Absence Of Electron Donors Research Articles

Electron paramagnetic resonance evidence of the generation of superoxide (O 2.− and hydroxyl ( .OH) radicals by irradiation of a new photodynamic therapy photosensitizer, Victoria Blue BO

Electron paramagnetic resonance (EPR) experiments were performed on Victoria Blue BO, a cationic dye whose photocytotoxicity has been studied against the human leukaemic cell lines K-562 and TF-1. EPR experiments with 2,2,6,6-tetramethyl-4-piperidone and spin trapping with 5,5-dimethyl-1-pyrroline- N-oxide showed that, on illumination in aerated aqueous solution or dl-α-dipalmitoylphosphatidylcholine liposomes, photoexcited Victoria Blue BO is unable to generate 1O 2, whereas O 2 .− and ·;OH are trapped by 5,5-dimethyl-1-pyrroline- N-oxide in the presence or absence of electron donors. The O 2 .− formed probably leads to the ·OH radical, with an efficiency which is increased by electron donors such as Fe 2+.

Dichromated polyvinyl alcohol as a real-time hologram recording material: some observations and discussions

The real-time hologram formation in dichromated polyvinyl alcohol films has been studied in the presence of external electron donors included in the formulation. The effect of different external electron donors on the photosensitivity and real-time diffraction efficiency of volume holographic transmission gratings has been reported. The electron donors studied were found to be detrimental to both holographic characteristics, even in small concentrations. Some parameters that influence the holographic performance are discussed, and results are given. The photochemical recording mechanism in dichromated polyvinyl alcohol in the presence and absence of electron donors is briefly discussed.

The interaction of short chain coenzyme Q analogs with different redox states of myoglobin.

Two-equivalent oxidation of metmyoglobin (MbIII) by hydrogen peroxide (H2O2) yields an oxoferryl moiety (MbIV) plus a protein radical which presumably originates from the conversion of tyrosines to tyrosyl radicals (-MbIV). In the absence of electron donors, MbIII oxidation is followed by (i) heme degradation or (ii) tyrosyl radical-dependent reactions, such as irreversible dimerization or covalent binding of the heme group to the apoprotein. Moreover, the oxidizing equivalents of H2O2-activated MbIII promote the peroxidative decomposition of polyunsaturated fatty acids. In this study, water-soluble short chain coenzyme Q analogs (CoQ1H2 and CoQ2H2) were found to reduce the oxoferryl moiety, preventing heme degradation and regenerating MbIII and, more slowly, MbIIO2. CoQ1H2 and CoQ2H2 were also found to reduce tyrosyl radicals generated by UV irradiation of tyrosine solutions. Accordingly, CoQ1H2 and CoQ2H2 effectively prevented tyrosyl radical-dependent reactions such as the dimerization of sperm whale myoglobin and heme-apoprotein covalent binding in horse heart myoglobin. By competing for the oxidizing equivalents of hypervalent myoglobin, CoQ1H2 and CoQ2H2 also prevented the peroxidation of arachidonic acid. Collectively, these studies suggest that the proposed function of coenzyme Q as a naturally occurring antioxidant might well relate to its ability of reducing H2O2-activated myoglobin. Coenzyme Q should therefore mitigate cardiac or muscular dysfunctions that are caused by an abnormal generation of H2O2.

Electron-transfer quenching of excited 1,6-diphenylhexatriene cation radicals

The photochemistry of the all-trans-1,6-diphenyl-1,3,5-hexatriene cation radical, t DPH +. , in air-saturated acetonitrile solution at room temperature has been investigated by two-laser flash photolysis. In the presence of anisole, 1,2-dimethoxybenzene, and biphenyl, 590-nm photolysis of t DPH +. resulted in electron-transfer quenching. In the presence of anisole and 1,2-dimethoxybenzene, this quenching was indicated by the absence of the trans→cis isomerized cation radical, c DPH +. , which is the primary photoproduct in the absence of electron donors. In the presence of biphenyl, the biphenyl cation radical with λ max 375 nm was observed

Photoinhibition of hydroxylamine-extracted photosystem II membranes: studies of the mechanism.

The effects of photosystem II (PSII) exogenous electron donors and acceptors on the kinetics of weak light photoinhibition of NH2OH/EDTA-extracted spinach PSII membranes were examined. Under aerobic conditions, Mn2+ (approximately 1 Mn/reaction center; Km approximately 400 nM) inhibited photoinactivation and approximately 1 Mn/reaction center plus 100 microM NH2NH2 gave almost complete protection. In the absence of electron donors, strict anaerobiosis greatly inhibited photoinactivation even in the presence of an electron acceptor. Under aerobic conditions, the addition of electron acceptors (FeCN, DCIP), oxyradical scavengers, or superoxide dismutase strongly suppressed rates of photodamages. Increase in the concentrations of superoxide above those produced by illuminated NH2OH/EDTA-photosystem II membranes increased the rates of damage in the light but gave no damage in the dark. Scavengers of hydroxyl radicals and singlet oxygen did not suppress the rates of aerobic photoinhibition. These findings, along with others, lead us to conclude that photodamage of the secondary donors of the PSII reaction center occurs by two mechanisms: (1) a rapid superoxide and tyrosine YZ+ dependent process and (2) a slower process in which P680+/Chl+ catalyze the damages.

