Secondary Electron Transfer Reactions Research Articles

One-Electron Redox Processes in a Cyclic Selenide and a Selenoxide: A Pulse Radiolysis Study

One-electron redox reactions of cyclic selenium compounds, DL-trans-3,4-dihydroxy-1-selenolane (DHS(red)), and DL-trans-3,4-dihydroxy-1-selenolane oxide (DHS(ox)) were carried out in aqueous solutions using nanosecond pulse radiolysis, and the resultant transients were detected by absorption spectroscopy. Both *OH radical and specific one-electron oxidant, Br(2)(*-) radical reacted with DHS(red) to form similar transients absorbing at 480 nm, which has been identified as a dimer radical cation (DHS(red))(2)(*+). Secondary electron transfer reactions of the (DHS(red))(2)(*+) were studied with 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS(2-)) and superoxide (O(2)(*-)) radicals. The bimolecular rate constants for the electron transfer reaction between (DHS(red))(2)(*+) with ABTS(2-) was determined as 2.4 +/- 0.4 x 10(9) M(-1) s(-1). From this reaction, the yield of (DHS(red))(2)(*+) formed on reaction with *OH radical was estimated in the presence of varying phosphate concentrations. (DHS(red))(2)(*+) reacted with O(2)(*-) radical with a bimolecular rate constant of 2.7 +/- 0.1 x 10(9) M(-1) s(-1) at pH 7. From the same reaction, the positive charge on (DHS(red))(2)(*+) was confirmed by the kinetic salt effect. HPLC analysis of the products formed in the reaction of (DHS(red))(2)(*+) with O(2)(*-) radicals showed formation of the selenoxide, DHS(ox). In order to know if a similar mechanism operated during the reduction of DHS(ox), its reactions with e(aq)(-) were studied at pH 7. The rate constant for this reaction was determined as 5.6 +/- 0.9 x 10(9) M(-1) s(-1), and no transient absorption could be observed in the wavelength region from 280 to 700 nm. It is proposed that the radical anion (DHS(ox))(*-) formed by a one-electron reduction would get protonated to form a hydroxyl radical adduct, which in presence of proton donors, would undergo dehydration to form DHS(*+). Evidence for this mechanism was obtained by converting DHS(*+) to (DHS(red))(2)(*+) with the addition of DHS(red) to the same system. Quantum chemical calculations provided supporting evidence for some of the redox reactions.

Vibrational Coherence in Polar Solutions of ZnII tetrakis(N-methylpyridyl)porphyrin with Soret-Band Excitation: Rapidly Damped Intermolecular Modes with Clustered Solvent Molecules and Slowly Damped Intramolecular Modes from the Porphyrin Macrocycle

Ground-state coherent wavepacket motions arising from intermolecular modes with clustered, first-shell solvent molecules were observed using the femtosecond dynamic absorption technique in polar solutions of Zn(II) meso-tetrakis(N-methylpyridyl)porphyrin (ZnTMPyP) with excitation in the Soret absorption band. As was observed previously in bacteriochlorophyll a solution, the pump-probe transients in ZnTMPyP solutions are weakly modulated by slowly damped (effective damping time gamma > 1 ps) features that are assigned to intramolecular modes, the skeletal normal modes of vibration of the porphyrin. The 40 cm(-1) and 215 cm(-1) modes from the metal-doming and metal-solvent-ligand modes, respectively, are members of this set of modulation components. A slowly damped 2-4 cm(-1) component is assigned to the internal rotation of the N-methylpyridyl rings with respect to the porphyrin macrocycle; this mode obtains strong resonance Raman intensity enhancement from an extensive delocalization of pi-electron density from the porphyrin in the ground state onto the rings in the pi* excited states. The dominant features observed in the pump-probe transients are a pair of rapidly damped (gamma < 250 fs) modulation components arising from intermolecular modes with solvent molecules. This structural assignment is supported by an isotope-dependent shift of the average mode frequencies in methanol and perdeuterated methanol. The solvent dependence of the mean intermolecular mode frequency is consistent with a van der Waals intermolecular potential that has significant contributions only from the London dispersion and induction interactions; ion-dipole or ion-induced-dipole terms do not make large contributions because the pi-electron density is not extensively delocalized onto the N-methylpyridyl rings. The modulation depth associated with the intermolecular modes exhibits a marked dependence on the electronic structure of the solvent that is probably related to the degree of covalency; the strongest modulations are observed in acetonitrile and dimethylsulfoxide. The results strongly support a structural assignment of the low-frequency modes that are coupled to the primary and secondary electron-transfer reactions in photosynthetic reaction centers to intermolecular modes between the redox-active chromophores and first-solvation shell groups from the surrounding protein, and an important additional function of the intermolecular modes in the stabilization of charged intermediates is suggested.

