QUINONES AS MEDIATORS OF ELECTRON TRANSFER FROM MEMBRANE-BOUND CHLOROPHYLL TO CYTOCHROME c IN THE AQUEOUS PHASE IN LIPID BILAYER VESICLES: SYNERGISTIC ENHANCEMENT OF CYTOCHROME c REDUCTION BY LIPOPHILIC/ HYDROPHILIC QUINONE PAIRS

Laser flash photolysis has been used to determine the kinetics of cytochrome c reduction by chlorophyll triplet state in negatively-charged lipid bilayer vesicles, as mediated by quinones. Large synergistic enhancements in the yield of reduced cytochrome were obtained using a pair of quinones, one of which was lipophilic (e.g. benzoquinone, 2,6-di-f-butylbenzoquinone) and the other of which was hydrophilic (e.g. l,2-naphthoquinone-4-sulfonate). The mechanism was shown to involve initial quenching of the triplet by the membrane-associated quinone to form chlorophyll cation radical and quinone anion radical. An interquinone electron transfer process followed this reaction, which occurred at the membrane-water interface, and greatly facilitated electron transport from within the bilayer to the aqueous phase. This process formed the basis of the synergistic effect. Cytochrome c reduction occurred in the water phase by reaction with the anion radical of the hydrophilic quinone. Finally, the reduced cytochrome was reoxidized by a slow reaction with chlorophyll cation radical. Under the most favorable conditions, we estimate that the quantum yield of conversion of triplet quenching events to reduction of cytochrome approached unity. The lifetime of the reduced protein and oxidized chlorophyll could be as long as 140 ms, under the best conditions. This system has properties which are thus quite favorable for solar energy conversion in a biomimetic process.     

ESR EVIDENCE FOR CHLOROPHYLL-PHOTOSENSITIZED ONE-ELECTRON OXIDATION OF WATER BY BENZOQUINONE*

Abstract —Illumination with red light of degassed solutions of chlorophyll and benzoquinone in dry acetone does not result in quinone radical formation, even though the presence of quinone greatly enhances the photoproduction of chlorophyll cation radical at low temperatures. However, when small amounts of water (or alcohols) are added to this system, large benzoquinone anion radical electron spin resonance (ESR) signals are generated. The effects on the low-temperature ESR signals of microwave power saturation, replacement of chlorophyll and quinone by their deuterated analogs, and replacement of H₂O by D₂O, lead to the suggestion that a one-electron oxidation of water is occurring. Pheophytin and bacteriochlorophyll are shown to sensitize a similar photoreaction. These results could lead to new insights into the mechanism of oxygen formation in photosynthesis.     

Charge localization in a 17-bond mixed-valence quinone radical anion

Optical spectra in dimethylformamide are reported for the radical anions of benzoquinone, its tetramethyl and tetrachloro analogues, and tetra-ortho-alkyl derivatives of biphenyl, stilbene, terphenyl, quadriphenyl, and 1,4-bis(2-phenylethenyl)benzene quinones. The first absorption bands for all but the quadriphenyl quinone show vibrational fine structure, demonstrating that they are delocalized (Class III) mixed-valence compounds. The quadriphenyl quinone radical anion shows a wide Gaussian-shaped band having a band maximum that is strongly dependent on solvent, typical of localized (Class II) mixed-valence compounds. The simple O charge-bearing unit of these compounds maintains charge delocalization in examples with unusually large bridges.     

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.     

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>) 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.     

Role of LHCII organization in the interaction of substituted 1,4-anthraquinones with thylakoid membranes

The chlorophyll fluorescence, photochemical activity and surface electric properties of thylakoid membranes with different stoichiometry of pigment-protein complexes and organization of the light-harvesting chlorophyll a/b protein complex of photosystem II (LHCII) were studied in the presence of substituted 1,4-anthraquinones. Data show strong dependence of the quenching of the chlorophyll fluorescence on the structural organization of LHCII. The increase of the LHCII oligomerization, which is associated with significant reduction of the transmembrane electric charge asymmetry and electric polarizability of the membrane, correlates with enhanced quenching effect of substituted 1,4-athraquinones. Crucial for the large quinone-induced changes in the membrane electric dipole moments is the structure of the quinone molecule. The strongest reduction in the values of the dipole moments is observed after interaction of thylakoids with 3-chloro-9-hydroxy-1,4-anthraquinone (TF33) which has the highest quenching efficiency. The quinone induced changes in the photochemical activity of photosystem II (PSII) correlate with the total amount of the supramolecular LHCII-PSII complex and depend on the number of substituents in the 1,4-anthraquinone molecule.     

