The primary processes in the OH* radical-induced oxidation mechanism of sulphoxides have been investigated by pulse radiolysis and, in particular, by an improved conductivity detection method with a time resolution of ca. 50 ns in aqueous solution. Electrophilic addition of the OH-radical to the sulphoxide group leads to a transient adduct, R,SO(OH). which decays unimolecularly with tl12 up to 100 ns into a sulphinic acid, RSO,H, and a radical R*.The various RS0,H have been identified by their pK, , and R* (including t-butyl and phenyl radicals) by direct optical measurement or chemical scavenging experiments. The probability of radical split-off from R,SO(OH)* for mixed sulphoxides depends on the stability of the radical leaving. Depending on the nature of the sulphoxide substituents two other OH* radical reactions compete with, and may even predominate over, the addition at the sulphoxide group. Thus hydrogen-atom abstraction readily occurs from longer chain and branched aliphatic groups and in the presence of aromatic substituents OH* radicals add to the n-system to form a hydroxycyclohexadienyl radical. The respective yields, kinetics and some physico-chemical properties of the primary species are presented and discussed.

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and 'C6H5 radicals (from the reaction of *OH with dimethyl and diphenyl sulphoxide) react with p-bentoquinone with k (4.5 f 1 .O) x 107 and (1.2 f 0.2) x 1 O9 dm3 mol-sl, respectively. In both case$ an immediate formation of a transient is observed which could be identified as the unsubstituted semiquinone radical. The reaction mechanism is considered to proceed via addition of the 'CH3 or 'C6H5 radicals to one p-benzoquinone molecule and immediate subsequent reaction of these radical adducts with a second p-benzoquinone molecule. The latter process is likely to be an electron transfer. The pulse radiolysis results are supported by steady-state irradiation product analysis.

The reactions of the one electron adduct of 2'-deoxyadenosine dA(-), in aqueous solutions have been studied using pulse radiolysis techniques with optical absorption and conductivity detection. The dA(-) radical anion itself shows a weak and featureless optical absorption at greater than 300 nm. It reacts rapidly with H20 (k equals 6 x 10(4)M(-1)s(-1); t1/2 equals 210 ns) to yield at least three different protonated structures (dAH., dA'H., dA"H.). In neutral solutions the most important of these (dAH., lambda(max) equals 315 nm) decays by a first order process (t1/2 approximately equal to 9 microseconds). In basic solutions dAH. undergoes on OH- catalysed rearrangement into another neutral radical (dA"'H., lambda(max) equals 355 nm; k(dAH. plus OH-) equals 1.7 x 10(8)M(-1)s(-1)). p-Nitroacetophenone (PNAP) reacts rapidly with the protonated electron adducts of 2'-deoxyadenosine (k equals 5 x 10(9)M(-1)s(-1)). An electron transfer occurs with dAH to yield PNAP and a reoxidized form of 2'-deoxyadenosine. As indicated by its pK value of 8.8 the latter is not, however, simply a repaired 2'-deoxyadenosine molecule, but is suggested to include the elements of water. Species dA"'H.(and dA'H.) react with PNAP in a process which is clearly not an electron transfer but likely an addition reaction.

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