Pariser-parr-pople calculations on different DNA constituents

Abstract

A semiempirical SCF LCAO MO-CI calculation has been performed for the nucleotide bases. According to the results we obtain a better agreement with the experimental singlet excitation energies, if we use the SCF eigenvectors of the Pople-type matrix in the CI calculation instead of the eigenvectors obtained after the first iteration step. On the basis of some parameter variation a set of integrals has been determined which yields as largest deviation between the experimental and theoretical excitation energies for the first two intensive excitations of the five bases (A, T, G, C and U) 1 eV. The possibilities of further improvements are discussed. In the second part of the calculations the oscillator strength values (f) of the G-C base pair and of the GpG, GpC, CpG and CpC dinucleotides have been determined using the first 16 singlet excited configurations of these composite systems for the CI calculation. The comparison of the results obtained with the appropriatef values of the constituent single bases shows a hypochromicity of the first absorption band system in the case of all the four dinucleotides and a slight hyperchromicity in the case of the G-C base pair.