STRUCTURE, ENERGETICS, AND HARMONIC VIBRATIONAL-SPECTRA OF THE ADENINE-THYMINE AND ADENINE-ASTERISK-THYMINE-ASTERISK BASE-PAIRS - GRADIENT NONEMPIRICAL AND SEMIEMPIRICAL STUDY

The structure, interaction energies, and harmonic vibrational spectra of adenine-thymine and adenine*-thymine* (imino-enol tautomers) base pairs were studied by the ab initio and semiempirical methods. Gradient optimization at the SCF/MINI-1 level has shown that the former complex is only about 10 kcal/mol more stable than the latter. We have demonstrated that single-point calculations with a larger basis set performed for the reference geometry optimized in the minimal basis set (SCF/MIDI-1//SCF/MINI-1) and those taking the correlation energy into account can lead to unrealistic interaction energy values. Hence, single-point calculations of this kind should be taken with great care for AT and A*T* complexes. The results obtained by the semiempirical PM3 method agree reasonably well with the ab initio values; this is not true of the AM1 method. In addition to the interaction energy, reorganization energy and basis set superposition error were also determined; both the latter terms (which are positive) are important and should not be neglected. The theoretical interaction enthalpy of the AT formation agrees fairly well with the corresponding experimental value. Knowledge of the harmonic vibrational spectra of isolated A, T, A*, and T* bases and the AT and A*T* base pairs enabled us to determine the frequency and intensity changes induced by the Watson-Crick type of hydrogen bonding, tautomeric transitions, and double proton transfer in the AT base pair.