Isotope effects redox reactions

As both elements are important environmental contaminants whose mobility and environmental impact depend on redox reactions, we expect that Cr and Se isotope analyses will be used widely as indicators of oxyanion reduction. It may also be possible to trace the origin of the Se or Cr via isotopic signatures, but only if the possible confounding effects of reduction can be constrained. 

Just as in the preceding examples, early indications of tunneling in enzyme-catalyzed reactions depended on the failure of experiments to conform to the traditional expectations for kinetic isotope effects (Chart 3). Table 1 describes experimental determinations of -secondary isotope effects for redox reactions of the cofactors NADH and NAD. The two hydrogenic positions at C4 of NADH are stereochemically distinct and can be labeled individually by synthetic use of enzyme-catalyzed reactions. In reactions where the deuterium label is not transferred (see below), an... 

Isotope effects on redox reactions of the type considered in Section 2.6 are of interest for a number of reasons. At a fundamental level, the magnitude of the effect provides an important clue to the electronic structure and vibrational properties of the species involved. From a practical point of view, a large deviation from unity for the equilibrium constant offers a convenient procedure for the enrichment of the isotopomer mixtures. 

Consistent with the results of this study is the outcome of the oxidation of 4-X-substituted phenols by use of PINO, generated from HPI with Pb(OAc)4 at 25 °C in MeCN containing 1% AcOH . The reactivity (fcn) of PINO towards phenolic O—H bonds (BDE 85-90 kcal moC ) was about one order of magnitude higher than that measured towards the C—H bond of benzyl alcohols (cf. Table 4). A p value of —3.1 was obtained from plotting log kn vs. for this reaction, where removal of H-atom from the phenolic O—H bond (which is weaker than the O—H bond of aliphatic or benzyl alcohols) induces an oxidative phenolic coupling with the PINO moiety. In view of the low redox potential of the substituted phenols (in the 0.8-1.1 V/NHE range), and of the substantial value of the kinetic isotope effect = 3.1-3.7 measured, ... 

The biochemical reduction of sulfate to sulfide by bacteria of the genus Desulfovibrio in anoxic waters is a significant process in terms of the chemistry of natural waters since sulfide participates in precipitation and redox reactions with other elements. Examples of these reactions are discussed later in this paper. It is appropriate now, however, to mention the enrichment of heavy isotopes of sulfur in lakes. Deevey and Nakai (13) observed a dramatic demonstration of the isotope effect in Green Lake, a meromictic lake near Syracuse, N. Y. Because the sulfur cycle in such a lake cannot be completed, depletion of 32S04, with respect to 34S04, continues without interruption, and 32S sulfide is never returned to the sulfate reservoir in the monimolimnion. Deevey and Nakai compared the lake to a reflux system. H2S-enriched 32S diffuses to the surface waters and is washed out of the lake, leaving a sulfur reservoir depleted in 32S. The result is an 34S value of +57.5% in the monimolimnion. 

Although at pH 8 the electron distribution favours the formation of flavin semiquinone and reduced iron-sulfur center, the magnetic moments of the two redox centers do not interact. At pH 10, however, 2-electron-reduced TMADH exhibits the EPR spectrum diagnostic of the spin-mteracting state. In a more detailed analysis using the pH-jump technique, the interconversion of three states of TMADH [state 1, dihy-droflavin-oxidised 4Fe-4S center (formed at pH 6) state 2, flavin semi-quinone-reduced 4Fe-4S center (formed at pH 8) state 3, spin interacting state (formed at pH 10)] were studied in both H2O and D2O (Rohlfs et al., 1995). The kinetics were found to be consistent with a reaction mechanism that involves sequential protonation/deprotonation and electron transfer events (Figure 6). Normal solvent kinetic isotope effects were observed and proton inventory analysis revealed that at least one proton is involved in the reaction between pH 6 and 8 and at least two protons are involved between pH 8 and 10. At least three protonation/... 

Lastly, electron transfer in D—[H]—A assemblies is not a perquisite of the excited states of metal complexes. Organic ensembles 38 and 39 (R = SiMe2 Bu), containing a dimethylaniline-anthracene redox pair, have been synthesized recently [124]. Preliminary time-resolved and steady-state fluorescence experiments indicate the occurrence of photoinduced electron transfer. In work related to Watson Crick base-paired systems, the excited state of the fluorescent pyrene derivative 40 is efficiently quenched (94-99 %) by 2 -deoxyguanosine (dG), 2 -deoxycytidine (dC), or 2 -deoxythymidine (dT) in aqueous solution [125]. A PCET mechanism is thought to be responsible for this process, as the thermodynamics of electron transfer are unfavorable unless coupled to a rapid proton-transfer step. The quenched lifetime of 40 in the presence of dC and dT in H2O is significantly extended by a factor of 1.5-2.0 in D2O this isotope effect is similar to that observed in the kinetics studies of 1 [70]. The invoked PCET reaction mechanism also accounts for the inability of dC and dT to quench the fluorescence of 40 in the aprotic organic solvent DMSO. 

However, with respect to oxidation mechanism a distinction can be made between hydrogen and electron transfer on the basis of the kinetic isotope effect for the rate of oxidation , which is expected to be large (fe/ D > 1-5) for k and small (A h/A d = 1-5) for k2-The redox reactions 1-3 may be reversible or proceed predominantly in one direction. Equilibrium reactions will be discussed in Section in.E. In what follows, reactions wUl be discussed that proceed essentially to completion. 

Nuclear tunneling is potentially a significant consideration in outer-sphere radical electron transfer reactions. The case of reduction of NO2 to NO2 is notable in that nuclear tunneling is predicted to increase the self-exchange rate constant by a factor of 79 relative to the classical value.75 Kinetic isotope effect measurements could provide experimental evidence for nuclear tunneling. 180/160 KIE measurements have indeed provided evidence for nuclear tunneling in reactions involving the O2/O2 redox couple.76... 

Phase 1 [identified as step 1 in Eq. (7)] is complete within the mixing time and gives a compound retaining absorbance at 340 nm, but with quenched NADH and protein fluorescence. Phase 2 is a first-order process in which absorbance at 340 nm is destroyed, protein fluorescence appears, and NADH fluorescence remains quenched. If this first-order process were after the redox step [step 4 in Eq. (7) ], then the first turnover of NADH would be very fast. This is not observed. If the first-order process were the redox step itself [step 3 in Eq. (7)] then it would be slower with NADD than with NADH by a factor of 6 to 7, as with alcohol dehydrogenase (293). No appreciable isotope effect is measured. Thus the first-order phase must be identified with an isomerization of the ternary complex with NADH [step 2 in Eq. (7)] before the redox step (269,279). Siidi (293a) has also observed two phases in the reverse reaction and has deduced the on rate for pyruvate. The kinetics do not indicate the... 

Once intrinsic isotope effects are determined, one is in a position to deduce transition state stmcture, just as the physical organic chemist does for nonen-zymic reactions. Unfortunately, in many cases workers have assumed, rather than proved, that commitments are zero and intrinsic isotope effects were being looked at. Transition state structures have been investigated in the formate and liver alcohol dehydrogenase reactions as the redox potential of the nucleotide substrate was changed (103, 118). Primary deuterium and C, secondary deuterium, and for formate dehydrogenase 0 isotope effects were determined. In both cases the transition states appeared to be late with NAD and to become earlier as the redox potential of the nucleotide became more positive. So far the conclusions from such studies have been qualitative in nature, and there is room for much more work on these systems. 

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