Photochemical conversion of oligonucleotides containing 2-chloro-2'-deoxyadenosine: enhanced UV sensitivity of a modified purine base induced by the nearest neighbor.

Irradiation of alternating dodecamers containing 2-chloro-2'-deoxyadenosine (Cl2dAdo) with ultraviolet light (254 nm) has been investigated. The photoconversion of Cl2dAdo into 2'-deoxyisoguanosine (isodGuo) and the completeness of the reaction were studied by reversed-phase HPLC and UV absorption spectra. The photosensitivity of Cl2dAdo within an oligonucleotide was found to be sequence-specific and depends on the nearest neighbor; 2'-deoxyguanosine residues accelerate the rate of photoconversion by a factor of 3.