Reactions of photolytically generated 'OH and SO, ' radicals with uridine, cytidine, 2'-deoxyuridine (dU) 2'-deoxycytidine (dC), and thymidine have been studied by e.s.r. spectroscopy under anoxic conditions. In the experiments with 'OH, the spectra of the uracil compounds were dominated by the signals of radicals originating from 'OH addition at the alkenic double bond of the nucleobase. No spectra were observed for the cytosine derivatives and thymidine. With SO, ' , base radicals were generated from the deoxyribonucleosides [C( 5) -OH-6-yl from dU, C(6) -OH-5-yl from thymidine, and a nitrogen-centred radical from dC] whereas the ribonucleosides lead t o t w o different types of sugar radical. One of them is derived from the 2'-hydroxyalkyl radical by heterolytic elimination of the nucleobase and the other is the 3'hydroxyalkyl radical which undergoes ring-opening by heterolytic cleavage of the C(LF')-oxygen bond at neutral and alkaline pH. Both the 'OH and SO; ' radicals add to the base moieties in the primary step. The adduct radicals formed with 'OH from uridine and dU are stable o n the millisecond time-scale of the e.s.r. experiment whereas the sulphate adducts are too short-lived to be detected b y e.s.r. In the deoxyribose derivatives they either hydrolyse (dU and thymidine) or eliminate SO:and a proton (dC) whereas in the ribonucleosides they induce intramolecular H abstraction from positions 2' and 3' of the sugar resi d ues.

Using time resolved ESR spectroscopy and photoflash conductivity the uncatalysed, first order decay of 2-hydroxy-propyl-2-peroxyl radicals and the uncatalysed, first order generation of O2·̅ and H+ were measured to have the same rate constants. The formation of O2·̅ and H+ was measured in H2O and D2O and a kinetic isotope effect of kH/k =D3.5 was obtained. Comparing the rate constants of the peroxyl radicals derived from methanol, ethanol and 2-propanol it was shown that the rate constant increases with increasing methyl-substitution. In 2-propanol water mixtures the rate constant of the 2-hydroxy-propyl-2-peroxyl radical increases only slightly with increasing polarity of the solvent. The experimental results are in accord with a mechanism which involves a cyclic transition state leading to a concerted elimination of HO2· followed by dissociation into H+ and O2·̅.

Radicals obtained by reaction of photolytically generated SO,-' with 1 -methyluracil (1 ), 1,3-dimethyluracil (2), 1 -methylthymine (3), 1,3-dimethylthymine (4), and 1,3,6-trimethyluracil (5) were studied by e.s.r. spectroscopy in aqueous solutions. In situ photolysis of neutral and acidic solutions containing (1 ) and persulphate in low concentrations ( 3 m ~) resulted in the e.s.r. spectrum of the C(5)-OH adduct radical. E.s.r. spectra obtained from ( 1) and ( 2) with high persulphate concentrations ( 3 0 m ~) were assigned t o the C(6)-OH adduct radicals. It is proposed that this change in radical population is due to a persulphate-induced chain reaction which results in selective oxidation of the C(5) -OH radicals and simultaneous accumulation of the C(6) -OH adducts generated in side reactions. When the persulphate concentration was raised t o 6 0 m ~, 5-0x0-6-yl radicals were formed in secondary processes from (1) and ( 2) besides the OH adducts. In contrast to these results the thymines (3) and (4) yielded only C(6)-OH adduct radicals. Addition of phosphate dianions t o the photolysis solutions containing persulphate and the pyrimidine bases (1)-(4) resulted in the e.s.r. spectra of the C(6)-phosphate adduct radicals (pH 6.5-9.5).Identical spectra were obtained by reaction of (1)-( 4) with HPO,-' radical anions generated by photolysis of Li , P, O, .The results of the experiments with I ,3,6-trimethyluracil (5) were completely different. First, reaction of SO,-' with (5), even at l o w persulphate concentration ( 3 m ~) ,

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α-Alkoxyalkyl radicals with a leaving group L = Cl or OCOCH3 in β-position are produced by H-abstraction from the corresponding saturated substrates by ·OH, SO·4- or (CH3)3CO· radicals. From ESR spectroscopic observations it is concluded that in aqueous solution at pH 5 -9 the following fast hydrolysis reactions take place: The rate constants of these reactions and for the hydrolysis of CH3O-ĊH-CH2Cl are k ≥ 106 s-1, whereas the rate constant for CH3O-ĊH-CH2OCOCH3 was determined to be ≈ 2 × 103 s-1 at room temperature. The radicals with L = Cl cannot be scavenged by O2 which fact leads to a value of k ≥ 2 × 10-7 s-1. α-Alkoxyalkene radical cations are assumed as intermediates in the hydrolysis reactions. The radicals with L = OCOCH3 and the radical CH3O--ĊH-CH2Cl are observable in acetone solution ESR spectroscopically. In aqueous solution at pH below 3 proton catalyzed reactions are observed by ESR spectroscopy: Radicals resulting from H-abstraction at the CH3O-groups of the substrates or at the 5-positions of the cyclic ethers are also observed. The ESR parameters and the pH-ranges of existence of the above radicals are given. Support of the reported reactions comes from quantitative analysis of stable products such as H+, Cl- or CH3OH after 60Co-γ-irradiation of N2O saturated aqueous solutions of the substrates. The behaviour of the radicals is used as a model to describe a modified version of the degradation of DNA-4′ radicals in aqueous solution in the absence of oxygen.