized and “clean” Pt nanoparticles on graphene was prepared by a facile photoreduction method, in which Hantzsch 1,4-dihydropyridine (HEH) was used as a reductant. The photocatalytic system, containing eosin Y (EY) as a sensitizer, GSO3Pt as a catalyst, and TEOA as a sacrificial electron donor, exhibited an 18 times increase in hydrogen (H2) production compared to when colloidal Pt rather than GSO3Pt was used to conduct the same photocatalytic reaction. The higher turnover number (TON), up to 6410 ± 770 based on platinum, is attributed to the efficient forward electron-transfer mediated by graphene, which was clearly verified for the first time by using the unique spectroscopic property of EY.

Mechanism for Repair of Thymine Dimers by Photoexcitation of Proximal 8-Oxo-7,8-dihydroguanine

A wide range of experimental data from earlier studies by other workers are combined with recent data from the Burrows group to interpret that group's thymine dimer (T = T) repair rate data for 8-oxo-7,8-dihydroguanine (OG)-containing DNA duplexes. The focus of this effort is to explain (i) how and why the repair rates vary as the sequence location and distance of the OG relative to the T═T is changed and (ii) why the spatial extent over which repair is observed is limited to OG-T═T distances of ~6 Å. It is proposed that, if the OG and T═T are within ~5-6 Å, a Coulomb potential moves the energy of the OG(+)···T═T(-) ion-pair state below the photoexcited OG*···T═T state, even in the absence of full solvent relaxation, thus enhancing forward electron transfer from OG* to T═T by allowing it to occur as a radiationless internal conversion process rather than by overcoming a solvation-related barrier. The rate of this forward electron transfer is estimated to be ~10% of the decay rate of the photoexcited OG*. For OG-to-T═T distances beyond 5-6 Å, electron transfer is still exothermic, but it must occur through solvent reorganization, overcoming an energy barrier, which presumably renders this rate too slow to be detected in the experiments under study here. Once an electron has been injected into the T═T, as many other workers have shown, the reaction proceeds through two low-energy barriers first connecting T═T(-) to an intermediate in which the C(5)-C(5') bond of the cyclobutane unit is cleaved, and onward to where the cyclobutane unit is fully broken and two intact thymine sites are established. Our ab initio data show that the energy landscape for these bond cleavages is altered very little by the presence of the proximal OG(+) cation, which therefore allows us to use data from the earlier studies to conclude that it takes ~100 ps for complete bond cleavage to occur. The experimentally determined overall T═T repair quantum yield of 1% then allows us to estimate the rate at which an electron is transferred from the T═T(-) anion back to the OG(+) cation as 10 times the rate of bond cleavage. The experimental variations in T═T repair rates among several sequences are shown to be reasonably consistent with an exponential OG-to-T═T distance dependence, e(-βR), with a decay parameter of β = 0.6 Å(-1). Finally, suggestions are offered for experimental studies that would test the predictions offered here and shed further light on the OG-induced T═T repair mechanism.

Ultrafast Dynamics of Nonequilibrium Electron Transfer in Photoinduced Redox Cycle: Solvent Mediation and Conformation Flexibility

We report here our systematic characterization of a photoinduced electron-transfer (ET) redox cycle in a covalently linked donor-spacer-acceptor flexible system, consisting of N-acetyl-tryptophan methylester as an electron donor and thymine as an electron acceptor in three distinct solvents of water, acetonitrile, and dioxane. With femtosecond resolution, we determined all the ET time scales, forward and backward, by following the complete reaction evolution from reactants to intermediates and finally to products. Surprisingly, we observed two distinct ET dynamics in water, corresponding to a stacked configuration with ultrafast ET in 0.7 ps and back ET in 4.5 ps and a partially folded C-clamp conformation with ET in 322 ps but back ET in 17 ps. In acetonitrile and dioxane, only the C-clamp conformations were observed with ET in 470 and 1068 ps and back ET in 110 and 94 ps, respectively. These relatively slow ET dynamics in hundreds of picoseconds all showed significant conformation heterogeneity and followed a stretched decay behavior. With both forward and back ET rates determined, we derived solvent reorganization energies and coupling constants. Significantly, we found that solvent molecules intercalated in the cleft of the C-clamp structure mediate electron transfer with a tunneling parameter (β) of 1.0-1.4 Å(-1) and the high-frequency vibration modes in the product(s) couple with the back ET process, leading to the ultrafast back ET dynamics in tens of picoseconds. These findings provide mechanistic insights of nonequilibrium ET dynamics modulated by conformation flexibility, mediated by unique solvent configuration, and accelerated by vibrational coupling.