Resonance Raman spectroscopy of carotenoids in Photosystem II core complexes

Resonance Raman (RR) spectroscopy has been used to examine the configuration of the carotenoids bound to Synechocystis PCC 6803 Photosystem II (PS II) core complexes. The excitation wavelengths used (514.5, 488.0, 476.5 and 457.9 nm) span the absorption bands of all of the approximately 12-17 neutral carotenoids in the PS II core complex. The RR spectra of the two carotenoids associated with the D1-D2 polypeptides (Car507 and Car489) of the reaction center are extracted via light versus dark difference experiments measured at 20 K. The RR results are consistent with all-trans configurations for both Car507 and Car489 and indicate that majority of the other carotenoids in the PS II core complex must also be in the all-trans configuration. The configuration of beta-carotene is relevant to its proposed function as a molecular wire in the secondary electron-transfer reactions of PS II.

Secondary stabilization reactions and proton-coupled electron transport in photosystem II investigated by electroluminescence and fluorescence spectroscopy.

The oxidized primary electron donor in photosystem II, P(680)(+), is reduced in several phases, extending over 4 orders of magnitude in time. Especially the slower phases may reflect the back-pressure exerted by water oxidation and provide information on the reactions involved. The kinetics of secondary electron-transfer reactions in the microseconds time range after charge separation were investigated in oxygen-evolving thylakoids suspended in H2O or D2O. Flash-induced changes of chlorophyll fluorescence yield and electric field-induced recombination luminescence were decomposed into contributions from oxidation states S(0), S(1), S(2), and S(3) of the oxygen-evolving complex and interpreted in terms of stabilization kinetics of the initial charge-separated state S(j)Y(Z)P(680)(+)Q(A)(-)Q(B). In approximately 10% of the centers, only charge recombination took place. Otherwise, no static heterogeneity was involved in the microsecond reduction of P(680)(+) by Y(Z) (stabilization) or Q(A)(-) (recombination). The recombination component in active centers occurs mainly upon charge separation in S(3), and, in the presence of D2O, in S(2) as well and is tentatively attributed to the presence of Y(Z)(ox)S(j-1) in equilibrium with Y(Z)S(j). A 20-30 micros stabilization occurs in all S-states, but to different extents. Possible mechanisms for this component are discussed. D2O was found to decrease: (i) the rate of the reaction Y(Z)(ox)S(1) to Y(Z)S(2), (ii) the equilibrium constant between P680(+)Y(Z)S(2) and P(680)Y(Z)(ox)S(2), (iii) the rate of the slow phase of P(680)(+) reduction for the S(3) --> S(0) transition, and (iv) the rate of electron transfer from Q(A)(-) to Q(B) /Q(B)(-). The increased 'miss probability' in D2O is due to (iii).

The deactivation of singlet excited all-trans-1,6-diphenylhexa-1,3,5-triene by intermolecular charge transfer processes. 1. Mechanisms of fluorescence quenching and of triplet and cation formation

Abstract The photophysical and fluorescence quenching properties of all-trans-1,6-diphenulhexa-1,3,5-triene (DPH) were investigated in toluene and acetonitrile solution and the influences of the external heavy-atom effect (HAE) and of charge transfer (CT) processes on the fluorescence quenching mechanism were characterized. Particular emphasis was placed upon the elucidation of the electron transfer reactions in the quenching of DPH fluorescence by p -dicyanobenzene (p-DCB) in acetonitrile. It was shown that the combination of stationary and time-resolved fluorescence measurements, laser flash photolysis and CIDNP experiments allows a comprehensive description of the primary and secondary electron transfer reactions in the fluorescence quenching process. In particular, the characterization of the kinetics of genuine charge recombination (CR) reactions of the DPH p-DCB singlet and triplet ion pairs, of the spin evolution process and of the degenerate electron transfer between neutral and cationic DPH was achieved.