Quinone‐Catalyzed Selective Oxidation of Organic Molecules

Quinones are common stoichiometric reagents in organic chemistry. Para‐quinones with high reduction potentials, such as DDQ and chloranil, are widely used and typically promote hydride abstraction. In recent years, many catalytic applications of these methods have been achieved by using transition metals, electrochemistry, or O₂ to regenerate the oxidized quinone in situ. Complementary studies have led to the development of a different class of quinones that resemble the ortho‐quinone cofactors in copper amine oxidases and mediate the efficient and selective aerobic and/or electrochemical dehydrogenation of amines. The latter reactions typically proceed by electrophilic transamination and/or addition‐elimination reaction mechanisms, rather than hydride abstraction pathways. The collective observations show that the quinone structure has a significant influence on the reaction mechanism and has important implications for the development of new quinone reagents and quinone‐catalyzed transformations.     

Reactivity of quinones as alkyl radical acceptors

The enthalpies of the addition of 11 alkyl radicals to ortho-and para-benzoquinones and substituted para-benzoquinones and the enthalpies of formation of various alkoxyphenoxyl radicals have been calculated. Experimental data for the addition of alkyl radicals to quinones are analyzed in terms of the intersection of two parabolic potential curves, and parameters characterizing this class of reactions are calculated. The classical potential barrier of the thermally neutral reaction of alkyl radical addition to benzoquinone is E _e,0 = 82.1 kJ/mol. This class of reactions is compared to other classes of free-radical addition reactions. The interaction between the electrons of the reaction center and the π electrons of the aromatic ring is a significant factor in the activation energy. Activation energies, rate constants, and the geometric parameters of the transition state have been calculated for 40 reactions of alkyl radical addition to quinones. Strong polar interaction has been revealed in the addition of polar macroradicals to quinones, and its contribution to the activation energy has been estimated. Kinetic parameters, activation energies, and rate constants have been calculated for the reverse reactions of alkoxyphenoxyl radical decomposition to quinone and alkyl. The competition between chain termination and propagation reactions in alkoxyphenol-inhibited hydrocarbon oxidation is discussed.     

Reaction of superoxide radical with quinone molecules

When the superoxide radical O(2)(?-) is generated on reaction of KO(2) with water in dimethyl sulfoxide, the decay of the radical is dramatically accelerated by inclusion of quinones in the reaction mix. For quinones with no or short hydrophobic tails, the radical product is a semiquinone at much lower yield, likely indicating reduction of quinone by superoxide and loss of most of the semiquinone product by disproportionation. In the presence of ubiquinone-10, a different species (I) is generated, which has the EPR spectrum of superoxide radical. However, pulsed EPR shows spin interaction with protons in fully deuterated solvent, indicating close proximity to the ubinquinone-10. We discuss the nature of species I, and possible roles in the physiological reactions through which ubisemiquinone generates superoxide by reduction of O(2) through bypass reactions in electron transfer chains.     

DPPH (= 2,2‐Diphenyl‐1‐picrylhydrazyl = 2,2‐Diphenyl‐1‐(2,4,6‐trinitrophenyl)hydrazyl) Radical‐Scavenging Reaction of Protocatechuic Acid Esters (= 3,4‐Dihydroxybenzoates) in Alcohols: Formation of Bis‐alcohol Adduct

