UV photoelectron and theoretical characterization of 2'-deoxyguanosine-5'-phosphate valence electronic properties: changes in structure associated with the B to Z-DNA conformational transition.
He(I) UV photoelectron spectroscopy and ab initio SCF molecular orbital calculations with the 4-31G basis set have been employed to characterize the valence electronic structures of 2'-deoxyguanosine-5'-phosphate (5'-dGMP-). In 5'-dGMP-, the electron distributions of the upper occupied orbitals are localized and similar to those appearing in 1,9-dimethylguanine (1), 3-hydroxytetrahydrofuran (2) and CH3HPO4- (3). Theoretical ionization potentials (IP's) of 5'-dGMP- (4) have been obtained by applying Koopmans' Theorem to the 4-31G SCF results. The IP's of seven orbitals in the base and sugar groups in 4, predicted from the 4-31G SCF calculations, have been individually corrected by comparison to results from 4-31G SCF calculations on neutral 5'-dGMP, and to Hel photoelectron spectra of the model compounds, 1 and 2. The IP's of six of the highest occupied orbitals of the phosphate group in 4 and in the model anion 3, predicted from 4-31G SCF calculations, have been corrected by comparing 4-31G SCF results for PO2- to theoretical IP's obtained from second-order Møller-Plesset perturbation calculations on PO2-. For 4 in the conformation occurring in B-DNA, the first IP's associated with the phosphate, base, and sugar groups occur at 5.1, 5.6 and 6.6 eV, respectively. A comparison of the valence electronic structures of 4 in geometries associated with the B and Z-DNA conformations indicates that in B-DNA the base and sugar orbitals have lower IP's than in Z-DNA, while the phosphate orbitals have higher IP's.