Kinetics and mechanism of electron transfer in intact photosystem II and in the isolated reaction center: Pheophytin is the primary electron acceptor

In order to study the interfacial electron transfer, selfassembled monolayers (SAMs) on electrode surface have been attractive as a model system because they provide a stable and structurally well-defined monolayer with an adjustable thickness and desirable function. This characteristic of SAMs affords an opportunity to study fundamental issues such as the effects of distance and interfacial structure on the long-range electron transfer kinetics between a redox active species and an electrode. Weaver and Li obtained the first evidence of the distance-dependence of heterogeneous electron transfer rate constant for reduction of pentaaminecobalt(III) complexes anchored to gold and mercury surface. After Chidsey and co-workers reported heterogeneous electron transfer rate and electron tunneling constant (β) for the ferrocene-terminated alkanethiol monolayers with different alkyl chain length, a number of groups have reported β values for SAMs containing redox couples such as pentaamine (pyridine) Ru(II) complex, Os(II) bipyridyl complex, viologen, naphtoquinone, azobenzene, and hydroquinone. All of these reports demonstrated that the logarithmic heterogeneous apparent rate constant (kapp) linearly decreases as the distance between the electroactive center and the electrode surface increases as expected by Marcus theory. Those investigations on the distance dependence of electron transfer gave β values which are roughly ranging from 0.7 to 1.3 per methylene unit in the alkyl chain spacer. It is interesting to note that these β values are quite similar each other in their magnitude even though the electroactive SAMs contain different redox molecules and they follow significantly different electron transfer mechanism each other. For example, the electron transfer of ruthenium complex containing pentaamine and pyridine tether is highly reversible due to one-electron outer-sphere redox couple. However, redox center such as azobenzene undergoes substantially slow heterogeneous electron transfer because of its protonation reaction and structural change during its 2e−, 2H redox process. Among these redox centers studied, the electron transfer kinetics of a hydroquinone (H2Q) is also quite complicated due to its 2e −, 2H transfer reaction though Laviron presented a theoretical treatment of proton-coupled electron transfer reaction based on the nine-member square scheme. The electrochemical properties of hydroquinone/benzoquinone derivatives have been extensively studied in solutions because of their important biological activities. Especially, Hubbard and Soriaga have studied on the orientation of various quinones and mercaptohydroquinone derivatives adsorbed on metal surfaces using thin-layer electrochemistry. And Uosaki and coworkers have investigated the pH dependent redox behaviors of mercaptohydroquinone adsorbed on gold surface. We have reported the distance dependence of heterogeneous proton-coupled electron transfer rate constant of H2Q redox center in ω-mercaptoalkylhydroquinone SAMs on gold in 0.1 M HClO4 solution a few years ago. The β value reported for the H2Q-SAMs was 1.04 ± 0.06 and it was in good agreement with the values for the electroactive SAM systems reported up to now. However, the electron transfer kinetics of the H2Q-SAMs on gold shows remarkably different kinetic behavior in 0.1 M NaOH solution. In this note, we report an unusual observation for long range electron transfer in the ω-mercaptoalkylhydroquinone SAMs on gold electrode in a strong basic media. At this moment, the distance dependence of electron transfer of H2Q group is not observed in basic solution unlike that in acidic condition. Almost zero value of β determined from this phenomenon, to the best of our knowledge, is the first one in investigation of electron tunneling constant for long range electron transfer in the electroactive SAMs so far. The present work provides some understandings on the controlling factors related to proton-coupled electron transfer reaction nature.

Rate constants of photoinduced electron transfer between spherical fullerenes were determined using triscandium nitride encapsulated C80 fullerene (Sc3N@C80) as an electron donor and the triplet excited state of lithium ion-encapsulated C60 fullerene (Li(+)@C60) as an electron acceptor in polar and less polar solvents by laser flash photolysis measurements. Upon nanosecond laser excitation at 355 nm of a benzonitrile (PhCN) solution of Li(+)@C60 and Sc3N@C80, electron transfer from Sc3N@C80 to the triplet excited state [(3)(Li(+)@C60)*] occurred to produce Sc3N@C80(•+) and Li(+)@C60(•-) (λ(max) = 1035 nm). The rates of the photoinduced electron transfer were monitored by the decay of absorption at λ(max) = 750 nm due to (3)(Li(+)@C60)*. The second-order rate constant of electron transfer from Sc3N@C80 to (3)(Li(+)@C60)* was determined to be k(et) = 1.5 × 10(9) M(-1) s(-1) from dependence of decay rate constant of (3)(Li(+)@C60)* on the Sc3N@C80 concentration. The rate constant of back electron transfer from Li(+)@C60(•-) to Sc3N@C80(•+) was also determined to be k(bet) = 1.9 × 10(9) M(-1) s(-1), which is close to be the diffusion limited value in PhCN. Similarly, the rate constants of photoinduced electron transfer from C60 to (3)(Li(+)@C60)* and from Sc3N@C80 to (3)C60* were determined together with the back electron-transfer reactions. The driving force dependence of log k(et) and log k(bet) was well fitted by using the Marcus theory of outer-sphere electron transfer, in which the internal (bond) reorganization energy (λi) was estimated by DFT calculations and the solvent reorganization energy (λs) was calculated by the Marcus equation. When PhCN was replaced by o-dichlorobenzene (o-DCB), the λ value was decreased because of the smaller solvation changes of highly spherical fullerenes upon electron transfer in a less polar solvent.