Secondary electron transfer reactions of the isolated Photosystem II reaction centre after reconstitution with plastoquinone-9 and diacylglycerolipids

A Photosystem II (PS II) reaction centre preparation isolated with Triton X-100 and stabilised by transfer to dodecylmaltoside has been used in a reconstitution procedure with diacyl lipids and the naturally occurring PS II quinone, plastoquinone-9 (PQ9). As shown previously, this isolated protein complex consists of the D1, D2, b -559 α and β polypeptides, plus the psb I gene product. Before reconstitution there was only about 0.004 mol PQ9 and 0.2 mol diacyl lipid per mole chlorophyll a associated with the isolated complex. After reconstitution and precipitation by centrifugation the complex was found to have an associated lipid matrix of 1.3 mol PQ9 and 3.7 mol diacyl lipid for each mole of chlorophyll a . After reconstitution, the presence of a functional quinone in the PS II reaction centre was indicated by a strong thermoluminescence signal with a peak emission at −60°C. This was recognised as the previously characterised Z v signal on the basis of the relationship between emission and excitation temperatures and a complete quenching by ethanol. This signal was essentially unaltered by addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and was the same after reconstitution with phosphatidylcholine or a thylakoid lipid extract. Calculation of the activation free energy for the charge separation reaction which gave rise to this signal suggested that this was 0.48 ± 0.01 eV and was the same as for a smaller Z v signal detected in the isolated complex before reconstitution with PQ9. When duroquinone was used this value was not significantly different, although a much smaller signal was observed. With decylplastoquinone the thermoluminescence peak was as large as with PQ9 but the difference in shape indicated that the activation free energy was significantly higher at 0.55 ± 0.02 eV. Photosynthetic electron transfer in the reconstituted complex was assayed by reduction of 2,6-dichlorophenolindophenol (DCPIP) with diphenylcarbazide (DPC) as an artificial electron donor. A low residual rate was strongly enhanced by reconstitution with PQ9. This activity was greater when reconstitution was with a thylakoid lipid extract than with phosphatidylcholine and was inhibited by addition of DCMU ( I 50 = 0.2 mM).

An electroluminescence study of stabilization reactions in the oxygen-evolving complex of Photosystem II

The stabilization of the photosynthetic charge separation in Photosystem II by secondary reactions was studied using chlorophyll luminescence induced by electric field pulses in a suspension of preilluminated osmotically swollen chloroplasts. This ‘electroluminescence’ was measured as a function of the delay time between illumination and field pulse, and as a function of the number of preilluminating flashes. The result is a survey of, in principle, all stabilization and deactivation processes beyond the state Z + Q − A , which is formed within the approx. 20 μs time resolution of the method. Most of these could be identified with known secondary electron transfer reactions. A 20-fold stabilization with a half-time of 330 μs is attributed to Q − A reoxidation. No further stabilization at the acceptor side seemed to occur and no flash number dependence was detected, although a normal Q B /Q − B oscillation was found in ultraviolet absorbance. With regard to the donor side, the data are consistent with the known S-state-dependent Z + reduction times and indicate values of 9, 5 and 65 for the equilibrium constants associated with this reaction on the transitions S 1 →S 2 , S 2 →S 3 and S 3 →S 0 (O 2 ) respectively. Z + reduction was found to be electrogenic and exposed to about 5% of the membrane potential. An 0.1 s phase in S 0 is attributed to oxygen release. S 2 and S 3 are further stabilized in two phases of unknown origin with half-times of 15 ms and 0.4 s, followed by a final 20 s phase attributed to deactivation. In S 1 , Z + reduction was probably hidden in an unresolved fast phase present on all transitions, but in addition a 350 μs phase was found, which might be related to proton release. In nearly 20% of the Photosystem II reaction centers electron transfer beyond Q − A was inhibited. In these centers Z + reduction appeared to take about 1 ms and charge recombination followed in phases of about 8, 80 and 800 ms half-time.

ChemInform Abstract: Second‐Order Combination Reactions of Phenoxyl Radicals.