Dynamics of excited state electron transfer at a liquid interface using time-resolved sum frequency generation

Femtosecond time resolved vibrational sum frequency generation has been used for the first time to probe a chemical reaction involving interfacial molecules pumped into their excited electronic states. The ultrafast dynamics of electron transfer from ground state N,N-dimethylaniline (DMA) to photoexcited coumarin 314 at a water/DMA monolayer interface was obtained. The forward electron transfer time constant is 16±2ps, which is faster than electron transfer in bulk DMA. The faster rate is attributed to a lower reorganization free energy, which is a consequence of lower interfacial polarity. The back electron transfer time constant is 174±21ps.

Dye-cosensitized graphene/Pt photocatalyst for high efficient visible light hydrogen evolution

In this work, a highly efficient and stable photocatalytic H2 evolution catalyst was constructed on Pt deposited graphene sheets cosensitized by Eosin Y (EY) and Rose Bengal (RB). Under two-beam monochromic light irradiation (520 and 550 nm), a high quantum yield (QY) up to 37.3% has been achieved owing to maximum utilization of incident visible light. As a result of the excellent electron transport properties of graphene, it can greatly facilitate the forward electron transfer from photoexcited dye molecules to Pt catalyst and suppress back electron transfer, which significantly enhances photocatalytic efficiency for H2 evolution. This efficient cosensitization strategy is also effective in enhancing H2 evolution efficiencies of dye sensitized TiO2 and multiwall carbon nanotubes (MWCNTs).

DNA photolyase: Is the nonproductive back electron transfer really much slower than forward transfer?

In a recent commentary, Stuchebrukhov (1) reviewed the progress in understanding DNA repair by photolyase, with special emphasis on a time-resolved study by Liu et al. (2). Among others, he raised the question of why back electron transfer (ET) to the semireduced flavin cofactor FADH° before splitting of the cyclobutane pyrimidine dimer (CPD) was slow (2.4 ns, compared with 250 ps for the preceding forward ET from the photoexcited fully reduced FADH− to the CPD), and suggested two possible reasons: the Marcus inverted region slow-down effect or the electron “is locked in a state that prevents back-transfer until the reaction is completely finished.” … [↵][1]1To whom correspondence should be addressed. E-mail: klaus.brettel{at}cea.fr. [1]: #xref-corresp-1-1

Electron Tunneling Pathways and Role of Adenine in Repair of Cyclobutane Pyrimidine Dimer by DNA Photolyase

Electron tunneling pathways in enzymes are critical to their catalytic efficiency. Through electron tunneling, photolyase, a photoenzyme, splits UV-induced cyclobutane pyrimidine dimer into two normal bases. Here, we report our systematic characterization and analyses of photoinitiated three electron transfer processes and cyclobutane ring splitting by following the entire dynamical evolution during enzymatic repair with femtosecond resolution. We observed the complete dynamics of the reactants, all intermediates and final products, and determined their reaction time scales. Using (deoxy)uracil and thymine as dimer substrates, we unambiguously determined the electron tunneling pathways for the forward electron transfer to initiate repair and for the final electron return to restore the active cofactor and complete the catalytic photocycle. Significantly, we found that the adenine moiety of the unusual bent flavin cofactor is essential to mediating all electron-transfer dynamics through a superexchange mechanism, leading to a delicate balance of time scales. The cyclobutane ring splitting takes tens of picoseconds, while electron-transfer dynamics all occur on a longer time scale. The active-site structural integrity, unique electron tunneling pathways, and the critical role of adenine ensure the synergy of these elementary steps in this complex photorepair machinery to achieve maximum repair efficiency which is close to unity. Finally, we used the Marcus electron-transfer theory to evaluate all three electron-transfer processes and thus obtained their reaction driving forces (free energies), reorganization energies, and electronic coupling constants, concluding that the forward and futile back-electron transfer is in the normal region and that the final electron return of the catalytic cycle is in the inverted region.

Dipole-Assisted Charge Separation in Organic–Inorganic Hybrid Photovoltaic Heterojunctions: Insight from First-Principles Simulations

It has been observed that the external quantum efficiency (EQE) of a hybrid organic/inorganic (P3HT/ZnO) solar cell can be tripled by inserting a monolayer of fullerene (PCBA) at the donor (P3HT) and acceptor (ZnO) interface. First-principles simulations are performed to understand the origin of the increased EQE, offering a complementary perspective to the experiment. The interfacial atomic and electronic structure as well as energy alignment are examined for both the bare and PCBA-modified interface. The presence of PCBA induces an interfacial dipole and shifts up the lowest-unoccupied-molecular-orbital (LUMO) level of P3HT relative to the conduction band edge of ZnO. Interfacial electron dynamics including forward and backward charge transfer time scales are estimated. We find that the two interfaces have similar forward electron transfer rates but their transfer mechanisms differ. The PCBA-modified interface exhibits a weaker adiabatic but a stronger nonadiabatic charge transfer. In contrast, the charge recombination time scale of the modified interface is 20 times slower than that of the bare interface, thanks to reduced donor/acceptor wave function coupling as well as increased donor/acceptor energy offset. By slowing down the charge recombination, the PCBA-modified heterojunction offers the superior performance observed in the experiment.