The intracomplex reaction of anthraquinone-2,6-disulfonate (AQDS) in triplet excited state (3AQDS) with beta-cyclodextrin (CD) and the reaction of 3AQDS:CD complex with external quencher, N-Acetyl-l-Tyrosine (TyrO−), have been studied using laser flash photolysis, 1H NMR and 1H CIDNP. The two reactions were found to be competing with each other. The intracomplex hydrogen abstraction previously proposed for 3AQDS:CD complex may represent a two-step process, where at first stage an electron is transferred from the –OH group of CD to 3AQDS followed by a proton transfer and recombination of AQDS and CD radicals resulting in a stable AQDS-CD product. The rate constant of the intracomplex electron transfer from CD to 3AQDS was found to be 6 ± 1 × 106 s−1 and the rate of subsequent proton transfer is 1 ± 0.1 × 106 s−1. The rate constant of electron transfer between TyrO− and 3AQDS:CD complex (0.4 ± 0.1 × 109 M−1 s−1) drops approx. by half compared to that in the reaction between TyrO− and free 3AQDS (0.8 ± 0.1 × 109 M−1 s−1).

Spinel metal oxides possess excellent magnetic, electrical, and optical properties. They take AB2O4 (face centered cubic) form with oxygen anions providing tetrahedral (Td) and octahedral (Oh) sites for A+2 and B+3 cations. Spinel can have a normal, inverse, or mixed form based on the occupancy of different cations in Td and Oh sites. The peculiarity of the spinel crystal structure is that its composition can be easily modified without affecting the crystal structure based on the type of cation. The type of cation in the composition defines if the spinel has a normal or inverse form [1]. Based on these premises, we have already synthesized ZnxNi1-xFe2O4 (x =0, 0.2, 0.4, 0.6, 0.8, 1) nanomaterials achieving a clear gradual transition from inverse (x=0) to normal (x=1) spinel. Synthesized nanomaterials were employed as mediators in electron transfer between the screen-printed carbon working electrode and paracetamol to understand the effect of chemical composition and crystal structure on the electron transfer at the electrochemical interface [1]. Normal spinel ZnFe2O4 was found to be the best nanomaterial in terms of sensitivity and kinetic rate constant. In further works, with the aim to understand the effect of ionic radii on sensitivity and electron transfer rate constant in electrochemical sensing of paracetamol, we have focused on the normal spinel structure and modified the composition by varying the concentration of Fe3+ with Cr3+ and Bi3+ [2,3]. This study also proved that the normal spinel ZnFe2O4 has the highest sensitivity and electron transfer rate constant towards paracetamol sensing.In this work, we will present the synergic effect that can be obtained by interfacing ZnFe2O4 with ZnO nanomaterials by tuning the band gap of the heterogeneous structure. The aim is to understand the effect of band gap on sensitivity and electron transfer rate constant in electrochemical sensing. ZnFe2O4, ZnO, and ZnO/ZnFe2O4 nanomaterials are synthesized by a simple, single step auto combustion technique using the respective metal nitrates as precursors. Nanomaterial morphology and particles size are investigated by scanning electron microscopy. X-ray diffraction technique is employed to analyze the crystal structure and identify different phases in the newly synthesized materials. Then, commercially available screen-printed carbon electrodes with carbon working electrode and carbon counter electrodes are used for the electrochemical measurements in combination with an external double junction Ag/AgCl as a reference electrode. The synthesized nanomaterials are mixed with 1-butanol and a 5 μL solution is used to modify the surface of the carbon working electrode to mediate the redox reactions between the carbon surface and the molecule of interest. Primarily the sensors are characterized using cyclic voltammetry with ferri/ferrocyanide redox couple as a probe molecule. Improvement in sensitivity is observed for ZnFe2O4 (8.85 ± 0.50 μA/mM), ZnO (8.50 ± 0.30 μA/mM), and ZnO/ZnFe2O4 (8.22 ± 0.16 μA/mM) sensors compared to the bare carbon one (6.30 ± 0.40 μA/mM). By performing cyclic voltammetry at different scan rates (ν) from 25 to 125 mV/s, a good linearity of redox currents with respect to v0.5 is observed and redox peak positions are varying linearly with ln(ν). Peak-to-peak separation (ΔEp) is reduced for ZnO/ZnFe2O4 sensors compared to the carbon one. All these results suggest a faster electron transfer at the interface when the modified electrodes are used. Laviron model is employed to calculate the electron transfer rate coefficient and constant. The rate constant for ZnFe2O4 (41.8 ± 2.6 ms-1), ZnO (46.0 ± 4.0 ms-1), and ZnO/ZnFe2O4 (33.1 ± 4.5 ms-1) sensors is 3 to 5 times higher as compared to the bare carbon one (9.97 ± 0.78 ms-1). We are currently studying the potential application of ZnO/ZnFe2O4 nanomaterials in electrochemical sensing of small molecules relevant in biomedical field (dissolved oxygen, pH) and pharmaceutical drugs (paracetamol) to assess the potential for their use in different clinical settings.

