CHLOROPHYLL-QUINONE PHOTOCHEMISTRY IN LIPOSOMES: MECHANISMS OF RADICAL FORMATION AND DECAY*

Abstract— Laser flash photolysis has been used to investigate the mechanism of formation and decay of the radical species generated by light-induced electron transfer from chlorophyll a (Chi) triplet to various quinones in egg phosphatidyl choline bilayer vesicles. Chlorophyll triplet quenching by quinone is controlled by diffusion occurring within the bilayer membrane (kq～ 10⁶M^?1 s^?1. as compared to ～ 10⁹ M^?1 s^?1 in ethanol) and reflects bilayer viscosity. Radical formation via separation of the intermediate ion pair is also inhibited by increased bilayer viscosity. Cooperativity is observed in the radical formation process due to an enhancement of radical separation by electron transfer from semiquinone anion radical to a neighboring quinone molecule. Two modes of radical decay are observed, a rapid (t_1/2= 150μ) recombination between Chi and quinone radicals occurring within the bilayer and a much slower (t_1/2= 1–100 ms) recombination occurring across the bilayer-water interface. The latter is also cooperative, which accounts for a t_1/2 which is dependent upon quinone concentration. The slow decay is only observed with quinones which are not tightly anchored into the bilayer, and is probably the result of electron transfer from semiquinone anion radical formed within the bilayer to a quinone molecule residing at the bilayer-water interface. Direct evidence for such a process has been obtained from experiments in which both ubiquinone and benzoquinone are present simultaneously. With benzo-quinone, approx. 60% of the radical decay occurs via the slow mode. Triplet to radical conversion efficiencies in the bilayer systems are comparable to those obtained in fluid solution (～ 60%). However, radical recombination, at least for the slow decay mechanism, is considerably retarded.     

CHLOROPHYLL ONE-ELECTRON PHOTOCHEMISTRY: FLASH PHOTOLYSIS STUDIES OF THE CHLOROPHYLL-QUINONE REACTION IN ALCOHOL SOLVENTS*

Abstract— Flash photolysis of chlorophyll a alone in CBE (cyclohexanol-t-butanol-ethanol) yields a difference spectrum similar to those obtained upon steady illumination of chlorophyll a-quinone mixtures in this solvent. Decay kinetics in CBE and dimethylsulfoxide are faster at the Soret band than at 460–580 nm and red band regions. This difference is not obtained in other solvents (CHCI₃, CCI₄, t-butanol, ethanol), implying that two or more species are obtained in CBE and DMSO. β-Carotene in CBE increases the rate of decay of the flash-induced chlorophyll transients at 430 and 660 nm but only decreases the magnitude of the signal at 470 nm. This implies that the 470 nm absorbance is due to a product formed from the triplet state. This effect is not observed in ethanol. Adding quinone to chlorophyll solutions results in slowly decaying species being generated by flash excitation in CBE. Three components can be distinguished: the first (t_1/2? 0.2 msec) corresponds to the triplet state; the second (t_1/2= 5–10 msec) is quinone concentration and species independent; the third (t_1/2= several seconds) is dependent upon quinone concentration and species (rate is faster for higher concentrations and lower potential quinones). The ESR signal decay rate is approximately equal to the third component flash decay rate when the chlorophyll and quinone concentrations are equal. With excess quinone, the flash decay rate becomes faster, and the ESR decay rate decreases slightly. These slowly-decaying species are not produced when quinone is added to chlorophyll a in ethanol or t-butanol, or to pheophytin in CBE. One observes merely a decrease in signal height with no accompanying increase in decay rate. Mechanisms to account for all of these phenomena are presented which involve an initial chlorophyll triplet-solvent reaction with the subsequent formation of several species of chloro-phyll-quinone radical complexes.     

CHLOROPHYLL PHOTOCHEMISTRY IN CONDENSED MEDIA—I. TRIPLET STATE QUENCHING AND ELECTRON TRANSFER TO QUINONE IN CELLULOSE ACETATE FILMS*

Chlorophyll-a was incorporated into cellulose acetate films and the triplet state decay kinetics and electron transfer from triplet to p-benzoquinone in aqueous solution was studied using laser flash photolysis and EPR. The triplet was found to decay by first order kinetics with a rate constant which was independent of Chl concentration. The triplet yield, however, was concentration dependent. These properties are due to quenching which occurs only at the singlet state level. In the presence of quinone, the triplet is quenched and, when the quinone is in an aqueous solution in contact with the film, Chl cation radical (C^±) as well as the semiquinone anion radical (Q^±) can be observed. The C decays by second order kinetics with a rate constant of 1.5 × 10⁶M^-1 s^-1. Although triplet conversion to radicals is slightly lower in the films as compared to fluid solutions (? 3 times), the lifetimes of the radicals are greatly increased (? 10³ times).     

