In this paper, we aim to determine whether the N7-methylation can
influence the base pairing properties of guanine by promoting the
formation of guanine enol-tautomers. The keto-to-enol-tautomerization of
N7-methylguanine (N7mG) and its base pairing patterns with all the
canonical DNA bases have been investigated at the M06-2X/6-311+G(d,p)
level of density functional theory. The barrier free energy calculations
reveal that N7-methylation does not promote the keto- to
enol-tautomerization of guanine. The Watson-Crick-like enol-N7mG:T1 or
enol-N7mG:T2 base pair similar to what is observed experimentally is
found to be energetically more stable than the keto-N7mG:T base pairs.
However, the keto-N7mG:C1 which is structurally similar to the canonical
G:C base pair is the most stable base pair among all the base pairs
studied here. Thus, our calculations predict that N7mG would pair
preferably with cytosine during DNA replication but there is also a
probability that it can cause mutation through mispairing with thymine,
in agreement with experimental observations.