Reaction mechanism in the photochemistry of the antileukaemic agents 2-chloro- and 2-bromo-2 '-deoxyadenosine, studied by nanosecond laser flash photolysis

The reaction mechanism in the UV photochemistry of 2-chloro-2'-deoxyadenosi ne (Cladibrine) and 2-bromo-2'-deoxyadenosine in aqueous solution has been studied by laser photolysis at nanosecond time resolution. It is found that excitation at 266 nn wavelength produces heterolytic cleavage of the halog en-carbon bond by one-photon absorption and formation of the unstable 2-hyd roxy tautomer of 2'-deoxy-isoguanosine as predominant 'primary' product. Th e 2-hydroxy tautomer then transforms in 10(-6)-10(-5) s into the stable 2-o xo tautomer in an acid-base-catalysed reaction. A reaction mechanism is pro posed and discussed in relation to previous UV low-intensity studies of the se halogenodeoxyadenosines. (C) 1999 Elsevier Science S.A. All rights reser ved.