2-mercaptoethanol thiyl radical

One further aspect besides H-atom abstraction must, however, be considered in the reactions of thiyl radicals with PUFAs, namely, the possibility of concurrent RS additions to the double bonds. By analyzing a particular system in which the thiyl radical from mercaptoethanol had reacted with linolenic acid, Schoneich at came to the conclusion that the abstraction of bisallylic hydrogen and thiyl addition occur with comparable rates. The C-centered adduct radical formed in the addition reaction is, of course, prone for repair in the presence of thiols, i.e., will subsequently regenerate thiyl radicals via the back reaction of equilibrium 14. These, in turn, will re-enter into the abstraction / addition competition cycle and eventually all of the thiyl radicals will appear to have reacted via the abstraction route. Experimentally, overall efficiencies of up to 85 % have been measured and the difference to the limiting 100% efficiency may be accounted for by side and termination processes. But looking into the details, the situation is, in fact, even more complex. As shown by Schwinn at al., there is still another process which readily occurs within the RS -adducts to the double bonds, namely, cis-trans isomerization. Quantification of this is of imminent interest for the biological community. 

Finke has established a chemical precedent for the proposed mechanism for thiyl radical formation in the Bi2-dependent ribonucleotide reductase. Thermolysis of AdoCbl with excess /3-mercaptoethanol under anaerobic conditions yielded 90% Co—C homolysis and 10% heterolysis, as determined by product characterization. The homolysis products were 5 -deoxyadenosine, cob(II)alamin, and the disulfide 2,2 -dithiodiethanol. Kinetic studies established a zero-order dependence on thiol at high [RSH], consistent with rate-limiting Co—C homolysis and formation of a discrete Ado- that subsequently abstracts an H atom from the thiol. Consequently, the... 

It was suggested that dimerization of thiyl radicals to give disulphide occurred mainly on dissolution of the irradiated solid. In the case of cysteamine hydrochloride, where there is no carbonyl group to trap the electron prior to deamination, it was found that (7(NH3)<0 1. In spite of this G(H2S) was only 1-2, lower than for cysteine, but the value of (Ha) of 5-1 was much higher. This is the same situation as was found for mercaptoethanol but not for cysteamine itself in aqueous solution. 

Tautomerism of the thiyl radical from 2-mercaptoethanol, forming a carbon-centred radical, similarly occurs on the millisecond timescale [97]. While such rearrangements can be rapidly locked in , for example by oxygen addition, they are probably too slow to be biologically important (see below. Section 6.2). 

The observed rate laws for the oxidation of mercaptoethanol by methylene blue under different reaction conditions are consistent with the steady-state rate laws derived from the proposed mechanism. If step 2 of the reaction sequence, namely formation of the disulfide radical anion from a thiyl radical and a sulfide ion, is the rate limiting step of the chain sequence, and therefore only termination by 6 (coupling of thiyl radicals) occurs, the derived rate law for the reaction is Eq. 11. This rate law, which takes.into account the distribution of the thiol and sulfide as determined by the of the thiol and the acidity of the medium, predicts that the observed rate law would be half order in MB and three halves order in mercaptoethanol. This is the rate law expected, however, only if the concentration of MB " is sufficiently high so that the second term in the denominator is neglible. At pH s below the pH maximum, and at a sufficiently... 

Rate constants on the order of several 10 M s and 10 -10 s for the forward and back reactions, respectively, yield equilibrium constants of lO -lO" M [39-41], Although this indicates reasonable stability for the thioperoxyl radical it may nevertheless easily escape detection. In air saturated solutions ([O2] ca. 2.5 X lO M), for example, formation of thioperoxyl radicals occurs with r,/2 ca. 1 ps, which is of the same magnitude as the halflife of its decay. This is of particular significance when a distinction between the reactions of RS and RSOO is at stake, and wrong assignments may be made unless absolute kinetic and thermodynamic data are known and taken into account. Two of the still small number of systems for which accurate measurements are available deal with the respective thiyl and thioperoxyl radicals of mercaptoethanol and glutathione. At least in case of the mercaptoethanol derived thioperoxyl radical, investigations have been facilitated by a weak but distinct optical absorption of RSOO with near 550 nm (c ca. 200-400 M cm ) [37,42]. 

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