LAUSR

Abstract It has been shown experimentally that the hydrolytic deamination of cytosine in single-stranded DNA occurs significantly more frequently than that of adenine, despite similarities in their structure. To provide insight into this difference, we compute mechanisms for these reactions at the ω B97X-D/pcseg-2 level of theory, and we include quantum tunneling effects of nuclei. We calculate an activation energy of 117.9 kJ/mol for the deamination of cytosine in the presence of three water molecules, and we calculate 147.1 kJ/mol for the analogous process with adenine. The rate-determining step for both cytosine and adenine deamination is the final hydrogen transfer from a water molecule to the amine which results in the formation of ammonia. Tunneling effects lowered the activation energy by only 1–2 kJ/mol, indicating that tunneling of nuclei does not play a significant role in these processes despite the prevalence of hydrogen-transfer steps.