Homolytic C−H Bond Activation by Phosphine−Quinone-Based Radical Ion Pairs

Herein, we present the formation of transient radical ion pairs (RIPs) by single-electron transfer (SET) in phosphine−quinone systems and explore their potential for the activation of C−H bonds. PMes₃ (Mes=2,4,6-Me₃C₆H₂) reacts with DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) with formation of the P−O bonded zwitterionic adduct Mes₃P−DDQ ( 1 ), while the reaction with the sterically more crowded PTip₃ (Tip=2,4,6-iPr₃C₆H₂) afforded C−H bond activation product Tip₂P(H)(2-[CMe₂(DDQ)]-4,6-iPr₂-C₆H₂) ( 2 ). UV/Vis and EPR spectroscopic studies showed that the latter reaction proceeds via initial SET, forming RIP [PTip₃]⋅⁺[DDQ]⋅⁻, and subsequent homolytic C−H bond activation, which was supported by DFT calculations. The isolation of analogous products, Tip₂P(H)(2-[CMe₂{TCQ−B(C₆F₅)₃}]-4,6-iPr₂-C₆H₂) ( 4 , TCQ=tetrachloro-1,4-benzoquinone) and Tip₂P(H)(2-[CMe₂{oQ^tBu−B(C₆F₅)₃}]-4,6-iPr₂-C₆H₂) ( 8 , oQ^tBu=3,5-di-tert-butyl-1,2-benzoquinone), from reactions of PTip₃ with Lewis-acid activated quinones, TCQ−B(C₆F₅)₃ and oQ^tBu−B(C₆F₅)₃, respectively, further supports the proposed radical mechanism. As such, this study presents key mechanistic insights into the homolytic C−H bond activation by the synergistic action of radical ion pairs.     

HETEROGENEITY OF THE QUINONE ELECTRON ACCEPTOR SYSTEM IN BACTERIAL REACTION CENTERS

Abstract— In reaction centers from Rhodopseudomonas viridis, biphasicity of the charge recombination kinetics between P⁺, the primary electron donor, and Q_A and Q_B^-, the primary and secondary quinone electron acceptors, respectively, have been analyzed by the flash-induced absorption change technique. We have studied the effect of quinone environment modifications on the ratio of the two phases for the P⁺Q_A- ([A_fast/A_slow]^a) and P⁺Q_B^- ([A_fast/A_slow]^b) charge recombination processes. In reaction centers from Rps. viridis reconstituted in phosphatidylcholine liposomes a notable influence of the nature of the Q_B pocket occupancy was observed on (A_fast/A_slow)^a. This ratio is much affected by the presence of o-phenanthroline compared to reaction centers with an empty Q_B pocket or with terbutryn present. Because o-phenanthroline was proposed to hydrogen bind His^L190, whereas terbutryn does not, we suggest that a His^LI90-Fe-His^M217 (the equivalent to His^LI90 in the Q_A pocket) “wire” may be involved in the existence of the two conformational states associated with the two phases of charge recombination. In chromat-ophores from the T₁ (Ser^L223→ Ala; Arg^L217→ His) and T₄ (Tyr^L222→ Phe) mutants no modification of the (A_fast/A_slow)^a ratio is detected, whereas the (A_fast/A_slow)^b ratios are substantially modified compared to the wild type (WT). In the T₃ mutant (Phe^L216→ Ser; Val^M263→ Phe [4.1 Å apart from Q_A]), (A_fast/A_slow)^a is notably changed compared to the WT. Our data show that any modification in the close protein environment of the quinones and/or of the His^L190 and His^M217 affects the equilibrium between the two reaction center states. This is consistent with the existence of two reaction center states from Rps. viridis, associated with two different conformations of the quinones-histidines-iron system. This “wire” allows both quinone protein pockets to interact over quite long distances.     