Enhanced decomposition of oxyferrous cytochrome P450CIA1 (P450 cam) by the chemopreventive agent 3-t-butyl-4-hydroxyanisole

The efficacy of 2(3)-t-butyl-4-hydroxyanisole (BHA) and other chemicals as chemopreventive agents against chemically induced cancer or toxicity may involve direct modulation of cytochrome P450 activity. Direct interaction of BHA with cytochrome P450 was investigated using substrate-bound, oxyferrous cytochrome P450CIA1 either in a reconstituted system containing cytochrome P450CIA1, putidaredoxin and putidaredoxin reductase with NADH as electron donor or in the absence of physiological electron donors. In the reconstituted system, BHA caused a concentration-dependent decrease in the production of 5- exo-hydroxycamphor and a substoichiometric increase in hydrogen peroxide production. However, BHA did not appreciably inhibit either NADH oxidation or oxygen utilization under conditions optimal for accumulation of oxyferrous cytochrome P450CIA1 during steady-state metabolism of camphor. In the absence of electron donor, BHA enhanced decomposition of the ternary oxyferrous substrate complex of cytochrome P450CIA1 without the formation of any apparent spectral intermediate(s). The rate of decomposition of the oxyferrous complex was pseudo-first order and was dependent upon the concentration of BHA present. Enhanced decomposition of the complex was not attributable to catalytic turnover of cytochrome P450CIA1 (i.e., acquisition of a second electron from an indeterminate source) since no appreciable metabolism of either camphor or BHA was observed. The enhanced decomposition was accompanied by a substoichiometric increase in hydrogen peroxide production, suggesting that BHA may facilitate four-electron reduction of molecular oxygen to water. These results indicate that BHA inhibits cytochrome P450 function, presumably by enhancing autoxidation of the substrate-bound oxyferrous complex.

Maltose transport in membrane vesicles of Escherichia coli is linked to ATP hydrolysis.

We examined the energy requirement for maltose transport in right-side-out membrane vesicles derived from Escherichia coli. When membrane vesicles were made from strains producing tethered maltose-binding proteins by dilution of spheroplasts into phosphate buffer, those from an F0F1 ATPase-containing (unc+) strain transported maltose in the presence of an exogenous electron donor, such as ascorbate/phenazine methosulfate, at a rate of 1-5 nmol/min per mg of protein, whereas those from an isogenic unc- strain failed to transport maltose. Transport in vesicles obtained from the latter strain could be restored in the presence of electron donors if the vesicles were made to contain NAD+ and either ATP or an ATP-regenerating system. ATP hydrolysis was apparently required for transport, since nonhydrolyzable ATP analogues did not sustain transport. Maltose transport significantly increased ATP hydrolysis in ATP-containing vesicles from unc- cells. Finally, ATP-containing vesicles from unc- strains producing normal maltose-binding proteins could accumulate maltose in the absence of electron donors. These results provide convincing evidence that it is the hydrolysis of ATP that drives maltose transport, and probably also other periplasmic-binding-protein-dependent transport systems.

Aniline hydroxylase activities of haemoglobin: Kinetics and mechanism

The mechanism of the aniline hydroxylase activity of methaemoglobin in a monooxygenase system consisting of NADH as electron donor, riboflavin, FAD, FMN or methylene blue as electron carrier and methaemoglobin as the terminal oxidase has been studied. Hydrogen peroxide is produced from oxygen in a methaemoglobin-independent process. 4-Aminophenol is subsequently produced peroxidatively by an NADH-dependent process; NADH prevents a further oxidation of 4-aminophenol in the presence of haemoglobin. In the absence of electron carrier, NADH slowly reduces haemoglobin and then oxyhaemoglobin reacts with aniline to give 4-aminophenol. In the absence of electron donor and electron carrier, oxyhaemoglobin and aniline give rise to the reversible production of 4-aminophenol.

A rapid vectorial back reaction at the reaction centers of Photosystem II in Tris-washed chloroplasts induced by repetitive flash excitation

