Nonempirical ab initio calculations on DNA base pairs Original Research Article

28 H-bonded DNA base pairs formed by guanine, adenine, thymine and cytosine were optimized by the gradient method at the ab initio HF/MINI-1 level. 10 pairs were found to be nonplanar with propeller structure. Harmonic vibration analysis indicates that all the stationary points found correspond to energy minima. The calculated interaction enthalpies (sum of the SCF interaction energy, the dispersion energy and the zero-point energy) are larger than the respective experimental data due to overestimation of the MINI-1 SCF interaction energy. Buckling and propeller vibrations were found to be the lowest for all the pairs; it is therefore possible to expect that all the pairs are flexible toward buckle and propeller vibrations. The H-bonded structure of the cytosine dimer was found to be considerably more stable than the stacked structures; both structures were studied at the higher MP2/6-31G∗ level. The reliability of the procedure used (MP2/6-31G∗) was tested for model clusters (formamide … formamidine, benzene … Ar) at higher levels (MP4, CCSD(T)).