Protocatechuic acid esters (= 3,4‐dihydroxybenzoates) scavenge ca. 5 equiv. of radical in alcoholic solvents, whereas they consume only 2 equiv. of radical in nonalcoholic solvents. While the high radical‐scavenging activity of protocatechuic acid esters in alcoholic solvents as compared to that in nonalcoholic solvents is due to a nucleophilic addition of an alcohol molecule at C(2) of an intermediate o‐quinone structure, thus regenerating a catechol (= benzene‐1,2‐diol) structure, it is still unclear why protocatechuic acid esters scavenge more than 4 equiv. of radical (C(2) refers to the protocatechuic acid numbering). Therefore, to elucidate the oxidation mechanism beyond the formation of the C(2) alcohol adduct, 3,4‐dihydroxy‐2‐methoxybenzoic acid methyl ester ( 4 ), the C(2) MeOH adduct, which is an oxidation product of methyl protocatechuate ( 1 ) in MeOH, was oxidized by the DPPH radical (= 2,2‐diphenyl‐1‐picrylhydrazyl) or o‐chloranil (= 3,4,5,6‐tetrachlorocyclohexa‐3,5‐diene‐1,2‐dione) in CD₃OD/(D₆)acetone 3 : 1). The oxidation mixtures were directly analyzed by NMR. Oxidation with both the DPPH radical and o‐chloranil produced a C(2),C(6) bis‐methanol adduct ( 7 ), which could scavenge additional 2 equiv. of radical. Calculations of LUMO electron densities of o‐quinones corroborated the regioselective nucleophilic addition of alcohol molecules with o‐quinones. Our results strongly suggest that the regeneration of a catechol structure via a nucleophilic addition of an alcohol molecule with a o‐quinone is a key reaction for the high radical‐scavenging activity of protocatechuic acid esters in alcoholic solvents.     

CHLOROPHYLL PHOTOCHEMISTRY IN CONDENSED MEDIA—II. TRIPLET STATE QUENCHING AND ELECTRON TRANSFER TO QUINONE IN LIPOSOMES*

The fate of excitation energy and electron transfer to quinones within Chl-a-containing phosphatidyl choline liposomes has been investigated. The bilayer membrane of the liposome stabilizes the Chl triplet state, as evidenced by a three-fold increase in the lifetime over that observed in ethanol solution. The relative triplet yield follows the relative fluorescence yield, indicative of quenching at the singlet level. Triplet state lifetimes are markedly shortened as the Chl concentration is increased, demonstrating that quenching occurs at the triplet level as well. This process is shown to be due to a collisional de-excitation. In the presence of quinones, the Chl triplet reduces the quinone resulting in production of long-lived electron transfer products. The percent conversion of Chl triplet to cation radical when benzoquinone is employed as acceptor is approximately 60 ± 10%, which is slightly less than in ethanol solution (70 ± 10%). The lifetime of the radical, however, can be as much as 1900 times longer. With respect to potentially useful photochemical energy conversion, the magnitude of this increased lifetime is far more significant than is the decreased radical yield.     

ESR studies of chlorophyll one-electron photochemistry in solution: kinetics of reversible quinone reduction and general redox mechanisms

Abstract— ESR studies have been made of the kinetics of semiquinone radical formation and disappearance resulting from the reversible photosensitization of reduction or oxidation, by chlorophyll, pheophytin or hematoporphyrin, of several quinone-hydroquinone pairs in various solvents. The rate of radical decay was found to be second order with respect to the radical concentration in all systems. Radical formation rates were determined by the initial production rate minus the decay rate. The kinetic constants for single electron transfer between triplet porphyrins and quinones or hydroquinones were determined usingβ-carotene as a quencher in aqueous pyridine, and by measuring the initial rate of radical formation at various concentrations of quinones and hydroquinones in methanol and ethanol. These constants were found to be approximately the same in a given solvent for benzoquinone and hydroquinone with all porphyrins, though the rates differed in different solvents: pyridine-water ～ 10⁶I./mole sec, and methanol and ethanol ～ 5X 10⁴l./mole sec. Trimethylquinone and its hydroquinone also give similar rate constants for radical formation in pyridine-water, ～ 10⁶ l./mole sec. The second order radical decay constants for both benzoquinone and hydroquinone in pyridine-water were the same, ～ 10⁵ I./mole sec, with either chlorophyll, pheophytin or hematoporphyrin as sensitizer. The same activation energy, 6900 cal/mole, was found for chlorophyll-benzoquinone and hydroquinone in aqueous pyridine; 5500 cal/mole was obtained for these systems in ethanol. In methanol and ethanol solutions of chlorophyll, the same radical decay rate constants, ～10⁶ I./mole sec, were observed for both benzoquinone and hydroquinone. Also, the same decay constants, ～ 10⁶ I./mole sec, were found for trimethylquinone and its hydroquinine in pyridine-water. These latter two compounds gave extremely small steady-state ESR signals in ethanol compared with aqueous pyridine. We have also observed that the steady-state signal obtained with chlorophyll-menadione in ethanol-water was much enhanced by the presence of NADH. In contrast, NAD⁺ was found to decrease radical production, by increasing the decay rate, in the chlorophyll-hydroquinone system in aqueous pyridine. These results are discussed in terms of possible mechanisms for radical formation and disappearance. The most likely possibility is considered to be a one-electron oxidation or reduction of the porphyrin triplet, followed by radical disproportionation and redox reactions between the disproportionation products.     

