UV photoelectron and ab initio quantum mechanical characterization of valence electrons in Na(+)-water-2'-deoxyguanosine 5'-phosphate clusters: electronic influences on DNA alkylation by methylating and ethylating carcinogens.

UV photoelectron data for 1,9-dimethylguanine, 3-hydroxytetrahydrofuran, and water, and results from ab initio self-consistent field (SCF) and post-SCF molecular orbital calculations were employed to describe valence electrons in clusters of 2'-deoxyguanosine 5'-phosphate (5'-dGMP-) with four water molecules and a phosphate-bound sodium ion. Two clusters (A and B) were examined. In A, Na+ is coordinated to 5'-dGMP-. In B, Na+ and 5'-dGMP- are separated by water. In 5'-dGMP- clusters, the nucleotide valence ionization potentials (IPs) are different from those previously reported for isolated 5'-dGMP-. The smallest IP in isolated 5'-dGMP- (4.6 eV) arises from the phosphate group; the smallest IPs in A (8.2 eV) and B (7.9 eV) arise from the base. In the clusters, the IPs associated with the seven upper occupied base orbitals differ from corresponding IPs in 1,9-dimethylguanine by less than 0.4 eV. In A and B, the smaller IP of the base, compared with the phosphate group, is consistent with reactivity data indicating that DNA and RNA are subject to electrophilic SN2 attack by carcinogens such as N-methyl-N-nitrosourea, dimethyl and diethyl sulfate, and methyl and ethyl methanesulfonate, in which more than 75% of nucleotide alkylation occurs at the bases.