Quantum mechanical effects in inorganic and bioinorganic electron transfer

The Endless World of Carotenoids-Structural, Chemical and Biological Aspects of Some Rare Carotenoids

Carotenoids are a large and diverse group of compounds that have been shown to have a wide range of potential health benefits. While some carotenoids have been extensively studied, many others have not received as much attention. Studying the physicochemical properties of carotenoids using electron paramagnetic resonance (EPR) and density functional theory (DFT) helped us understand their chemical structure and how they interact with other molecules in different environments. Ultimately, this can provide insights into their potential biological activity and how they might be used to promote health. In particular, some rare carotenoids, such as sioxanthin, siphonaxanthin and crocin, that are described here contain more functional groups than the conventional carotenoids, or have similar groups but with some situated outside of the rings, such as sapronaxanthin, myxol, deinoxanthin and sarcinaxanthin. By careful design or self-assembly, these rare carotenoids can form multiple H-bonds and coordination bonds in host molecules. The stability, oxidation potentials and antioxidant activity of the carotenoids can be improved in host molecules, and the photo-oxidation efficiency of the carotenoids can also be controlled. The photostability of the carotenoids can be increased if the carotenoids are embedded in a nonpolar environment when no bonds are formed. In addition, the application of nanosized supramolecular systems for carotenoid delivery can improve the stability and biological activity of rare carotenoids.

Antioxidant Activity in Supramolecular Carotenoid Complexes Favored by Nonpolar Environment and Disfavored by Hydrogen Bonding

Carotenoids are well-known antioxidants. They have the ability to quench singlet oxygen and scavenge toxic free radicals preventing or reducing damage to living cells. We have found that carotenoids exhibit scavenging ability towards free radicals that increases nearly exponentially with increasing the carotenoid oxidation potential. With the oxidation potential being an important parameter in predicting antioxidant activity, we focus here on the different factors affecting it. This paper examines how the chain length and donor/acceptor substituents of carotenoids affect their oxidation potentials but, most importantly, presents the recent progress on the effect of polarity of the environment and orientation of the carotenoids on the oxidation potential in supramolecular complexes. The oxidation potential of a carotenoid in a nonpolar environment was found to be higher than in a polar environment. Moreover, in order to increase the photostability of the carotenoids in supramolecular complexes, a nonpolar environment is desired and the formation of hydrogen bonds should be avoided.

Synthesis, crystal structure, spectroscopic investigations, thermal behavior and DFT calculations of pentacarbonyl(2-methylpyrazine)chromium(0)

Pentacarbonyl(2-methylpyrazine)chromium(0) [Cr(CO)(5)(2mpyz)], complex was isolated from its n-hexane solution as orange plate-like crystals which were characterized by IR, NMR spectroscopies and X-ray crystallography. The crystallographic results show that the complex was crystallized in the monoclinic system with the unit cell parameters of a=7.176 (5), b=12.045 (3), c=14.461 (3)Å, β=90.44 (3)° and space group 2/M. The single crystal structure of the complex shows the bonding of chromium metal to 2-methylpyrazine through the less sterically hindered nitrogen-4 lone pair. The pyrazine ring plane makes an angle of 179.58° (19) with COCrN bond axis. The four carbonyl groups are slightly bent away from pyrazine with the angle of 91.28° (17) for C(5)CrN1 bond axis. The DFT calculations run out using the Gaussian 03 PC program show good agreement with the experimental results. The thermal properties of the complex were also investigated by differential thermal analysis and thermogravimetry techniques.

Photoinduced electron transfer and geminate recombination for photoexcited acceptors in a pure donor solvent

Photoinduced electron transfer and geminate recombination are studied for the systems rhodamine 3B (R3B(+)) and rhodamine 6G (R6G(+)), which are cations, in neat neutral N,N-dimethylaniline (DMA). Following photoexcitation of R3B(+) or R6G(+) (abbreviated as R(+)), an electron is transferred from DMA to give the neutral radical R and the cation DMA(+). Because the DMA hole acceptor is the neat solvent, the forward transfer rate is very large, approximately 5x10(12) s(-1). The forward transfer is followed by geminate recombination, which displays a long-lived component suggesting several percent of the radicals escape geminate recombination. Spectrally resolved pump-probe experiments are used in which the probe is a "white" light continuum, and the full time-dependent spectrum is recorded with a spectrometer/charge-coupled device. Observations of stimulated emission (excited state decay-forward electron transfer), the R neutral radical spectrum, and the DMA(+) radical cation spectrum as well as the ground-state bleach recovery (geminate recombination) make it possible to unambiguously follow the electron transfer kinetics. Theoretical modeling shows that the long-lived component can be explained without invoking hole hopping or spin-forbidden transitions.