CARBODIPHOSPHAN (2.4.6-tBu3C6H2[sbnd]P[dbnd]2)C: REAKTION MIT ELEKTROPHILEN

Abstract

The reaction of Ar[sbnd]P[dbnd]C[dbnd]P[sbnd]Ar (Ar=2.4.6-^tBu₃C₆H₂) with electrophiles (H⁺, S₈) proceeds at the phosphorus atom with subsequent cyclisation of an o-^tbutyl group.     

Mixing of triply degenerated molecular orbitals in C602− and C603−

Mixing of triply degenerated lowest unoccupied molecular orbitals (LUMO; t_1u) and the next LUMO (NLUMO; t_1g) of a neutral C₆₀ molecule was estimated when it becomes dianionic (C₆₀²⁻) and trianionic (C₆₀³⁻) species. The electronic structure of the basic C₆₀ was obtained by a semiempirical (INDO type) Hartree-Fock scheme and the mixing of the t_1u and t_1g MOs by the conventional configuration interaction (CI) method assuming I_h structural symmetry of a C₆₀ for the sake of simplicity. The most favorable electronic states of C₆₀²⁻ and C₆₀³⁻ are predicted to be triplet and doublet, respectively. Furthermore, in C₆₀²⁻, the energy difference of this triplet state and the first excited singlet state is very close, which agrees well with the experimental observation. © 1997 John Wiley & Sons, Inc.     

Four‐Center Oxidation State Combinations and Near‐Infrared Absorption in [Ru(pap)(Q)2]n (Q=3,5‐Di‐tert‐butyl‐N‐aryl‐1,2‐benzoquinonemonoimine,pap=2‐Phenylazopyridine)

The complex series [Ru(pap)(Q)₂]ⁿ ([ 1 ]ⁿ–[ 4 ]ⁿ; n=+2, +1, 0, ?1, ?2) contains four redox non‐innocent entities: one ruthenium ion, 2‐phenylazopyridine (pap), and two o‐iminoquinone moieties, Q=3,5‐di‐tert‐butyl‐N‐aryl‐1,2‐benzoquinonemonoimine (aryl=C₆H₅ ( 1⁺ ); m‐(Cl)₂C₆H₃ ( 2⁺ ); m‐(OCH₃)₂C₆H₃ ( 3⁺ ); m‐(tBu)₂C₆H₃ ( 4 ⁺)). A crystal structure determination of the representative compound, [ 1 ]ClO₄, established the crystallization of the ctt‐isomeric form, that is, cis and trans with respect to the mutual orientations of O and N donors of two Q ligands, and the coordinating azo N atom trans to the O donor of Q. The sensitive C? O (average: 1.299(3) Å), C? N (average: 1.346(4) Å) and intra‐ring C? C (meta; average: 1.373(4) Å) bond lengths of the coordinated iminoquinone moieties in corroboration with the N?N length (1.292(3) Å) of pap in 1 ⁺ establish [Ru^III(pap⁰)(Q^.?)₂]⁺ as the most appropriate electronic structural form. The coupling of three spins from one low‐spin ruthenium(III) (t_2g⁵) and two Q^.? radicals in 1 ⁺– 4 ⁺ gives a ground state with one unpaired electron on Q^.?, as evident from g=1.995 radical‐type EPR signals for 1 ⁺– 4 ⁺. Accordingly, the DFT‐calculated Mulliken spin densities of 1 ⁺ (1.152 for two Q, Ru: ?0.179, pap: 0.031) confirm Q‐based spin. Complex ions 1 ⁺– 4 ⁺ exhibit two near‐IR absorption bands at about λ=2000 and 920 nm in addition to intense multiple transitions covering the visible to UV regions; compounds [ 1 ]ClO₄–[ 4 ]ClO₄ undergo one oxidation and three separate reduction processes within ±2.0 V versus SCE. The crystal structure of the neutral (one‐electron reduced) state ( 2 ) was determined to show metal‐based reduction and an EPR signal at g=1.996. The electronic transitions of the complexes 1 ⁿ– 4 ⁿ (n=+2, +1, 0, ?1, ?2) in the UV, visible, and NIR regions, as determined by using spectroelectrochemistry, have been analyzed by TD‐DFT calculations and reveal significant low‐energy absorbance (λ_max>1000 nm) for cations, anions, and neutral forms. The experimental studies in combination with DFT calculations suggest the dominant valence configurations of 1 ⁿ– 4 ⁿ in the accessible redox states to be [Ru^III(pap⁰)(Q^.?)(Q⁰)]²⁺ ( 1 ²⁺– 4 ²⁺)→[Ru^III(pap⁰)(Q^.?)₂]⁺ ( 1 ⁺– 4 ⁺)→[Ru^II(pap⁰)(Q^.?)₂] ( 1 – 4 )→[Ru^II(pap^.?)(Q^.?)₂]^? ( 1 ^?– 4 ^?)→[Ru^III(pap^.?)(Q^2?)₂]^2? ( 1 ^2?– 4 ^2?).     