Ultrafast Transient Absorption Studies on Photosystem I Reaction Centers from Chlamydomonas reinhardtii. 2: Mutations near the P700 Reaction Center Chlorophylls Provide New Insight into the Nature of the Primary Electron Donor

Structure–function relationships in Anabaena ferredoxin/ferredoxin:NADP + reductase electron transfer: insights from site-directed mutagenesis, transient absorption spectroscopy and X-ray crystallography

The heterogeneous electron-transfer properties of flavin adenine dinucleotide (FAD) were studied at self-assembled monolayer (SAM)-modified gold electrodes and compared to ferrocene-modified SAMs as a reference system. A modified form of FAD, N6-(2-aminoethyl)-FAD, was covalently immobilized onto a mixed self-assembled monolayer (SAMs) formed by two series of alkanethiols (HS−(CH2)n−COOH with a diluent HS−(CH2)n−CH3 where n = 5, 7, 10, or 15 and HS−(CH2)n−COOH with a diluent HS−(CH2)n−CH2OH where n = 5, 7, 10, or 11). The electron-transfer (ET) rates followed an exponential decay with the increasing thickness of the SAM (ket = A exp(−βn); n = number of bonds) with an attenuation factor (β) of 1.0 per bond for FAD with both diluents. Despite the similarity in β the apparent rate constant for electron transfer was 2 orders of magnitude greater with the alcohol-terminated diluent relative to the methyl-terminated SAM. In comparison the ferrocenyl SAMs also had β values of 1.0−1.2 per bond, depending on the composition of the diluent layer, with the apparent rate constant for electron transfer being significantly faster with the alcohol-terminated diluent layer. The reconstitution of apo-glucose oxidase (apo-GOx) onto a FAD-modified gold electrode was also carried out; however, no biocatalytic activity was observed. Based on the β value for the FAD-modified electrodes and the magnitude of the apparent rate of electron transfer, rate constants for direct electron transfer to glucose oxidase at carbon nanotube-modified electrodes and graphite electrodes suggest the enzyme has been partially denatured from its native configuration, thus bringing the redox-active center of the enzyme closer to the electrode and allowing appreciable ET to be observed.

