No abstract

With the technique of pulse radiolysis we have measured the redox midpoint potentials of the tryptophan side chain neutral indolyl radical (1.05 +/- 0.01 V vs NHE, pH 7.0 and 25 degrees C) and the tyrosine side chain neutral phenoxy radical (0.94 +/- 0.01 vs NHE, pH 7.0 and 25 degrees C). These potentials were obtained by using a variety of inorganic reference compounds in both kinetic and equilibrium protocols. We compare these results with others already in the literature, and we also present data useful in establishing a pulse radiolysis redox reference scale over the range 0.42-1.28 V.

The reduction of the disulfide zrazii-4,5-dihydroxy-l,2-dithiane (oxidized dithiothreitol, ox-DTT) by the radicals derived from methanol, ethanol, 2-propanol, and formate has been investigated by pulse radiolysis and steady-state "Co -radiolysis. By following pulse radiolytically the rate of formation of the disulfide radical anion, ox-DTT", in N20-saturated aqueous solutions of ox-DTT (5 X 10"1 234 mol dm"3) in the presence of an excess of the radical-forming substrate (e.g., 0.5 mol dm'3 methanol) at different pH values, it was found that essentially only the anions of the -hydroxyalkyl radical reduce ox-DTT to ox-DTT" (e.g., ¿(*CH20") = 2.3 X 108 dm3 mol'1 s'1 at 25 °C) while the neutral forms react with ox-DTT with rate constants < 106 dm3 mol'1 s'1. The temperature dependence of the rate of ox-DTT" formation by the radical anions has been measured. With all systems studied, Arrhenius plots are strongly curved which indicates that the redox process cannot be adequately described by a simple one-step mechanism. It is suggested that the alcohol radical anions react by reversible addition to form an adduct radical with ox-DTT followed by decomposition into ox-DTT" and carbonyl compounds (k 1.3 X 106 s'1 in the case of the 2-propanol radical anion, at 20 °C). With "Co -radiolysis at pH 4 where the alcohol radical anions no longer play a role (e.g., p.8Ta(*CH2OH) = 10.7) reduction of ox-DTT also occurs. The estimated overall rate constant of the reaction of the -hydroxyalkyl radicals with ox-DTT is on the order of 105 dm3 mol'1 s'1.

The polynucleotides poly(U), poly(C), poly(A) and poly(G) have been y-irradiated in N2O- and N2O/O2 (4:1)-saturated aqueous solutions. Hydroxyl radicals from the radiolysis of water react with the polynucleotides thereby producing among other lesions strand breaks. Strand breakage is connected with the formation of phosphomonoester end groups. Such end groups have been determined by measuring inorganic phosphate after a three hour incubation at 37 °C with acid or alkaline phosphatase. In the absence of oxygen G(phosphomonoester end groups) (in units of μmol J-1) are 0.47 (poly(U)), 0.17 (poly(C)) and ≤ 0.04 (poly(A) and poly(G)). In the case of poly(U) and poly(C) on heating the sample for one hour at 95 °C prior to incubation with phosphatases the above values increased by 0.14 and 0.07 μmol J-1, resp., whereas such treatment of the purine polynucleotides still did not produce a measurable yield of phosphomonoester end groups. Comparing these values with G values for strand breakage taken from the literature, about two phosphomonoester end groups are formed per strand break in poly(U) while for poly(C) this ratio is about unity. The purine polynucleotides show very low yields of strand breakage in agreement with the negligible phosphomonoester yields. In the presence of oxygen G(phosphomonoester end groups) are 0.46 (poly(U)), 0.21 (poly(C)), and ≤ 0.04 (poly(A) and poly(G)). On heating, these values increase, most markedly for poly(U) and poly(C). This is possibly linked to the decomposition of unstable hydroperoxides which are formed in high yields in poly(U) and poly(C) (G = 0.7 and 0.19 μmol J-1 resp.). It is known that at least in the case of poly(U), base radicals attack a sugar moiety and are the main precursors of these lesions. G(phosphomonoester end groups) are considerably lower in the case of the purine polynucleotides. Whether this is due to an inability of the base radicals to attack a sugar moiety or has other reasons must remain an open question.