Photoinduced electron transfer and geminate recombination in liquids on short time scales: Experiments and theory

The coupled processes of intermolecular photoinduced forward electron transfer and geminate recombination between the (hole) donor (Rhodamine 3B) and (hole) acceptors (N,N-dimethylaniline) are studied in three molecular liquids: acetonitrile, butyronitrile, and benzonitrile. Two color pump-probe experiments on time scales from approximately 100 fs to hundreds of picoseconds give information about the depletion of the donor excited state due to forward electron transfer and the survival kinetics of the radicals produced by forward electron transfer. The data are analyzed with a model presented previously that includes distance dependent forward and back electron transfer rates, donor and acceptor diffusion, solvent structure, and the hydrodynamic effect in a mean-field theory of through solvent electron transfer. The forward electron transfer is in the normal regime, and the Marcus equation for the distance dependence of the transfer rate is used. The forward electron transfer data for several concentrations in the three solvents are fitted to the theory with a single adjustable parameter, the electronic coupling matrix element Jf at contact. Within experimental error all concentrations in all three solvents are fitted with the same value of Jf. The geminate recombination (back transfer) is in the inverted region, and semiclassical treatment developed by Jortner [J. Chem. Phys. 64, 4860 (1976)] is used to describe the distance dependence of the back electron transfer. The data are fitted with the single adjustable parameter Jb. It is found that the value of Jb decreases as the solvent viscosity increases. Possible explanations are discussed.

A Proton-Induced N-1 to eta(2) Migration of the Fluxional Pyrazine in the [Ru(II)(hedta)(pz)](-) Complex

[Ru(II)(hedta)(D(2)O)](-), hedta(3)(-) = N-(hydroxyethyl)ethylenediaminetriacetate, reacts with pyrazine in D(2)O at 25 degrees C to yield several isomers and </=20% of the pyrazine-bridged binuclear complex. Two isomers of 56.2% combined abundance have differentiated alpha (near) and beta (remote) (1)H NMR pyrazine resonances at 9.09 ppm (alpha or H2, H6 pair) and 8.33 ppm (beta or H3, H5 pair). The other isomer of ca. 24% abundance exhibits only a singlet at 8.76 ppm, indicative of fluxional pyrazine movement from N-1 to N-4. This is believed to be the cis-polar isomer. Within 24 h the differentiated isomers convert to the fluxional isomer, which remains fluxional in 50% D(2)O/50% CD(3)OD down to 237 K. The fluxional isomer has all equivalent (13)C NMR resonances at 152.44 ppm, 5.18 ppm downfield of free pyrazine. Comparison with the bridged binuclear ion {[Ru(hedta)](2)(pz)}(2)(-) revealed fortuitously similar shifts; the (1)H NMR spectrum shows a singlet at 8.76 ppm, and the (13)C NMR spectrum, a singlet at 152.40 ppm. These species have different electrochemical signatures, however, with the 1:1 fluxional complex having a Ru(II/III) wave at 0.20 V that shifts to 0.35 V upon protonation of the N-4 position, whereas the binuclear complex has two waves at 0.18 and 0.33 V which are independent of pH. (1)H NMR indicates stereochemically rigid coordination of 2-methylpyrazine (2-CH(3)pz) at N-4 with the 2-CH(3) position remote in the major species (65.5%) and at N-2 with the 2-CH(3) site adjacent (34.4%) in the lesser isomer. The proton resonances are as follows. Remote isomer (N-4): H2, 8.22 ppm; H3, 8.97 ppm; H5, 8.88 ppm; CH(3), 2.53 ppm. Adjacent (N-1) isomer: H2, 8.73 ppm; H3, 8.42 ppm, H5, 8.50 ppm; CH(3), 2.45 ppm. A slow conversion of the strained adjacent isomer to the remote isomer is observed. Remote and adjacent isomers were also prepared for [(NH(3))(5)Ru(2-CH(3)pz)](2+) in 87.7% and 12.3% yield. Protonation of [Ru(hedta)(pz)](-) yields nonfluxional complexes: a major species (63%) bound eta(2)(1,2) with four complicated resonance patterns at 8.72, 8.57, 8.00, and 7.91 ppm, each signal having across-the-ring couplings which require couplings to at least two different ring protons and a minor N-1-coordinated N-4 protonated species (16%) with alpha and beta proton pairs resonating at 8.81 and 8.25 ppm. An eta(2)(2,3) isomer is also detectable (20%) which has four types of (1)H resonances at 9.51, 9.01, 8.85, and 8.15 ppm. The weakened sigma bonding from pzH(+) and enhanced pi-acceptor capacity power of pzH(+) combine to induce a switch in coordination to either eta(2)(1,2) or eta(2)(2,3).