In Tris-washed chloroplasts, completely lacking the oxygen-evolving capacity, absorption changes in the range of 420–560 nm induced by repetitive flash excitation have been measured in the presence and absence of electron donors. It was found: 1. (1) At 520 nm flash-induced absorption changes are observed, which predominantly decay via a 100–200-μs exponential kinetics corresponding to that of the back reaction between the primary electron donor and acceptor of Photosystem II (Haveman, J. and Mathis, P. (1976) Biochim. Biophys. Acta 440, 346–355; Renger, G. and Wolff, Ch. (1976) Biochim. Biophys. Acta 423, 610–614). In the presence of hydroquinone/ascorbate as donor couple the amplitude is nearly doubled and the decay becomes significantly slowed down. 2. (2) The difference spectrum of the absorption changes obtained in the presence of hydroquinone/ascorbate, which are sensitive to ionophores, is nearly identical with that of normal chloroplasts in the range of 460–560 nm (Emrich, H.M., Junge, W. and Witt, H.T. (1969) Z. Naturforsch. 24b, 1144–1146). In the absence of hydroquinone/ascorbate the difference spectrum of the absorption changes, characterized by a 100–200-μs decay kinetics, differs in the range of 460–500 nm and by a hump in the range of 530–560 nm. The hump is shown to be attributable to the socalled C550 absorption change, which reflects the turnover of the primary acceptor of Photosystem II (van Gorkom, H.J. (1976) Thesis, Leiden), while the deviations in the range of 460–500 nm are understandable as to be due to the overlapping absorption changes of chlorophyll a + II. The problems arising with the latter explanation are discussed. 3. (3) The electron transfer due to the rapid turnover at Photosystem II, which can be induced by flash groups with a short dark time between the flashes, is not able to energize the ATPase and to drive photophosphorylation. On the basis of the present results it is inferred, that in Tris-washed chloroplasts under repetitive flash excitation a rapid transmembrane vectorial electron shuttle takes place between the primary acceptor (X320) and donor (Chl a II) of Photosystem II, which is not able to energize the photophosphorylation. Furthermore, the data are shown to confirm the localization of X320 and Chl a II within the thylakoid membrane at the outer and inner side, respectively.

The involvement of iron and ubiquinone in electron transfer reactions mediated by reaction centers from photosynthetic bacteria

Reaction centers from Rhodopseudomonas sphaeroides strain R-26 were prepared with varying Fe and ubiquinone (Q) contents. The photooxidation of P-870 to P-870 + was found to occur with the same quantum yield in Fe-depleted reaction centers as in control samples. The kinetics of electron transfer from the initial electron acceptor (I) to Q also were unchanged upon Fe removal. We conclude that Fe has no measurable role in the primary photochemical reaction. The extent of secondary reaction from the first quinone acceptor (Q A) to the second quinone acceptor (Q B) was monitored by the decay kinetics of P-870 + after excitation of reaction centers with single flashes in the absence of electron donors, and by the amount of P-870 photooxidation that occurred on the second flash in the presence of electron donors. In reaction centers with nearly one iron and between 1 and 2 ubiquinones per reaction center, the amount of secondary electron transfer is proportional to the ubiquinone content above one per reaction center. In reaction centers treated with LiClO 4 and o-phenanthroline to remove Fe, the amount of secondary reaction is decreased and is proportional to Fe content. Fe seems to be required for the secondary reaction. In reaction centers depleted of Fe by treatment with SDS and EDTA, the correlation between Fe content and secondary activity is not as good as that found using LiClO 4. This is probably due in part to a loss of primary photochemical activity in samples treated with SDS; but the correlation is still not perfect after correction for this effect. The nature of the back reaction between P-870 + and Q − B was investigated using stopped flow techniques. Reaction centers in the P-870 + Q − B state decay with a 1-s half-time in both the presence and absence of o-phenanthroline, an inhibitor of electron transfer between Q − B and Q B. This indicates that the back reaction between P-870 + and Q − A is direct, rather than proceeding via thermal repopulation of Q − A. The P-870 + Q − B state is calculated to lie at least 100 mV in free energy below the P-870 + Q − A state.

Photoreduction of Ferricytochrome C in the Presence and Absence of Electron-Donor

Photoreduction of Ferricytochrome C in the Presence and Absence of Electron-Donor

The effect of the mixing of π -and σ -orbitals on the Fermi surface in the l.c.a.o. model for graphite

The common energy of the two principal conduction bands in graphite along the triad axes in k-space is recalculated using an l.c.a.o. model based on sixteen orbitals per unit cell; four 2 s and 2 p ( π and σ ) orbitals centred on each of the four atomic sites in the unit cell. (An earlier calculation took account only of the four 2 p z orbitals.) The results of a group-theoretical analysis are used to show that only 15 of the 136 elements of the resulting 16 x 16 secular determinant are required to compute the doubly degenerate root corresponding to the common energy of the two conduction bands on the triad axes. This energy is found to vary periodically along the triad axes with an amplitude of 0.013 eV. The corresponding overlap of the two conduction bands is about three times larger than the overlap found in the earlier calculation. The resulting correction to the energy surfaces near the triad axes is too small to affect the calculation of the Hall coefficient of graphite for room temperatures based on the earlier calculation, but it is significant at low temperatures. For temperatures below about 50° K and in the absence of electron donors or traps, the conduction region in k-space is confined to the top of the lower conduction band where the density of states per unit energy range is estimated to be 1.1 x 10 -2 √(0.0135 — ϵ ) electronic states per atom per electron-volt, and to the bottom of the upper conduction band where the density of states is found to be 5.8 x 10 -2 √ ϵ . Formulae are given for the shape of the energy surfaces in these regions.