Phenoxyl radicals, when produced pulse radiolytically at concentrations > 10{sup {minus}4} M, combine in second-order processes to give 2,2{prime}-, 2,4{prime}-, and 4,4{prime}-dihydroxybiphenyl as the predominant products. The ratios of these products observed under a variety of conditions, 0.73:1.73:1.00, indicate that radical combination at the ortho and para positions of the ring occurs statistically with, however, reaction at the para site favored by a factor of 2.4 over that at the ortho site. This ratio is considerably greater than the ratio of 1.55 of the unpaired spin populations at the two sites as indicated by ESR data. There is essentially no reaction at the meta position where the unpaired spin population is negative. Approximately 10% of the radicals couple at oxygen to yield 2- and 4-phenoxyphenol. The yields of these two products are, however, considerably lower than might be expected from the unpaired spin population of 0.2 on the oxygen atom of phenoxyl. It is clear that factors in addition to the unpaired spin population are important in controlling the combination processes. Under optimum conditions these five products account for {approx}90% of the phenoxyl radicals initially produced so that electronic disproportionation or coupling to form diphenyl peroxide is relatively unimportant. Multiple-pulsemore » experiments show that after products build up to concentrations comparable to that of the phenoxyl radical, secondary electron-transfer reactions lead to more complex products. In studies with {gamma}-rays at low-dose rates phenoxyl radicals predominantly react with the products so that the yields of simple combination products are very low.« less

Second-order combination reactions of phenoxyl radicals

Phenoxyl radicals, when produced pulse radiolytically at concentrations > 10{sup {minus}4} M, combine in second-order processes to give 2,2{prime}-, 2,4{prime}-, and 4,4{prime}-dihydroxybiphenyl as the predominant products. The ratios of these products observed under a variety of conditions, 0.73:1.73:1.00, indicate that radical combination at the ortho and para positions of the ring occurs statistically with, however, reaction at the para site favored by a factor of 2.4 over that at the ortho site. This ratio is considerably greater than the ratio of 1.55 of the unpaired spin populations at the two sites as indicated by ESR data. There is essentially no reaction at the meta position where the unpaired spin population is negative. Approximately 10% of the radicals couple at oxygen to yield 2- and 4-phenoxyphenol. The yields of these two products are, however, considerably lower than might be expected from the unpaired spin population of 0.2 on the oxygen atom of phenoxyl. It is clear that factors in addition to the unpaired spin population are important in controlling the combination processes. Under optimum conditions these five products account for {approx}90% of the phenoxyl radicals initially produced so that electronic disproportionation or coupling to form diphenyl peroxide is relatively unimportant. Multiple-pulsemore » experiments show that after products build up to concentrations comparable to that of the phenoxyl radical, secondary electron-transfer reactions lead to more complex products. In studies with {gamma}-rays at low-dose rates phenoxyl radicals predominantly react with the products so that the yields of simple combination products are very low.« less

ELECTRON TRANSFER REACTIONS OCCURRING UPON LASER FLASH PHOTOLYSIS OF PHOSPHOLIPID BILAYER SYSTEMS CONTAINING CHLOROPHYLL, BENZOQUINONE AND CYTOCHROME c‐II. ELECTRICALLY NEUTRAL AND POSITIVELY‐CHARGED VESICLES*

Abstract— The primary and secondary electron transfer reactions which occurred upon laser flash photolysis of electrically neutral and positively‐charged lipid bilayer vesicles containing chlorophyll, benzoquinone and cytochrome c were determined by time‐resolved difference spectral and kinetic measurements, and compared with previous results obtained with negatively‐charged vesicles (Y. Fang and G. Tollin, Photochem. Photobiol. 1988). The extent to which oxidized cytochrome c could function as an electron acceptor from triplet state chlorophyll, and reduced cytochrome c could act as an electron donor to chlorophyll cation radical, decreased from negatively‐charged to electrically neutral to positively‐charged vesicles, in agreement with expectations based on changes in the ability of cytochrome c to bind to the bilayer. In all three types of vesicles, cytochrome c reduction by benzoquinone anion radical occurred in the aqueous phase.

