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.     

CHLOROPHYLL ONE-ELECTRON PHOTOCHEMISTRY-I. PHOTOPRODUCTION OF THE CATION RADICALS OF CHLOROPHYLL AND BACTERIOCHLOROPHYLL IN SOLUTIONS AT LOW TEMPERATURES*

Abstract— Electron spin resonance studies have shown that chlorophyll and bacteriochlorophyll can be photo-oxidized in a variety of solvents via their lowest excited singlet states to produce cation radicals. Pheophytin does not undergo this reaction. The mechanism of this photoprocess and its implications for photosynthesis are discussed.     

Origin of substituent effects in the absorption spectra of peroxy radicals: time dependent density functional theory calculations.

We investigate the assignment of electronic transitions in alkyl peroxy radicals. Past experimental work has shown that the phenyl peroxy radical exhibits a transition in the visible region; however, previous high level calculations have not reproduced this observed absorption. We use time dependent density functional theory (TDDFT) to characterize the electronic excitations of the phenyl peroxy radical as well as other hydrocarbon substituted peroxy radicals. TDDFT calculations of the phenyl peroxy radical support an excitation in the visible spectrum. Further, we investigate the nature of this visible absorption using electron attachment/detachment density diagrams of the peroxy radicals and present a qualitative picture of the origin of the visible absorption based on molecular orbital perturbations. The peroxy radical substituent is also compared against isoelectronic radical groups. The visible absorption is determined to be dependent on mixing of the alkyl and radical substituent orbitals.     

CHLOROPHYLL-PHOTOSENSITIZED OXIDATION OF HYDROQUINONE AND p-PHENYLENEDIAMINE DERIVATIVES

Abstract— ESR and photovoltaic studies on light-induced one-electron transfer between chlorophyll a and electron donors in the absence of oxygen show (1) the possible conversion of photo-reduced chlorophyll a and p-benzosemiquinone ion radicals to their non-ionic radicals in methanol solutions of low pH, (2) the production of ESR absorption of tetrachloro-p-benzosemi-quinone even in benzene, enhanced by the addition of triethylamine or methanol, and (3) the transfer of one electron from tetramethyl-p-phenylenediamine to either excited chlorophyll a or pheophytin a in methanol at pH above 3.6 but not to pheophytin a at pH below 1 0 where its radical cation appears to accept an electron from excited pheophytin a . Bacteteriochloro-phyll is also shown to be capable of photooxidizing hydroquinones and tetramethyl-p-phenyl-enediamine.
The presence of oxygen enhances chlorophyll a -photosensitized oxidation of hydroquinone and tetrachloro-hydroquinone by one-electron transfer to oxygen and of trimethylhydro-quinone probably by two-electron trnasfer to oxygen. A free radical from excited chlorophyll a-oxygen interaction is formed in these reactions, but rapidly quenched in the case of trimethyl-hydroquinone. This, kind of free radical is not formed in pheophytin a . Tetramethyl- p -phenyl-enediamine readily undergoes chlorophyll a-photosensitized oxidation by oxygen in any pH region.     

Multiplet splitting of core level binding energies in paramagnetic species and the unpaired orbital spin density distribution

The relationship between the multiplet splitting in core electron binding energies of radicals and the unpaired orbital spin density distribution is developed and an explicit formula derived. The atomic intershell exchange interaction is shown to be important to determining magnitudes and trends in the multiplet splitting. The information obtained from an electron spin resonance spectroscopy experiment through the isotropic electron-nuclear hyperfine interaction and an X-ray photoelectron spectroscopy experiment is shown to be complementary.     

Structure,epr parameters,and reactivity of organic free radicals from a density functional approach

Summary A recently introduced density functional incorporating gradient corrections and some Hartree-Fock exchange has been used to study the structures, properties, and reactivity of representative organic free radicals. A general theoretical model has been introduced, in which standardized grid, functional, and orbital basis set are used to compute geometrical parameters, vibrational frequencies, and one-electron properties. The results are compared with available experimental data from diatomic to polyatomic radicals. All the geometric and electronic parameters compare favourably with available experimental data and with the results of refined post Hartree-Fock computations. Also the thermodynamics and kinetics of a representative unimolecular reaction (isomerization of formaldehyde radical cation) are well reproduced. These findings together with the very favourable scaling of the computations with the number of electrons suggest that the density functional approach is a promising theoretical tool for the study of relationships between structure and properties of large free radicals.     