CHLOROPHYLL ONE-ELECTRON PHOTOCHEMISTRY: LIGHT-INDUCED ABSORBANCE CHANGES AND ESR SIGNALS FOR VARIOUS PORPHYRIN-QUINONE AND HYDROQUINONE SYSTEMS IN ALCOHOL SOLVENTS*

Abstract— Light-dark optical difference spectra of degassed ethanol or pyridine solutions of chlorophyll and benzoquinone or hydroquinone at temperatures above — 50°C show only the semiquinone absorbance band. Decay of the signals is second order, with a rate constant in agreement with earlier ESR results. Light-induced optical changes due to chlorophyll can be elicited by lowering the temperature of ethanol solutions of chlorophyll and benzoquinone to a region of high viscosity. Hydroquinone is not effective in producing these optical changes. Similar results are achieved at room temperature by using as solvent a degassed mixture of the alcohols: cyclohexanol, tert-butanol, and ethanol (CBE). Difference spectra show bleaching of the chlorophyll bands and increased absorbance in the intermediate wavelength region (460–580 nm). Decay kinetics are first order, while the rise is complicated (probably biphasic). ESR signals have no hyperfine structure and also decay by first order kinetics, at a rate which is faster than that of the optical changes. The ESR signals reach a steady state more rapidly than the optical signals, without biphasic kinetics. These results demonstrate that at least two species are generated. Addition of acid increases the amount of bleaching in CBE, while small amounts of base decrease it. Larger amounts of base cause chlorophyll bleaching to completely disappear and only the semiquinone anion is observed. Activation energies for the chlorophyll a-benzoquinone photoreaction in CBE are 10–14 kcal/mole. Lower potential quinones give lower activation energies. The rate constant for quenching of the triplet state of chlorophyll a by β-carotene in CBE is 7.5±0.5×10⁸ (M set)^-1. β-carotene also quenches photoproduct formation. The bimolecular rate constant for formation of the photoproduct with benzoquinone was calculated to be 7×10⁸ (Msec)^-1. The redox potential of the quinone affects both the magnitude of the chlorophyll absorbance changes and the rate of decay. The higher the potential, the larger the changes and the slower the decay. Other porphyrin systems show similar photoreactions only if they are chelated with a group II metal, such as Mg²⁺, Cd⁺², or Zn⁺². The results are interpreted in terms of the formation, by a triplet-sensitized one-electron transfer from solvent to quinone, of a chlorophyll-semiquinone complex which is stabilized via coordination with the chelated metal.     

Destabilization of the Oxygen Evolving Complex of Photosystem II by Al3+

The inhibitory effect of Al³⁺ on photosynthetic electron transport was investigated in isolated thylakoid membranes of spinach. A combination of oxygen evolution, chlorophyll fluorescence induction (FI) and decay and thermoluminescence measurements have been used to characterize photosystem II (PSII) electron transport in the presence of this toxic metal cation. Our results show that below 3 mm , Al³⁺ already caused a destabilization of the Mn₄O₅Ca cluster of the oxygen evolving complex (OEC). At these concentrations, an increase in the relative amplitude of the first phase (OJ) of FI curve and retardation of the fluorescence decay kinetics following excitation with a single turnover flash were also observed. A transmembrane structural modification of PSII polypeptides due to the interaction of Al³⁺ at the OEC is proposed to retard electron transfer between the quinones Q_A and Q_B. Above 3 mm , Al³⁺ strongly retarded fluorescence induction and significantly reduced F_v/F_m together with the maximal amplitude of chlorophyll fluorescence induced by a single turnover flash. This chlorophyll fluorescence quenching was attributed to the formation of P680⁺ due to inhibition of electron transfer between tyrosine 161 of D1 subunit and P680.     

Visual pigments. V. Ground and excited-state acid dissociation constants of protonated all-trans retinal schiff base and correlation with theory.