ELECTRON TRANSFER REACTIONS OCCURRING UPON LASER FLASH PHOTOLYSIS OF PHOSPHOLIPID BILAYER SYSTEMS CONTAINING CHLOROPHYLL, BENZOQUINONE AND CYTOCHROME c‐I. NEGATIVELY‐CHARGED VESICLES*

Abstract— In negatively‐charged lipid bilayer vesicles prepared in deionized water from egg phosphatidylcholine and 25 mol % of α‐eleostearic acid, and containing chlorophyll a, benzoquinone, and cytochrome c, primary electron transfer after a laser flash occurred principally from chlorophyll triplet to benzoquinone, and to a smaller extent from chlorophyll triplet to oxidized cytochrome c. Several secondary electron transfer reactions occurred subsequent to this. The most rapid of these was electron transfer from reduced cytochrome c, which was bound to the outer surface of the negatively‐charged vesicle, to chlorophyll cation radical (k= 3.9 times 103 s‐1). Subsequent to this, the cation radical was reduced by benzoquinone anion radical (k= 1.6 times 102 s‐1&gt;) and bound oxidized cytochrome c was reduced by the remaining anion radical which was expelled into the aqueous phase by the negative charge on the vesicle surface. This latter reaction occurred at the membrane‐solution interface with an observed rate constant (k= 60 s‐1) two orders of magnitude smaller than cytochrome oxidation. Net reduced cytochrome c was produced in this process. The reduced cytochrome c was slowly reoxidized by benzoquinone (k= 17 s‐1) and the system was returned to its original state. When the vesicle system was made slightly basic by adding tris(hydroxymethyl)aminomethane, the rates of both the reverse electron transfer between chlorophyll cation radical and benzoquinone anion radical (k= 5 times 102 s‐1) and the oxidation of reduced cytochrome c by chlorophyll cation radical (k= 9.4 times 103 s‐1) were accelerated. The rate of reduction of oxidized cytochrome c by benzoquinone anion radical remained approximately the same.

Effect of positive ion scavengers on recombination times: An attempt to use secondary electron transfer reactions as a probe for the existence of a high mobility hole in cyclohexane

In order to probe for the existence of a high mobility hole in cyclohexane, secondary electron transfer reactions between CH 3Cl − and other electron scavenging solutes have been studied in the absence and presence of positive ion scavengers. There is no evidence in these experiments for a change in the time scale for neutralization of the negative ions which can be attributed to trapping of the positive ions.

Redistribution of electric charge accompanying photo-synthetic electron transport in Chromatium

The isoionic pH of Chromatium chromatophores is 5.2±0.1. At pH 7.7, the net charge on the chromatophore is approx. −1·10 4. If a change in this charge accompanies the oxidation of an electron carrier, the midpoint redox potential ( E m) of that carrier should be a function of the solution ionic strength ( I). of that carrier should be a function of the solution ionic strength ( I). The E m values of P870 and cytochrome c-555 increase strongly with increasing I at low values of I. The E m of cytochrome c-552 also increases with increasing I, though not so strongly. These effects probably cannot be attributed to an influence of I on the activity coefficient of a dissociable ion. We conclude that, when either P870 or cytochrome c-555 loses an electron, no specific ions (including protons) are bound or released in significant amounts, and the absolute value of the charge on the chromatophore decreases. The E m values of the primary and secondary electron acceptors, X and Y, do not depend on I. Because these E m values have been shown previously to depend on pH, we conclude that the uptake of a proton keeps the charge on the chromatophore constant when either X or Y accepts an electron. This means that the primary and secondary electron transfer reactions in Chromatium result in a net decrease in the charge on the photosynthetic membrane. They do not result in the translocation of protons across the membrane. The E m of the soluble flavocytochrome c-552 from Chromatium depends only weakly on I, but depends strongly on the pH. The uptake of a proton appears to keep the net charge on this cytochrome constant upon reduction.

The P700-chlorophyll a-protein of a blue-green alga

The previously isolated chlorophyll a-protein of blue-green algae has been shown to contain P700 in a ratio of 1 reaction center molecule per 100 light-harvesting chlorophyll molecules. One-fifth of the molecules in the preparation contain P700 together with some 20 light-harvesting molecules, whereas the other molecules contain bulk chlorophyll only. Both pigment-protein entities are considered to be essentially the same and cannot be fractionated. An aggregate containing both types probably makes up the photochemical portion of the algal Photosystem I in vivo. The absorption and emission spectra of the pigment-protein are reported, as well as the spectral changes associated with the photochemical reaction. In addition to chlorophyll, carotenoid and protein the complex contains a quinone, which is not a plastoquinone. This unidentified quinone appears to participate in secondary electron transfer reactions occurring in the complex. Horse cytochrome c can be bound to the complex and will donate electrons to P +700 upon illumination. Current hypotheses for the identity of the primary electron acceptor were tested. It appears unlikely that flavins, pteridines or iron fill this role.