Photochemical (8→8) coupling of purine nucleosides to guanosine
Abstract
IN comparison with the pyrimidine bases of nucleic acids, the purines are relatively inert to photochemical alteration. No photoproducts derived specifically from adenine or guanine have been isolated from ultraviolet-irradiated DNA or RNA. However, purine bases and nucleosides can be substituted at C8 by free radical species generated photochemically from simple alcohols, amines, and ethers, and these reactions may be relevant to the radiation-induced cross-linking of proteins, and other biological molecules, to nucleic acids1. We report here that guanosine is substituted in a similar manner by the free radicals produced on photolysis of 8-bromopurine nucleosides to give compounds in which two purine nucleoside moieties are coupled together through their respective C8 positions.