The electron-transfer and hydride-transfer properties of an isolated manganese(V)−oxo complex, (TBP8Cz)Mn(V)(O) (1) (TBP8Cz = octa-tert-butylphenylcorrolazinato) were determined by spectroscopic and kinetic methods. The manganese(V)−oxo complex 1 reacts rapidly with a series of ferrocene derivatives ([Fe(C5H4Me)2], [Fe(C5HMe4)2], and ([Fe(C5Me5)2] = Fc*) to give the direct formation of [(TBP8Cz)Mn(III)(OH)]− ([2-OH]−), a two-electron-reduced product. The stoichiometry of these electron-transfer reactions was found to be (Fc derivative)/1 = 2:1 by spectral titration. The rate constants of electron transfer from ferrocene derivatives to 1 at room temperature in benzonitrile were obtained, and the successful application of Marcus theory allowed for the determination of the reorganization energies (λ) of electron transfer. The λ values of electron transfer from the ferrocene derivatives to 1 are lower than those reported for a manganese(IV)−oxo porphyrin. The presumed one-electron-reduced intermediate, a Mn(IV) complex, was not observed during the reduction of 1. However, a Mn(IV) complex was successfully generated via one-electron oxidation of the Mn(III) precursor complex 2 to give [(TBP8Cz)Mn(IV)]+ (3). Complex 3 exhibits a characteristic absorption band at λ(max) = 722 nm and an EPR spectrum at 15 K with g(max)′ = 4.68, g(mid)′ = 3.28, and g(min)′ = 1.94, with well-resolved 55Mn hyperfine coupling, indicative of a d3 Mn(IV)S = 3/2 ground state. Although electron transfer from [Fe(C5H4Me)2] to 1 is endergonic (uphill), two-electron reduction of 1 is made possible in the presence of proton donors (e.g., CH3CO2H, CF3CH2OH, and CH3OH). In the case of CH3CO2H, saturation behavior for the rate constants of electron transfer (k(et)) versus acid concentration was observed, providing insight into the critical involvement of H+ in the mechanism of electron transfer. Complex 1 was also shown to be competent to oxidize a series of dihydronicotinamide adenine dinucleotide (NADH) analogues via formal hydride transfer to produce the corresponding NAD+ analogues and [2-OH]−. The logarithms of the observed second-order rate constants of hydride transfer (k(H)) from NADH analogues to 1 are linearly correlated with those of hydride transfer from the same series of NADH analogues to p-chloranil.

The standard oxidation potential and the electron transfer (ET) rate constants of two silicon-based hybrid interfaces, Si(111)/organic-spacer/Ferrocene, are theoretically calculated and assessed. The dynamics of the electrochemical driven ET process is modeled in terms of the classical donor/acceptor scheme within the framework of “Marcus theory”. The ET rate constants, , are determined following calculation of the electron transfer matrix element, , together with the knowledge of the energy of the neutral and charge separated systems. The recently introduced Constrained Density Functional Theory (CDFT) method is exploited to optimize the structure and determine the energy of the charge separated species. Calculated ET rate constants are and , in the case of the short and long organic-spacer, respectively.

Acceleration of electron transfer from organic molecular crystals to Fe(CN) 63− and Mo(CN) 83− through mono-, di-, and trivalent cations and protons

Acceleration of electron transfer from organic molecular crystals to Fe(CN)63− and Mo(CN)83− through mono-, di-, and trivalent cations and protons

A large scale mutation of the Rhodobacter capsulatus reaction center M-subunit gene, sym2-1, has been constructed in which amino acid residues M205-M210 have been changed to the corresponding L subunit amino acids. Two interconvertable spectral forms of the initial electron donor are observed in isolated reaction centers from this mutant. Which conformation dominates depends on ionic strength, the nature of the detergent used, and the temperature. Reaction centers from this mutant have a ground-state absorbance spectrum that is very similar to wild-type when measured immediately after purification in the presence of high salt. However, upon subsequent dialysis against a low ionic strength buffer or the addition of positively charged detergents, the near-infrared spectral band of P (the initial electron donor) in sym2-1 reaction centers is shifted by over 30 nm to the blue, from 852 to 820 nm. Systematically varying either the ionic strength or the amount of charged detergent reveals an isobestic point in the absorbance spectrum at 845 nm. The wild-type spectrum also shifts with ionic strength or detergent with an isobestic point at 860 nm. The large spectral separation between the two dominant conformational forms of the sym2-1 reaction center makes detailed measurements of each state possible. Both of the spectral forms of P bleach in the presence of light. Electrochemical measurements of the P/P+ midpoint potential of sym2-1 reaction centers show an increase of about 30 mV upon conversion from the long-wavelength form to the short-wavelength form of the mutant. The rate constant of initial electron transfer in both forms of the mutant reaction centers is essentially the same, suggesting that the spectral characteristics of P are not critical for charge separation. The short-wavelength form of P in this mutant also converts to the long-wavelength form as a function of temperature between room temperature and 130 K, again giving rise to an isobestic point, in this case at 838 nm for the mutant. A similar, though considerably less pronounced spectral change with temperature occurs in wild-type reaction centers, with an isobestic point at about 855 nm, close to that found by titrating with ionic strength or detergent. Fitting the temperature dependence of the sym2-1 reaction center spectrum to a thermodynamic model resulted in a value for the enthalpy of the conformational interconversion between the short- and long-wavelength forms of about -6 kJ/mol and an entropy of interconversion of about -35 J/(K mol). Similar values of enthapy and entropy changes can be used to model the temperature dependence in wild-type. Thus, much of the temperature dependence of the reaction center special pair near-infrared absorbance band can be described as an equilibrium shift between two spectrally distinct conformations of the reaction center.

