Analysis of the reaction between 2'-deoxycytidine and 4-oxo-2-nonenal by LC/MS revealed the presence of three major products (adducts A(1), A(2), and B; [M + H](+) = 364). Adducts A(1) and A(2) were isomeric, and each dehydrated to form adduct B. The structure of adduct B was shown by LC/MS and NMR spectroscopy to be an etheno-2'-deoxycytidine adduct 1' '-[1-(2'-deoxy-beta-d-erythro-pentofuranosyl)-1H-imidazo[2,1-c]pyrimidin-2-oxo-4-yl]heptane-2' '-one. A time course experiment performed at 65 degrees C (pH 5-8) showed that the transformation of both A(1) and A(2) was pH-dependent. In acidic conditions, adducts A(1) and A(2) dehydrated primarily to adduct B. In contrast, in basic conditions, adducts A(1) and A(2) hydrolyzed primarily to dCyd. The data are consistent with adducts A(1) and A(2) being substituted ethano adducts that dehydrate to adduct B, a substituted 3,N(4)-etheno-2'-deoxycytidine adduct.