UV PHOTOELECTRON AND ABINITIO QUANTUM-MECHANICAL CHARACTERIZATION OF NUCLEOTIDES - THE VALENCE ELECTRONIC-STRUCTURES OF 2'-DEOXYCYTIDINE-5'-PHOSPHATE

He 1 UV photoelectron spectroscopy and ab initio molecular orbital calculations with the 4-31G basis set have been employed to characterize the valence electronic structures of neutral and anionic 2'-deoxycytidine-5'-phosphate (5'-dCMP). Theoretical ionization potentials (IP's) of neutral molecules and of anions have been obtained by applying Koopmans' theorem to ab initio SCF results. The ionization potentials predicted from the SCF calculations have been compared to He I photoelectron spectra of the neutral model compounds, 1-methylcytosine, tetrahydrofuran, cyclopentanol, and the trialkylphosphate esters, trimethyl, triethyl, and tributylphosphate. The SCF calculations predict a value (8.94 eV), for the highest occupied w orbital in 1-methylcytosine, which agrees well with the experimental vertical IP (8.65 eV). However, IP's for the highest occupied lone-pair orbitals in tetrahydrofuran, cyclopentanol, and the trialkylphosphates are predicted to be higher than the experimental IP's by 1.51,0.84, and 1.18-1.41 eV, respectively. Ionization potentials obtained from SCF calculations on the model anions CH3O-, PO2-, and PO3- have been compared with experimental adiabatic IP's. The results of the SCF calculations for these model anions and for H2PO4- have also been compared with results from third-order Moller-Plesset perturbation calculations. The comparison of 4-31G SCF results with experimental data and with results from post-SCF calculations on the model compounds and anions permits an individual orbital evaluation of the accuracy of IP's calculated for neutral and anionic 5'-dCMP. In both the neutral and the anion, the electron distributions for all of the upper occupied orbitals are localized and similar to those appearing in 1-methylcytosine, 2'-deoxyribose, and methyl phosphate or H2PO4-. In neutral 5'-dCMP, the first and second to fourth highest occupied molecular orbitals, with IP's of 8.7 and 9.4-9.7 eV, respectively, are orbitals located on the base. The fifth to eighth highest occupied orbitals, with IP's of 9.7, 10.2, 10.2-11.1, and 11.1 eV, respectively, are located on the sugar group. The ninth and tenth highest occupied orbitals, both of which have IP's in the range 11.2-11.3 eV, are associated with the phosphate group. In anionic 5'-dCMP, the first ionization potentials associated with the phosphate, base, and sugar groups occur at 4.6, 6.1, and 6.8 eV, respectively. The present description of orbital structure in anionic 5'-dCMP suggests that in electrophilic attack of nucleotides, product distributions of electrostatically controlled reactions will be different from product distributions of orbitally controlled reactions. A comparison of DNA and RNA alkylation patterns in methylation and ethylation reactions supports this conclusion. © 1990, American Chemical Society. All rights reserved.