In this paper, we propose to study the dynamics and spectroscopy of photosynthetic reaction centers (RCs). We shall analyze the experimental femtosecond spectra of Rb. sphaeroides (R-26) which usually consist of contributions from populations and coherences. We shall extract information on vibrational relaxation, electron transfer (ET), and the vibrational modes involved in ET from the analysis of the femtosecond time-resolved spectra with pronounced coherence contributions. Conventional ET theory assumes that vibrational relaxation (VR) is much faster than ET so that the vibrational equilibrium is established before ET takes place. However, the primary ET in RCs occurs in 1-4 ps. This implies that this assumption should be examined. We shall study theoretically the effect of the excitation wavelength; that is, we shall study how the ET in RCs is affected by exciting to different vibronic states of RCs. In the dynamic aspect of primary ET in RCs, these experiments will provide us not only with the mechanism of ET but also with the vibrational relaxation of various modes involved in ET. In this case, it is necessary to know the expressions of single-vibronic level ET rate constants. This will also be presented in this paper. 45 refs., 6 figs., 1 tab.

This chapter uses transition-state theory to examine the transfer of electrons in homogeneous systems, which include oxidation–reduction reactions in solution. It introduces Marcus theory, which establishes a relation between the activation parameters and the rate constant of electron transfer and can be expressed in terms of structural parameters of the species involved. It also explores how electron transfer reactions between protein-bound cofactors or between proteins play an important role in a variety of biological processes. The chapter discusses the rate constant of electron transfer in a donor–acceptor complex that depends on the distance between electron donor and acceptor, the standard reaction Gibbs energy, and the energy needed to reach a particular arrangement of atoms. It makes use of transition-state theory and the concept of tunnelling, steady-state approximation, and the Franck–Condon principle.

The substantial electronic distinctions between bacteriochlorophyll (BChl) and its Mg-free analogue bacteriopheophytin (BPh) are exploited in two sets of Rhodobacter capsulatus reaction center (RC) mutants that contain a heterodimeric BChl-BPh primary electron donor (D). The BPh component of the M-heterodimer (Mhd) or L-heterodimer (Lhd) obtains from substituting a Leu for His M200 or for His L173, respectively. Lhd-β and Mhd-β RCs serve as the initial templates in the two mutant sets, where β denotes that the L-side BPh acceptor (HL) has been replaced by a BChl (due to substituting His for Leu M212). Three variants each of Lhd-β and Mhd-β mutants were constructed: (1) a swap (denoted YF) of the native Phe (L181) and Tyr (M208) residues, which flank D and the nearby M- and L-side monomeric BChl cofactors, respectively, giving Tyr (L181) and Phe (M208); (2) addition of a hydrogen bond (denoted L131LH) to the ring V keto group of the L-macrocycle of D, via replacing the native Leu at L131 with His; (3) the combination of 1 and 2. A low yield of electron transfer (ET) to the M-side BPh (HM) is observed in all four Lhd-containing RCs. Comparison with the yield of ET to β on the L-side shows that electron density on the L-macrocycle of D* favors ET to the M-side cofactors and vice versa. Increasing or decreasing the electronic asymmetry of D* via the YF, L131LH mutations or the combination results in consistent trends in the characteristics of the long-wavelength ground state absorption band of D, the rate constant of internal conversion of D* to the ground state, and the rate constants for ET to both the L- and M-side cofactors. A surprising correlation is that an increase in the charge asymmetry in D* not only increases the D* internal-conversion rate constant, but also the rate constants for ET to both the L- and M-side cofactors, spanning time scales of tens of picoseconds to several nanoseconds. The YF swap has a previously unrecognized effect on the electronic asymmetry of D*, resulting in increased charge asymmetry for the Mhd and decreased charge asymmetry for the Lhd. This result indicates that the native Tyr (M208) and Phe (L181) in the wild-type RC promote an electron distribution in P* that is the reverse of that favorable for ET to the photoactive L-branch. This conclusion reinforces the view that the native configuration of these residues promotes ET to the L branch primarily by poising the free energies of the charge-separated states. Overall, this work addresses the extent to which electronic couplings complement energetics in underpinning the directionality of ET in the bacterial RC.