EFFECT OF CHLORIDE ON CHLOROPHYLL PHOTOCHEMISTRY IN SOLUTION: ENHANCEMENT OF CATION RADICAL AND SEMIQUINONE YIELDS

Abstract—Illumination with red light at low temperatures of a degassed ethanol solution of chlorophyll and ferric chloride reversibly produces an EPR signal due to the chlorophyll cation radical (Chl+). The magnitude of this signal is about ten times larger than is obtained with chlorophyll alone. When benzoquinone is also present, an EPR signal due to Chl+ and the neutral semiquinone radical (QH.) is photoproduced. The semiquinone signal is about five times larger than in the absence of ferric chloride. Both of these systems show first order radical decay when the light is turned off, suggesting that radical complexes are being formed. These results indicate that iron is capable of serving as an electron transfer bridge between chlorophyll and quinone.     

Preparation and electrochemical properties of palladium(0) complexes coordinated by quinones and 1,5-cyclooctadiene

The complexes Pd(quinone)(COD) (COD = 1,5-cyclooctadiene) are prepared by a ligand substitution reaction of Pd₂(DBA)₃ (DBA = dibenzylideneacetone) in the presence of both quinone and COD. Palladium(0) complexes coordinated by quinones only are formed in the reaction in the absence of COD. The cyclic voltammetric behavior of Pd(quinone)(COD) has been studied. The reduction potentials for quinones shifted toward negative values on coordination to palladium(0). The oxidation potentials for the central palladium(0) in Pd(quinone)(COD) depend on the electron-withdrawing ability of the free quinones, and are in the following series: quinone = p-benzoquinone < 5,8-dihydro-1,4-naphthoquinone ～ 1,4-naphthoquinone < duroquinone. The shift of oxidation potentials for Pd(quinone)(COD) on changing the quinones as ligands is in contrast to that of Pd(quinone)(triphenylphosphine)₂.     

Type 1 reaction center of photosynthetic heliobacteria

The reaction center (RC) of heliobacteria contains iron-sulfur centers as terminal electron acceptors, analogous to those of green sulfur bacteria as well as photosystem I in cyanobacteria and higher plants. Therefore, they all belong to the so-called type 1 RCs, in contrast to the type 2 RCs of purple bacteria and photosystem II containing quinone molecules. Although the architecture of the heliobacterial RC as a protein complex is still unknown, it forms a homodimer made up of two identical PshA core proteins, where two symmetrical electron transfer pathways along the C2 axis are assumed to be equally functional. Electrons are considered to be transferred from membrane-bound cytochrome c (PetJ) to a special pair P800, a chlorophyll a-like molecule A0, (a quinone molecule A1) and a [4Fe-4S] center Fx and, finally, to 2[4Fe-4S] centers FA/FB. No definite evidence has been obtained for the presence of functional quinone acceptor A1. An additional interesting point is that the electron transfer reaction from cytochrome c to P800 proceeds in a collisional mode. It is highly dependent on the temperature, ion strength and/or viscosity in a reaction medium, suggesting that a heme-binding moiety fluctuates in an aqueous phase with its amino-terminus anchored to membranes.     

