3-(2'-DeOXY-beta-D-erythro-pentofuranosyl)pyrimido[1,2-alpha]purin-10(3H )-one (M(1)dG) is the major product of the reaction of cleoxyguanosine with malondialdehyde (MDA). M(1)dG blocks replication by DNA polymerases in vitro and is mutagenic in vivo. M(1)dG reacts with hydroxide to form the N-2-(3-oxo-1-propenyl)deoxyguanosine anion (N(2)POPdG(-)). This reaction is pH-dependent and reverses under neutral and acidic conditions to form M1dG. Here we describe the kinetics and mechanism of the ring-closure reaction in both the nucleoside and oligonucleotides. Kinetic analysis of absorbance and fluorescence changes demonstrates that ring-closure is biphasic, leading to the rapid formation of an intermediate that slowly converts to M1dG in a general-acid-catalyzed reaction. The dependence of the rate of the rapid phase on pH reveals the pK(a) for protonated N(2)OPdG is 6.9. One-dimensional H-1 NMR and DQF-COSY experiments identified two distinct intermediates, N(2)OPdG-H and 8-hydroxy-6,7-propenodeoxyguanosine (HO-Pr-ene-dG) that are formed upon acidification of N(2)OPdG(-). Characterization of ring-closure in single-stranded and in melted duplex oligonucleotides shows M(1)dG formation is also acid-catalyzed in single-stranded oligonucleotides and that the denaturation of an oligonucleotide duplex enhances ring-closure. This work details the complexity of ring-closure in the nucleoside and oligonucleotides and provides new insight into the role of duplex DNA in catalyzing ring-opening and ring-closing of M(1)dG and N(2)OPdG.