Mechanistic studies on the formation of deoxyribonolactone by UV irradiation of hexamer d(GCl(Br)UGC)(2) (ODN 2a) containing 1'-deuterio-2'-deoxyadenosine 1 was examined. The kinetic isotope effect (k(H)/k(D)) for the formation of Cl' oxidation product 3 was 1.7 +/- 0.1. ESI-MS of d(pUGC) (7) obtained from the photoreaction of ODN 2a indicated that V deuterium was abstracted by uracilyl-5-yl radical and incorporated into 7 in 96% yield. (C) 2002 Published, by Elsevier Science Ltd.