Structure and Conformation of Photosynthetic Pigments and Related Compounds. 12. A Crystallographic Analysis of Porphyrin-quinones and Their Precursors

Abstract. A comprehensive crystallographic analysis of 10 porphyrin quinone precursors (dimethoxybenzene derivatives), and six porphyrin quinones has been performed. The free bases and zinc(II) complexes of the porphyrin quinones are of the 5,10,15-triaryl/alkyl-20-quinone-porphyrin type and carry various bridging and quinone units. The structural and conformational parameters were determined for all compounds; the donor-acceptor separation distances range from 6.3 to 10.9 Å. Knowledge of these data is a prerequisite for a detailed interpretation of theoretical and spectroscopic studies on such systems. Despite the obvious influence of the type and geometry of the bridging unit and quinone on the spatial arrangement of the donor and acceptor components, a large variety of different packing arrangements in the crystal were observed. These include π stacking, aggregate formation and axial ligation in the zinc(II) porphyrins. The latter often utilized the quinone (or dimethoxy) oxygen atoms for coordination to zinc(II) centers leading to porphyrin quinone dimers and even polymers.     

Reaction of hydroquinone with hematite; I. Study of adsorption by electrochemical-scanning tunneling microscopy and X-ray photoelectron spectroscopy

The reaction of hematite with quinones and the quinone moieties of larger molecules may be an important factor in limiting the rate of reductive dissolution, especially by iron-reducing bacteria. Here, the electrochemical and physical properties of hydroquinone adsorbed on hematite surfaces at pH 2.5-3 were investigated with cyclic voltammetry (CV), electrochemical-scanning tunneling microscopy (EC-STM), and X-ray photoelectron spectroscopy (XPS). An oxidation peak for hydroquinone was observed in the CV experiments, as well as (photo)reduction of iron and decomposition of the solvent. The EC-STM results indicate that hydroquinone sometimes forms an ordered monolayer with approximately 1.1 QH(2)/nm(2), but can be fairly disordered (especially when viewed at larger scales). XPS results indicate that hydroquinone and benzoquinone are retained at the interface in increasing amounts as the reaction proceeds, but reduced iron is not observed. These results suggest that quinones do not adsorb by an inner-sphere complex where adsorbate-surface interactions determine the adsorbate surface structure, but rather in an outer-sphere complex where interactions among the adsorbate molecules dominate.     

Primary processes in plant photosynthesis: photosystem I reaction center

The photosystem I (PSI) pigment-protein complex of plants converts light energy into a transmembrane charge separation, which ultimately leads to the reduction of carbon dioxide. Recent studies on the dynamics of primary energy transfer, charge separation, and following electron transfer of the reaction center (RC) of the PSI prepared from spinach are reviewed. The main results of femtosecond transient absorption and fluorescence spectroscopies as applied to the P700-enchied PSI RC are summarized. This specially prepared material contains only 12–14 chlorophylls per P700, which is a special pair of chlorophyll a and has a significant role in primary charge separation. The P700-enriched particles are useful to study dynamics of cofactors, since about 100 light-harvesting chlorophylls are associated with wild PSI RC and prevent one from observing the elementary steps of the charge separation. In PSI RC energy and electron transfer were found to be strongly coupled and an ultrafast up-hill energy equilibration and charge separation were observed upon preferential excitation of P700. The secondary electron-transfer dynamics from the reduced primary electron acceptor chlorophyll a to quinone are described. With creating free energy differences (ΔG₀) for the reaction by reconstituting various artificial quinones and quinoids, the rate of electron transfer was measured. Analysis of rates versus ΔG₀ according to the quantum theory of electron transfer gave the reorganization energy, electronic coupling energy and other factors. It was shown that the natural quinones are optimized in the photosynthetic protein complexes. The above results were compared with those of photosynthetic purple bacteria, of which the structure and functions have been studied most.     

Overview of the Synthesis and Structure of Calix[n]quinones (n=4, 6, 8)

Calix[n]quinones, a class of cyclic oligomers composed of p‐benzoquinone structures connected by methylene, have multi‐conjugated carbonyl structures and adjustable cavities, which make their synthesis extremely attractive. In this minireview, synthetic methods of calix[n]quinones and recent synthetic experience of our group are summarized. The merits and demerits of various synthetic methods are briefly reviewed as well. When synthesizing calix[n]quinone (n≥6) with a larger ring, the reduction‐oxidation method is considered to be the most recommended.