Exploring the Electron Transfer Pathways in Photosystem I by High-Time-Resolution Electron Paramagnetic Resonance: Observation of the B-Side Radical Pair P700+A1B– in Whole Cells of the Deuterated Green Alga Chlamydomonas reinhardtii at Cryogenic Temperatures

Crystallographic models of photosystem I (PS I) highlight a symmetrical arrangement of the electron transfer cofactors which are organized in two parallel branches (A, B) relative to a pseudo-C2 symmetry axis that is perpendicular to the membrane plane. Here, we explore the electron transfer pathways of PS I in whole cells of the deuterated green alga Chlamydomonas reinhardtii using high-time-resolution electron paramagnetic resonance (EPR) at cryogenic temperatures. Particular emphasis is given to quantum oscillations detectable in the tertiary radical pairs P700(+)A1A(-) and P700(+)A1B(-) of the electron transfer chain. Results are presented first for the deuterated site-directed mutant PsaA-M684H in which electron transfer beyond the primary electron acceptor A0A on the PsaA branch of electron transfer is impaired. Analysis of the quantum oscillations, observed in a two-dimensional Q-band (34 GHz) EPR experiment, provides the geometry of the B-side radical pair. The orientation of the g tensor of P700(+) in an external reference system is adapted from a time-resolved multifrequency EPR study of deuterated and 15N-substituted cyanobacteria (Link, G.; Berthold, T.; Bechtold, M.; Weidner, J.-U.; Ohmes, E.; Tang, J.; Poluektov, O.; Utschig, L.; Schlesselman, S. L.; Thurnauer, M. C.; Kothe, G. J. Am. Chem. Soc. 2001, 123, 4211-4222). Thus, we obtain the three-dimensional structure of the B-side radical pair following photoexcitation of PS I in its native membrane. The new structure describes the position and orientation of the reduced B-side quinone A1B(-) on a nanosecond time scale after light-induced charge separation. Furthermore, we present results for deuterated wild-type cells of C. reinhardtii demonstrating that both radical pairs P700(+)A1A(-) and P700(+)A1B(-) participate in the electron transfer process according to a mole ratio of 0.71/0.29 in favor of P700(+)A1A(-). A detailed comparison reveals different orientations of A1A(-) and A1B(-) in their respective binding sites such that formation of a strong hydrogen bond from A1(-) to the protein backbone is possible only in the case of A1A(-). We suggest that this is relevant to the rates of forward electron transfer from A1A(-) or A1B(-) to the iron-sulfur center F(X), which differ by a factor of 10. Thus, the present study sheds new light on the orientation of the phylloquinone acceptors in their binding pockets in PS I and the effect this has on function.

On the Role of Hydrogen Bonds in Photoinduced Electron‐Transfer Dynamics between 9‐Fluorenone and Amine Solvents

Using ultrafast fluorescence upconversion and mid-infrared spectroscopy, we explore the role of hydrogen bonds in the photoinduced electron transfer (ET) between 9-fluorenone (FLU) and the solvents trimethylamine (TEA) and dimethylamine (DEA). FLU shows hydrogen-bond dynamics in the methanol solvent upon photoexcitation, and similar effects may be anticipated when using DEA, whereas no hydrogen bonds can occur in TEA. Photoexcitation of the electron-acceptor dye molecule FLU with a 400 nm pump pulse induces ultrafast ET from the amine solvents, which is followed by 100 fs IR probe pulses as well as fluorescence upconversion, monitoring the time evolution of marker bands of the FLU S(1) state and the FLU radical anion, and an overtone band of the amine solvent, marking the transient generation of the amine radical cation. A comparison of the experimentally determined forward charge-separation and backward charge-recombination rates for the FLU-TEA and FLU-DEA reaction systems with the driving-force dependencies calculated for the forward and backward ET rates reveals that additional degrees of freedom determine the ET reaction dynamics for the FLU-DEA system. We suggest that hydrogen bonding between the DEA molecules plays a key role in this behaviour.

