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'-deoxyadenosine (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 nm wavelength produces heterolytic cleavage of the halogen-carbon bond by one-photon absorption and formation of the unstable 2-hydroxy tautomer of 2'-deoxyisoguanosine as predominant 'primary' product. The 2-hydroxy tautomer then transforms in 10(-6)-10(-5) s into the stable 2-oxo tautomer in an acid-base-catalysed reaction. A reaction mechanism is proposed and discussed in relation to previous UV low-intensity studies of these halogenodeoxyadenosines.