Abstract— The acid dissociation constants of protonated all-trans retinal Schiff base (SB-H⁺) in a 50% water-methanol solution at 0°C is 6–95 for the ground state and nominally 16–65 for the first excited singlet state, with a potential range of ? 12–21. These values are in qualitative agreement with the results of semiempirical MO calculations, which indicate that the total charge density on nitrogen is greater in the first excited singlet than in the ground state (Q_N* > Q_N). However, pertinent to vision, CNDO/2 calculations on all-trans and 11-cis Schiff base and SB-H⁺ indicate that, for torsional angles of approximately 80–100° around the 11–12 double bond, Q_N* < Q_N. This result suggests that it may be possible for the proton to come off the imine nitrogen during isomerization from 11 -cis SB-H⁺ to all-trans SB-H⁺. The potential consequence of this during isomerization of rhodopsin is the initiation of unfolding of the protein opsin.     

Chromium(III)-isonicotinate Complexes. Kinetics and Mechanism of Isonicotinate Ligand Liberation in HClO4 Solutions

Chromium(III)-isonicotinate complexes, cis-[Cr(C₂O₄)₂(N-inic)(H₂O)]^- and [Cr(C₂O₄)(H₂O)₃-OH-Cr(C₂O₄)₂(O-inic)]^-(N-inic)(H₂ (N-inic = N-bonded and O-inic = O-bonded isonicotinic acid) were obtained and characterized in solution. Kinetics of acid-catalyzed isonicotinate ligand liberation were studied spectrophotometrically in the 0.1–1.0 m HClO₄ range, at I=1.0 m. The dependencies of the pseudo-first order rate constant on [H⁺] were established: k_obs = k₀+k_HQ_H[H⁺] and k_obs = k_HQ_H[H⁺] for the N-inic and O-inic complex, respectively, where k₀ and k_H are the rate constants of the spontaneous and the acid-catalyzed reaction paths, and Q_H is the protonation constant of the carboxylic group in isonicotinic ligand. The obtained results indicate that N-bonded isonicotinic acid liberation occurs mainly via a spontaneous reaction path and is much slower than O-bonded inic liberation. The mechanisms for these processes are proposed.     

Dynamics of photoexcited donor-acceptor complexes between C60 andN,N-diethylaniline. Polarization picosecond spectroscopy study

The kinetics of the formation and decay of photoexcited radical ion pairs of donoracceptor charge-transfer complexes between C₆₀ andN,N-diethylaniline (DEA) in chlorobenzene was studied by picosecond laser-induced diffraction gratings. It was established that the anisotropy of polarization of the diffraction signal decreases as the concentration of DEA increases. The radical ion states of the photoexcited C₆₀ ⁻...DEA⁺ complex have zero anisotropy. This effect is likely due to the isotropic intracomplex transfer of an electron from the local excited state to the radical ion state. The rate constant of quenching of the singlet excited C₆₀ byN,N-diethylaniline (1.4·10¹⁰ L mol⁻¹ s⁻¹) and the lifetimes of the solventseparated C₆₀ ⁻...DEA⁺ and tight [C₆₀ ⁻...DEA⁺] (95±7 and 31±4 ps, respectively) radical ion pairs were measured. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1605–1610, September, 1997.     

Iron-Catalyzed Intermolecular C−H Amination Assisted by an Isolated Iron-Imido Radical Intermediate

Here we report the use of a base metal complex [(^tBupyrpyrr₂)Fe(OEt₂)] ( 1 -OEt₂) (^tBupyrpyrr₂²⁻=3,5-tBu₂-bis(pyrrolyl)pyridine) as a catalyst for intermolecular amination of C_sp3−H bonds of 9,10-dihydroanthracene ( 2 a ) using 2,4,6-trimethyl phenyl azide ( 3 a ) as the nitrene source. The reaction is complete within one hour at 80 °C using as low as 2 mol % 1 -OEt₂ with control in selectivity for single C−H amination versus double C−H amination. Catalytic C−H amination reactions can be extended to other substrates such as cyclohexadiene and xanthene derivatives and can tolerate a variety of aryl azides having methyl groups in both ortho positions. Under stoichiometric conditions the imido radical species [(^tBupyrpyrr₂)Fe{=N(2,6-Me₂-4-tBu-C₆H₂)] ( 1 -imido) can be isolated in 56 % yield, and spectroscopic, magnetometric, and computational studies confirmed it to be an S = 1 Fe^IV complex. Complex 1 -imido reacts with 2 a to produce the ferrous aniline adduct [(^tBupyrpyrr₂)Fe{NH(2,6-Me₂-4-tBu-C₆H₂)(C₁₄H₁₁)}] ( 1 -aniline) in 45 % yield. Lastly, it was found that complexes 1 -imido and 1 -aniline are both competent intermediates in catalytic intermolecular C−H amination.     