Electron transfer reactions in condensed phase: Effect of reversibility

We propose a generalized one-dimensional kinetic equation for multidimensional reversible electron transfer (ET) reaction with a nonequilibrium situation as the initial condition. The rate constant for the forward reversible ET reaction obtained here consists of the rate for the corresponding irreversible ET reaction, and an extra term due to reversibility of the ET process which includes the rates of diffusion dynamics in the reactant and product wells. In order to understand the effect of reversibility, we consider back ET reaction in a system consisting of an electron donor-acceptor pair in a solvent modeled through low frequency solvent collective coordinates (multidimensional) characterized by the orientational polarization and slowly relaxing one-dimensional vibrational mode. We propose here a new generalized polarization energy functional corresponding to the extension of the continuum version for the same, which has opened up the possibility of inclusion of molecular nature of the solvent into the solvent reorganization energy. We then derive an exact expression for the ET rate for this model system. The numerical results calculated by using the proposed one-dimensional approach are shown to be in good agreement with the available experimental results. Non-Marcus free energy gap dependence of the rates observed here for the reversible and irreversible ET reactions are very close to each other in the barrierless region, while for other situations, the rate for the former process is found to be less than the latter. The extra term, which makes the difference between the rate constants for irreversible and reversible ET reactions, is found to be contributed by the diffusion dynamics from both reactant and product wells but the dominating contribution is provided mainly by the product well.

Multiple electron injection dynamics in linearly-linked two dye co-sensitized nanocrystalline metal oxide electrodes for dye-sensitized solar cells

Understanding the electron transfer dynamics at the interface between dye sensitizer and semiconductor nanoparticle is very important for both a fundamental study and development of dye-sensitized solar cells (DSCs), which are a potential candidate for next generation solar cells. In this study, we have characterized the ultrafast photoexcited electron dynamics in a newly produced linearly-linked two dye co-sensitized solar cell using both a transient absorption (TA) and an improved transient grating (TG) technique, in which tin(IV) 2,11,20,29-tetra-tert-butyl-2,3-naphthalocyanine (NcSn) and cis-diisothiocyanato-bis(2,2'-bipyridyl-4,4'-dicarboxylato)ruthenium(II) bis(tetrabutylammonium) (N719) are molecularly and linearly linked and are bonded to the surface of a nanocrystalline tin dioxide (SnO(2)) electrode by a metal-O-metal linkage (i.e. SnO(2)-NcSn-N719). By comparing the TA and TG kinetics of NcSn, N719, and hybrid NcSn-N719 molecules adsorbed onto both of the SnO(2) and zirconium dioxide (ZrO(2)) nanocrystalline films, the forward and backward electron transfer dynamics in SnO(2)-NcSn-N719 were clarified. We found that there are two pathways for electron injection from the linearly-linked two dye molecules (NcSn-N719) to SnO(2). The first is a stepwise electron injection, in which photoexcited electrons first transfer from N719 to NcSn with a transfer time of 0.95 ps and then transfer from NcSn to the conduction band (CB) of SnO(2) with two timescales of 1.6 ps and 4.2 ps. The second is direct photoexcited electron transfer from N719 to the CB of SnO(2) with a timescale of 20-30 ps. On the other hand, back electron transfer from SnO(2) to NcSn is on a timescale of about 2 ns, which is about three orders of magnitude slower compared to the forward electron transfer from NcSn to SnO(2). The back electron transfer from NcSn to N719 is on a timescale of about 40 ps, which is about one order slower compared to the forward electron transfer from N719 to NcSn. These results demonstrate that photoexcited electrons can be effectively injected into SnO(2) from both of the N719 and NcSn dyes.

Computational Analysis of Electron Tunneling Pathways of Blue-Light Photoreceptors

DNA phtolyases and cryptochrome DASH (cry-DASH) enzymes in the blue-light photoreceptor family repair UV-damaged DNA by photo-induced electron transfer reaction. Recently (Miyazawa et al., 2008) we analyzed the electron tunneling pathways in a class I CPD photolyase derived from A. nidulans and identified a key residue, Met-353, which is perfectly conserved in the class I CPD photolyases. Interestingly, this site is switched to histidine in 6-4 photolyases, and to glutamine in cry-DASH enzymes. For instance, Met-353 is replaced to Gln-395 in cry-DASH. It is likely that the amino acid residue at the site controls the enzymatic functions of the blue-light photoreceptor family. Until now, the electron transfer pathways in cry-DASH enzymes remain unclear, while those of CPD photolyases are studied well.To characterize the roles of the amino acid residue at the site, we performed molecular dynamics simulation and electronic state calculations of the CPD photolyase of A. nidulans and the cry-DASH. We analyzed the electron tunneling pathways in the forward and backward electron transfer reactions between FADH- and CPD of these two enzymes. As a result, we observed busy trafficking of electron-tunneling current at both Met-353 of the CPD photolyase and Gln-395 of the cry-DASH.