Spectroscopic and density functional theory studies of the molecular geometry and electronic structure of classical and nonclassical radical ions derived from 7-benzhydrylidenenorbornene analogues

A spectroscopic study, using nanosecond time-resolved laser flash photolysis and gamma-irradiation of low-temperature matrices, was undertaken along with a theoretical study using density functional theory (DFT) and time-dependent (TD)-DFT calculations to gain insight into the molecular geometry and electronic structure of radical cations and radical anions of 7-benzhydrylidenenorbornene (4) and its derivatives 6-8. The radical ions 4(.+), 6(.+), 7(.+), 8(.+), 4(.-), 6(.-), 7(.-), and 8(.-) exhibited clear absorption bands in the 350-800 nm region, which were reproduced successfully from the electronic transitions calculated with TD-UB3LYP/cc-pVDZ. Radical cations 4(.+) and 8(.+) are consistent with a bent structure having a delocalized electronic state where the spin and charge are delocalized not only in the benzhydrylidene subunit but also in the residual subunit. In contrast, 6(.+) and 7(.+) have nonbent structures with a localized electronic state where their spin and charge are localized in the benzhydrylidene subunit only. Therefore, 4(.+) and 89(.+) have a nonclassical nature, with 6(.+) and 7(.+) possessing a classical nature. In contrast, in the radical anion system, 7(.-) and 8(.-) are considered nonclassical, and 4(.-) and 6(.-) are classical. Orbital interaction theory and DFT calculations can account fully for the spectroscopic features, molecular geometries, and electronic structures of the radical ions. For example, the shift of the absorption bands and the nonclassical nature of 4(.+) are due to the antibonding character of the highest occupied molecular orbital (HOMO) of 4, and those of 7(.-) arise from the bonding character of the lowest unoccupied molecular orbital (LUMO) of 7. A topological agreement of p-orbitals at C-2, C-3 (or C-5, C-6), and C-7 produces strong electronic coupling with an antibonding or a bonding character in the frontier orbitals. It is the ethylene and butadiene skeleton at C-2-C-3 (or C-5-C-6), with its contrasting topology in the HOMO and LUMO of the neutral precursor, that holds the key to deducing the nonclassical nature of the 7-benzhydrylidenenorbornene-type radical cation and radical anion systems.     

Influence of Pb(II) on the radical properties of humic substances and model compounds

The influence of Pb(II) ions on the properties of the free radicals formed in humic acids and fulvic acids was investigated by electron paramagnetic resonance spectroscopy. It is shown that, in both humic acid and fulvic acid, Pb(II) ions shift the radical formation equilibrium by increasing the concentration of stable radicals. Moreover, in both humic acid and fulvic acid, Pb(II) ions cause a characteristic lowering of the stable radicals' g-values to g = 2.0010, which is below the free electron g-value. This effect is unique for Pb ions and is not observed with other dications. Gallic acid (3,4,5-trihydroxybenzoic acid) and tannic acid are shown to be appropriate models for the free radical properties, i.e., g-values, Pb effect, pH dependence, of humic and fulvic acid, respectively. On the basis of density functional theory calculations for the model system (gallic acid-Pb), the observed characteristic g-value reduction upon Pb binding is attributed to the delocalization of the unpaired spin density onto the Pb atom. The present data reveal a novel environmental role of Pb(II) ions on the formation and stabilization of free radicals in natural organic matter.     

Theoretical analysis of the structural and electronic properties of metalloporphyrin pi-cation radicals

A method for analyzing the A(1u)/A(2u) contents of metalloporphyrin pi-cation radicals is developed and applied to a series of unsubstituted planar metalloporphines (MPs) (M=Cr, Mn, Fe, Co, Ni, Cu, and Zn). The structures and electronic properties of the MPs and their cation radicals were calculated by density functional theory (DFT) and subsequently analyzed. It was found that the MPs with small core sizes have a tendency to form A(1u)-type radicals, while the MPs with large core size have a preference for an A(2u)-type. Neither of these pure-state species, however, is stable under the D(4)(h) symmetry, and both radical cation types are subject to pseudo-Jahn-Teller (pJT) distortion. The pJT distortion leads to structures with lower symmetry and states that have mixed character with respect to the A(1u) and A(2u) components. The degree of mixing could be estimated by employing orbital projection technique or a complementary spin density decomposition. Both techniques produce very similar results, pointing out that the frontier orbital, which becomes empty upon electron removal, plays a critical role in determining electronic properties.     