Uniform quality gaussian basis sets for molecular calculations. II. C2 hydrocarbons

This investigation is a continuation of a study on the optimality of MO basis sets of Gaussian functions, when constructed from AO basis sets optimized for the neutral atom or for ions. A formal charge parameter Q is used to adjust AO basis sets to the molecular environment, by virtue of a simple quadratic equation. Calculations are performed on a series of seven C₂ hydrocarbons (C₂H₂, C₂H₄, C₂H₆, C₂H₃⁺ (open), C₂H₃⁺ (bridged), C₂H₅⁺ (bridged), and C₂H₄^? radical anion). A simple rule is formulated to give approximate values of the charge parameter Q.     

Singlet-triplet splitting of divalent five-membered fused rings C<Subscript>12</Subscript>H<Subscript>8</Subscript>M (M = C,Si, Ge,Sn, and Pb): Application in nanocomplex synthesis

Energy differences, ΔX _s−t (X = E, H, and G) (ΔX _s−t = X(singlet) − X(triplet)) between singlet (s) and triplet (t) states of C₁₂H₈M were calculated at B3LYP/6-311+G*. The DFT calculations indicated that the ΔG _s−t between singlet (s) and triplet (t) states of C₁₂H₈M were increased from M = C to M = Pb. The ΔG _s−t of C₁₂H₈M was compared with its analogue C₄H₄M through replacement of heavy atoms from M = C to M = Pb. Configurations of the electrons in orbitals (σ² or π²) for the singlet state of C₁₂H₈M were discussed.     

MODELS FOR BACTERIAL PHOTOSYNTHESIS: ELECTRON TRANSFER FROM PHOTOEXCITED SINGLET BACTERIOPHEOPHYTIN TO METHYL VIOLOGEN AND m-DINITROBENZENE

Abstract. As a model for the primary reactions of photosynthesis, we studied photochemical electron transfer from bacteriopheophytin (BPh) to methyl viologen (MVC1₂) and to m-dinitrobenzene (m-DNB) in solution. Both MVC1₂ and m-DNB cause reductions in the lifetime of the first excited singlet state of BPh (BPh*), in the fluorescence quantum yield, and in the quantum yield of the triplet state, BPh ⁺. The quenching of BPh* probably results from electron transfer, which generates short-lived radical pairs involving the BPh radical cation (BPh⁺) and the reduced form of the quencher. Electron transfer from BPh* is thermodynamically favorable, but that from BPh^T is not. From the magnitude of the quenching, we calculate rate constants for electron transfer in collision complexes formed between BPh* and MVC1₂ or m-DNB. Measurements of the quantum yield of the free BPh⁺ radical indicate that about 3/4 of the [BPh⁺ MV⁺] radical pairs decay by reverse electron transfer, rather than dissociating to give the free radicals. Essentially all of the [BPh⁺m-DNB ⁺] radical pairs must decay by reverse electron transfer, because free BPh⁺ cannot be detected in this case. From these data, we estimate the rate constants for the reverse electron transfer reactions. The higher probability of dissociation in the [BPh⁺ MV⁺] radical pair can be explained by coulombic repulsion. The rate of the primary electron transfer reaction in photosynthetic bacteria is comparable to that of forward electron transfer in the BPh* collision complexes. Reverse electron transfer, however, is at least 10³-times slower in the radical pair formed in the bacterial reaction center than it is in [BPh⁺m-DNB^?], and more than 10⁴-times slower than in [BPh⁺ MV⁺]. The explanation for this dramatic and crucially important difference remains unclear, but several possibilities are discussed.     

