Lack Of Electron Transfer Research Articles

Synergistic Effects of Reduced Nontronite and Organic Ligands on Cr(VI) Reduction.

Widespread iron-bearing clay minerals are potential materials that can reduce and immobilize Cr(VI) as insoluble Cr2O3/Cr(OH)3. The kinetics of this process is controlled by various environmental factors, yet the effects of such factors on Cr(VI) transformation by iron-bearing clays are poorly understood. Herein, we report the synergistic effects of reduced nontronite (rNAu-2) and environmentally prevalent organic ligands on Cr(VI) reduction under near-neutral pH conditions. The presence of ligands belonging to α-hydroxyl or carbonyl carboxylates, such as tartrate, malate, lactate, pyruvate, and mandelate, significantly promoted the rate and extent of Cr(VI) reduction by rNAu-2, likely because of the formation of Cr(V)-ligand complexes and resulting electron transfer from the ligand to Cr(V). In contrast, ligands containing carboxyl groups only, such as succinate and propionate, had a slightly inhibitory or no effect, likely because of their weak complexing ability with Cr(V) and lack of electron transfer from the ligand to Cr(V). In addition, α-hydroxyl carboxylates are probably more easily oxidized by Cr(V)/Cr(IV) than carboxylates. Soluble Cr(III)-organic complexes were the dominant products of Cr(VI) reduction in the presence of tartrate and malate. This study highlights the importance of organic ligands in the biogeochemical cycling of chromium and has significant implications for chromium remediation in contaminated environments.

Evidence for Fast Electron Transfer between the High-Spin Haems in Cytochrome bd-I from Escherichia coli.

Cytochrome bd-I is one of the three proton motive force-generating quinol oxidases in the O2-dependent respiratory chain of Escherichia coli. It contains one low-spin haem (b558) and the two high-spin haems (b595 and d) as the redox-active cofactors. In order to examine the flash-induced intraprotein reverse electron transfer (the so-called ''electron backflow''), CO was photolyzed from the ferrous haem d in one-electron reduced (b5583+b5953+d2+-CO) cytochrome bd-I, and the fully reduced (b5582+b5952+d2+-CO) oxidase as a control. In contrast to the fully reduced cytochrome bd-I, the transient spectrum of one-electron reduced oxidase at a delay time of 1.5 μs is clearly different from that at a delay time of 200 ns. The difference between the two spectra can be modeled as the electron transfer from haem d to haem b595 in 3–4% of the cytochrome bd-I population. Thus, the interhaem electron backflow reaction induced by photodissociation of CO from haem d in one-electron reduced cytochrome bd-I comprises two kinetically different phases: the previously unnoticed fast electron transfer from haem d to haem b595 within 0.2–1.5 μs and the slower well-defined electron equilibration with τ ~16 μs. The major new finding of this work is the lack of electron transfer at 200 ns.

Stimulation of Cytochrome P450 Reactions by Apo-cytochromeb5: EVIDENCE AGAINST TRANSFER OF HEME FROM CYTOCHROME P450 3A4 TO APO-CYTOCHROME b5 OR HEME OXYGENASE

Many cytochrome P450 (P450)-dependent reactions have been shown to be stimulated by another microsomal protein, cytochrome b(5) (b(5)). Two major explanations are (i) direct electron transfer from b(5) and (ii) a conformational effect in the absence of electron transfer. Some P450s (e.g. 3A4, 2C9, 17A, and 4A7) are stimulated by either b(5) or b(5) devoid of heme (apo-b(5)), indicating a lack of electron transfer, whereas other P450s (e.g. 2E1) are stimulated by b(5) but not by apo-b(5). Recently, a proposal has been made by Guryev et al. (Biochemistry 40, 5018-5031, 2001) that the stimulation by apo-b(5) can be explained only by transfer of heme from P450 preparations to apo-b(5), enabling electron transfer. We have repeated earlier findings of stimulation of catalytic activity of testosterone 6beta-hydroxylation activities with four P450 preparations, in which nearly all of the heme was accounted for as P450. Spectral analysis of mixtures indicated that only approximately 5% of the heme can be transferred to apo-b(5), which cannot account for the observed stimulation. The presence of the heme scavenger apomyoglobin did not inhibit the stimulation of P450 3A4-dependent testosterone or nifedipine oxidation activity. Further evidence against the presence of loosely bound P450 3A4 heme was provided in experiments with apo-heme oxygenase, in which only 3% of the P450 heme was converted to biliverdin. Finally, b(5) supported NADH-b(5) reductase/P450 3A4-dependent testosterone 6beta-hydroxylation, but apo-b(5) did not. Thus, apo-b(5) can stimulate P450 3A4 reactions as well as b(5) in the absence of electron transfer, and heme transfer from P450 3A4 to apo-b(5) cannot be used to explain the catalytic stimulation.

PREPARATION AND PHOTOPHYSICAL STUDIES OF PORPHYRIN‐C60 DYADS

Abstract Porphyrin‐C60 dyads in which the two chromophores are linked by a bicyclic bridge have been synthesized using the Diels‐Alder reaction. The porphyin singlet lifetimes of both the zinc (Pzn‐C60) and free base (P‐C60) dyads, determined by time‐resolved fluorescence measurements, are ≦17 ps in toluene. This substantial quenching is due to singlet‐singlet energy transfer to C60 The lifetime of Pzn‐1C60 is ‐5 ps in toluene, whereas the singlet lifetime of an appropriate C60 model compound is 1.2 ns. This quenching is attributed to electron transfer to yield Pznbull;+‐C60bull;‐. In toluene, P‐1C60 is unquenched; the lack of electron transfer is due to unfavorable thermodynamics. In this solvent, a transient state with an absorption maximum at 700 ran and a lifetime of‐10 μs was detected using transient absorption methods. This state was quenched by oxygen, and is assigned to the C60 triplet. In the more polar benzonitrile, P‐1C60 underoes photoinduced electron transfer to give P•+‐C60bull;‐. The electron transfer rate constant is −2 × 1011 s−1.