Spin distributions, ring conformations, and spiroconjugation in "phosphaverdazyl" radicals

Reaction of P-dimethylaminophosphonic acid bis(1-methylhydrazide) (6) with trimethyl orthobenzoate gave 1,2,5,6-tetrahydro-1,5-dimethyl-6-(N,N-dimethylamino)-6-phenyl-1,2,4,5,6-tetrazaphosphorine-6-oxide (7), which was subsequently oxidized to the corresponding P-diemthylamino-6-phosphaverdazyl (5) as a persistent radical. Analysis of the electron paramagnetic resonance spectrum of 5 revealed significant spin density on the exocyclic nitrogen but very little spin density on the phosphorus, in contrast to the previously reported P-phenyl-6-phosphaverdazyl (4). Density functional theory calculations on simplified models of 4, 5, and related radicals were performed and revealed that spin polarization effects and the nature of the substituents on phosphorus have significant effects on the structures and spin distributions of these radicals. The spin transfer to the dimethylamino group in 5 was revealed to arise from spiroconjugation-type overlap between the nitrogen 2p orbital with the verdazyl radical singly occupied molecular orbital.     

Ab initio calculations on the electronic structure of the divalent lead-water complex

We studied the electronic structure of the Pb (2+)-4H 2O system. Analysis of the complex orbital evidenced no mixing between the 6s lone pair orbital of the lead and the 6p orbital components. Moreover, we found that the HOMO is widely described by the mixture of the 6p components with the 7s valence orbital of the lead. This orbital shows an important elliptical electron charge density around the lead ion and opposite the direction of the short lead-water bonds. From these results, we demonstrated that the hemidirected conformation of the Pb (2+)-4H 2O system could be easily explained by the shape of the electron charge density distribution of the HOMO rather than by the stereochemically active character of the 6s (2) lone pair of lead electrons.     

Synergistic Effects of Lewis Bases and Substituents on the Electronic Structure and Reactivity of Boryl Radicals

Boryl radicals have the potential for the development of new molecular entities and for application in new radical reactions. However, the effects of the substituents and coordinating Lewis bases on the reactivity of boryl radicals are not fully understood. By using first‐principles methods, we investigated the spin‐density distribution and reactivity of a series of boryl radicals with various substituents and Lewis bases. The substituents, along with the Lewis bases, only affect the radical reactivity when an unpaired electron is in the boron p_z orbital, that is, for three‐coordinate radicals. We found evidence of synergistic effects between the substituents and the Lewis bases that can substantially broaden the tunability of the reactivity of the boryl radicals. Among Lewis bases, pyridine and imidazol‐2‐ylidene show a similar capacity for stabilization by delocalizing the spin density. Electron‐donating substituents, such as nitrogen, more efficiently stabilize boryl radicals than oxygen and carbon atoms. The reactivity of a boryl radical is always boron based, irrespective of the spin density on boron.     

Excited state relaxation dynamics of the zinc(II) tetraphenylporphine cation radical

A new technique employed to study the photophysical properties of the zinc(II) tetraphenylporphine cation radical is reported. It employs a combination of controlled potential coulometry and femtosecond absorption spectrometry. The fast transient lifetime of 17 ps of the pi-cation species originates in very efficient mixing of the a(2u) HOMO cation orbital that places electronic density mainly on pyrrolic nitrogens and metal d-orbitals. That explains the lack of any emission of the cationic species. This nonradiative decay process might elucidate the processes taking place in photosynthetic systems when an electron is removed from the porphyrinic moiety and the hole is produced.     

A theoretical study of the addition of silyl radicals to olefinic monomers

In this paper, the high reactivity of silyl macroradicals toward double bonds of olefinic compounds has been explained by means of quantum‐mechanical calculations through their frontier orbital characteristics. In this way, the main orbital interaction corresponds to the overlapping between the SOMO of the disilyl radical and the LUMO of the olefin. In order to obtain more accurate results of differential reactivity, an orbitalic SOMO‐HOMO interaction should be included in addition to the main SOMO‐LUMO one. Also, we theoretically studied the regioselectivity of the addition of silyl radicals to double bonds obtaining similar results as for carbon centered radicals where the reaction takes place on the less hindered carbon of the olefin. Regarding to the geometrical and electronic parameters, it has been shown that carbon radicals have a sp² geometry and a negative charge on the radical center whilst silyl radicals have a sp³ goemetry and a positive charge. Both factors contribute to the enhanced reactivity of silyl radicals with respect to carbon ones.     

Unrestricted Hartree-Fock calculations of spin densities with orthogonalized atomic orbitals: Proton and 13C hyperfine splittings in some pi hydrocarbon radicals

The spin density distribution in a few hydrocarbon radicals has been calculated using orthogonalized atomic orbitals in the Unrestricted Hartree-Fock formalism of Amos and Snyder and including certain more important two-electron hybrid and exchange integrals and all the core-resonance integrals. Our calculated spin densities for the cation and anion radicals of alternant hydrocarbons, which are now different due to the breakdown of the pairing theorem, are, in general, of the right relative order so that even the simple McConnell type of relation can account partly for the observed differences in the proton splittings between cations and anions. The proton splittings for position 2 of naphthalene and anthracene radical ions are correctly predicted, thus clearing up the well-known cation-anion anomaly for this position. Comparative calculations have been made to show that the spin density results are worsened with the neglect of the integrals of the type mentioned before. An empirical analysis correlating the observed ¹³C splittings and the spin density results over a non-orthogonal basis set shows that the available ¹³C splittings in alternant hydrocarbon radical ions can be explained with a set of sigma-pi parameters which are consistent with the theory. It is shown that even though the spin densities in cations and anions may be different, these can lead to similar ¹³C splittings.     

