Some new results in the quantum mechanical investigation of DNa

Abstract

As a first step in testing the possibility of correlation calculations of extended systems using localized orbitals, localization of the canonical Hartree-Fock orbitals of the four nucleotide bases was performed. The results show an excellent localization of the occupied orbitals and a rather good one for the virtuals (both for the IT-and --orbitals). Interaction energy calculations of the stacked nucleotide bases show a few kcaldmol repulsion at the Hartree-Fock level, but they become attractive (again a few kcaldmol) if dispersion energy computed with the aid of London's formula is added. The internal charge transfer of -0.2 e from sugar to the nucleotide base (C, T, or A) found in largescale ab inirio homopolynucleotide calculations raises the question of whether there are free charge carriers in DNA B. Though in the Hartree-Fock level the answer is negative, one cannot rule out this possibility if, using a high-quality basis set, correlated band structures (using an mproved version of the electronic polaron model) will be computed. Interaction energy calculations between a polyglycine chain in different conformation and polynucleotide chains show a few kcaldmol attraction per base and glycine unit and a very small (a fraction of a kcal/mol) additional attraction due to the dispersion correction.