CHLOROPHYLL PHOTOSENSITIZED VECTORIAL ELECTRON TRANSPORT ACROSS PHOSPHOLIPID VESICLE BILAYERS: KINETICS AND MECHANISM*

Abstract— The characterization and kinetic analysis by laser Rash photolysis of an improved model system for observing chlorophyll a photosensitized electron transfer across a lipid bilayer membrane is described. In this system, the electron acceptor is a water-soluble naphthoquinone, S-(2-methyl-l,4-naphthoquinonyl-3)-glutathione (MGNQ) which is dissolved in the inner aqueous compartments of phospholipid bilayer vesicles, and the electron donor is glutathione (GSH) which is dissolved in the outer aqueous phase. Chlorophyll (Chl) is dissolved in the membrane. Oxidative quenching of the triplet state of Chl by the quinone at the inner surface of the vesicle produces the Chl⁺ and MGNQ^- radicals. Chi⁺ is reduced by GSH at the outer surface of the vesicle (k= 2.6 × 10⁶M^-1 s^-1) in competition with the recombination between Chl⁺. and MGNO^- (k= 2.5 × 10³ S^-1). It is shown that a kinetic mechanism involving competition between recombination, electron transfer across the bilayer, and reduction by donor at the opposite surface can quantitatively account for the decay of Chl⁺. Electron transport across the bilayer is postulated to occur by a two-step mechanism involving electron exchange between Chl and Chl⁺ within the lipid monolayer (k= 3.2 × 10⁶ M^-1 s^-1) and across the bilayer. The rate constant for the latter exchange process approaches 10⁴ s^-1 as the concentration of Chl in the bilayer increases. Under appropriate conditions, approximately 20% of all photons absorbed by the vesicle system result in electron transfer across the mcmbrane from GSH to MGNQ.     

The Remarkable Effect of the Manganese Ion with Dioxygen on the Stability of π‐Conjugated Radical Cations

In this paper, nanosecond laser flash photolysis has been used to investigate the influence of metal ions on the kinetics of radical cations of a range of carotenoids (astaxanthin (ASTA), canthaxanthin (CAN), and β‐carotene (β‐CAR)) and various electron donors (1,4‐diphenyl‐1,3‐butadiene (14DPB), 1,6‐diphenyl‐1,3,5‐hexatriene (16DPH), 4‐methoxy‐trans‐stilbene (4 MeOSt), and trans‐stilbene (trans‐St)) in benzonitrile. Radical cations have been generated by means of photosensitized electron‐transfer (ET) using 1,4‐dicyanonaphthalene (14DCN) and biphenyl (BP). The kinetic decay of CAR ^{. +} shows a strong dependence on the identity of the examined metal ion. For example, whereas NaClO₄ has a weak effect on the kinetics of CAR ^{. +}, Ni(ClO₄)₂ causes a strong retardation of the decay of CAR ^{. +}. It is also interesting to note that Mn²⁺, which is a biologically relevant metal ion, shows the strongest effect of all the investigated metal ions (e.g., in the presence of Mn²⁺ ions, the half‐life (t_1/2) of CAN ^{. +} (t_1/2>90 ms) is more than three orders of magnitude higher than in the absence of the metal ions (t_1/2≈16 μs)). Furthermore, the influence of metal‐ion and oxygen concentrations on the kinetics of CAR ^{. +} reveals their pronounced effect on the kinetic decay of CAR ^{. +}. However, these remarkable effects are greatly diminished if either oxygen or metal ions are removed from the investigated solutions. Therefore, it can be concluded that oxygen and metal ions interact cooperatively to induce the observed substantial effects on the stabilities of CAR ^{. +}. These results are the first direct observation of the major role of oxygen in the stabilization of radical cations, and they support the earlier mechanism proposed by Astruc et al. for the role of oxygen in the inhibition of cage reactions. On the basis of these results, the factors that affect the stability of radical cations are discussed and the mechanism that shows the role of oxygen and metal ions in the enhancement of radical‐cation stability is described.     

High-level ab initio calculations on CH+2(2A1) + PO(2II) reactions

We present ab initio calculations carried out in the framework of the G 2 theory on the singlet and triplet potential energy surfaces corresponding to the gas-phase between CH⁺₂ and PO. The global minimum of both potential energy surfaces is a cyclic singlet-state cation. Oxygen attachment of PO to CH⁺₂ in a triplet configuration is accompanied by a P(SINGLEBOND)O bond fission, with the result that the corresponding global minimum is an ion-dipole complex between P⁺(³P) and formaldehyde. This is also consistent with the fact that our results predict the formation of formaldehyde to be highly exothermic, either as a neutral or as radical cation. Both charge-transfer processes yielding CH₂(³B₁) or CH₂(¹A₁) are also exothermic. The formation of other carbon and oxygen containing species are endothermic. © 1996 John Wiley & Sons, Inc.