Spin densities from subsystem density-functional theory: assessment and application to a photosynthetic reaction center complex model

Subsystem density-functional theory (DFT) is a powerful and efficient alternative to Kohn-Sham DFT for large systems composed of several weakly interacting subunits. Here, we provide a systematic investigation of the spin-density distributions obtained in subsystem DFT calculations for radicals in explicit environments. This includes a small radical in a solvent shell, a π-stacked guanine-thymine radical cation, and a benchmark application to a model for the special pair radical cation, which is a dimer of bacteriochlorophyll pigments, from the photosynthetic reaction center of purple bacteria. We investigate the differences in the spin densities resulting from subsystem DFT and Kohn-Sham DFT calculations. In these comparisons, we focus on the problem of overdelocalization of spin densities due to the self-interaction error in DFT. It is demonstrated that subsystem DFT can reduce this problem, while it still allows to describe spin-polarization effects crossing the boundaries of the subsystems. In practical calculations of spin densities for radicals in a given environment, it may thus be a pragmatic alternative to Kohn-Sham DFT calculations. In our calculation on the special pair radical cation, we show that the coordinating histidine residues reduce the spin-density asymmetry between the two halves of this system, while inclusion of a larger binding pocket model increases this asymmetry. The unidirectional energy transfer in photosynthetic reaction centers is related to the asymmetry introduced by the protein environment.     

EPR spectra and structure of the chlorotrifluoroethylene and bromotrifluoroethylene radical anions

EPR spectra attributable to the halogenotrifluoroethylene radical anions have been generated by electron attachment in solid solutions at low temperatures. The spin density distribution in these radicals strongly suggests that the unpaired electron occupies a σ^* orbital, a conclusion which is supported by CNDO/2 calculations.     

1H NMR study of bacteriochlorophyll a radical anion: Use of difference spectra

The radical anion of bacteriochlorophyll a has been monitored by observing differential electron transfer broadening in the ¹H NMR spectrum of diamagnetic bacteriochlorophyll in the presence of a small amount of chemically produced anion. The use of NMR difference spectra clearly indicates which sites in the anion have greatest spin density; these data are compared with other studies on the cation and anion.     

Formation and stabilization model of the 42-mer Abeta radical: implications for the long-lasting oxidative stress in Alzheimer's disease

Amyloid fibrils mainly consist of 40-mer and 42-mer peptides (Abeta40, Abeta42). Abeta42 is believed to play a crucial role in the pathogenesis of Alzheimer's disease because its aggregative ability and neurotoxicity are considerably greater than those of Abeta40. The neurotoxicity of Abeta peptides involving the generation of free radicals is closely related to the S-oxidized radical cation of Met-35. However, the cation's origin and mechanism of stabilization remain unclear. Recently, structural models of fibrillar Abeta42 and Abeta40 based on systematic proline replacement have been proposed by our group [Morimoto, A.; et al. J. Biol. Chem. 2004, 279, 52781] and Wetzel's group [Williams, A. D.; et al. J. Mol. Biol. 2004, 335, 833], respectively. A major difference between these models is that our model of Abeta42 has a C-terminal beta-sheet region. Our biophysical study on Abeta42 using electron spin resonance (ESR) suggests that the S-oxidized radical cation of Met-35 could be generated by the reduction of the tyrosyl radical at Tyr-10 through a turn structure at positions 22 and 23, and stabilized by a C-terminal carboxylate anion through an intramolecular beta-sheet at positions 35-37 and 40-42 to form a C-terminal core that would lead to aggregation. A time-course analysis of the generation of radicals using ESR suggests that stabilization of the radicals by aggregation might be a main reason for the long-lasting oxidative stress of Abeta42. In contrast, the S-oxidized radical cation of Abeta40 is too short-lived to induce potent neurotoxicity because no such stabilization of radicals occurs in Abeta40.     

Free radicals and radical anions in a series of telluroxanthene derivatives

In the one-electron reduction of telluroxanthyl or telluroxanthone perchlorates, stable 9-aryltelluroxanthyl radicals and telluroxanthone radical anion are formed. An EPR study of the distribution of spin density in such radicals gives evidence that the nature of the heteroatoms in the xanthyl fragments has an insignificant influence on the distribution of the spin density.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1196–1100, September, 1988.