Protein conformational perturbations affect the photoreduction of native cytochrome c peroxidase (III) at alkaline pH

Ferric cytochrome c peroxidase (CCP) undergoes a ligation-state transition from a pentacoordinate, high-spin (5c/hs) heme to a hexacoordinate, low-spin (6c/1s) heme when titrated over a pH range of 7.30-9.70. This behavior is similar to that exhibited by the ferrous form of the enzyme. However, the photodissociation of the low-spin, axial ligand, exhibited by ferrous CCP at alkaline pH, is not observed for ferric CCP. Instead, a photoinduced reduction of the ferric heme is apparent in the pH range 7.90-9.70. In the absence of O2 and redox mediators such as methyl viologen (MV2+), the reoxidation of the photoreduced enzyme is very slow (tau 1/2 approximately 3 min). F(-)-bound CCP(III) (6c/hs) displays similar pH-dependent photoreduction. Horseradish peroxidase, however, does not. The formation of 6c/1s heme coincides with the onset of appreciable photoreduction (between laser pulses, > 60 ms) of CCP (III) at alkaline pH, suggesting a global protein conformational rearrangement within or around its heme pocket. Photoreduction of alkaline CCP(III) most likely involves intramolecular electron transfer (ET) from the aromatic residue in the proximal heme pocket to the photoexcited heme. We speculate that the kinetics of electron transfer are affected by changes in the orientation of Trp-191.

Long range electron transfer along an alpha-helix

The many observations of long range electron transfer in proteins raises the question of whether a protein's structure can influence the rate or path of such transfers, and if so, then how. To answer these questions requires information on which of the various structural elements composing proteins support long range electron transfer. In this report, we present evidence for long range electron transfer along the alpha-helix of a synthetic leucine zipper dimer. We also present electron transfer rate data obtained with other helical peptides.

Long range electron transfer between tyrosine and tryptophan in hen egg-white lysozyme

The azide, dibromide and dichloride radicals oxidize one or more tryptophan side chains in hen egg-white lysozyme. The indolyl radical produced in this second-order 1-electron oxidation subsequently oxidizes a tyrosine side chain to the phenoxy radical in an intramolecular reaction with a rate constant of 130 +/- 10 s-1 at pH 7, 25 degrees C. The final indolyl and phenoxy equilibrium mixture then decays with a t1/2 approximately 2 s. The faster intramolecular reaction exhibits a pH dependence; on decreasing the pH from 9 the first-order rate constant increases to a maximum near pH 5.4 and then declines as the pH is lowered further. In contrast, the first-order rate constant for the intramolecular electron transfer between the tyrosine and tryptophan of the peptide trpH-pro-tyrOH remains unchanged between approx. pH 11 and 6.5 and then increases as the pH is lowered further. This difference in the observed pH dependence suggests that changes in structure or ionization state influence the protein electron transfer rate. We also discuss the radiation inactivation of lysozyme in light of these observations.

Solvent dependence of pyrimidine dimer splitting in a covalently linked dimer-indole system

Cyclobutadipyrimidines (pyrimidine dimers) undergo splitting that is photosensitized by indole derivatives. We have prepared a compound in which a two-carbon linker connects a dimer to an indolyl group. Indolyl fluorescence quenching indicated that the two portions of the molecule interact in the excited state. Intramolecular photosensitization of dimer splitting was remarkably solvent dependent, ranging from phi spl = 0.06 in water to a high value of phi spl = 0.41 in the least polar solvent mixture examined, 1,4-dioxane-isopentane(5 : 95). A derivative with a 5-methoxy substituent on the indolyl ring behaved similarly. These results have been interpreted in terms of electron transfer from the excited indolyl group to the dimer, which would produce a charge-separated species. The dimer anion within such a species could split or undergo back electron transfer. The possibility that back electron transfer is in the Marcus inverted region can be used to rationalize the observed solvent dependence of splitting. In the inverted region, the high driving force of a charge recombination exceeds the reorganization energy of the solvent, which is less for solvents of low polarity than those of high polarity. If this theory is applicable to the hypothetical charge-separated species, a slower back electron transfer, and consequently higher splitting efficiencies, would be expected in solvents of lower polarity. Photolyases may have evolved in which a low polarity active site retards back transfer of an electron and thereby contributes to the efficiency of the enzymatic dimer splitting.