d(A)3d(T)3 and d(G)3d(C)3 B-DNA mini-helixes: the DFT/M06-2x and DFT/B97-D3 comparison of geometrical and energetic characteristics.

We report the comprehensive DFT based comparison of geometrical and energetic parameters of the d(A)3·d(T)3 and d(G)3·d(C)3 nucleic acid mini-helixes performed at B97-D3 and M06-2× levels of theory. We studied the ability of mini-helixes to retain the conformation of B-DNA in the gas phase and under the influence of water bulk, uncompensated charges, and counter-ions. The def2-SV(P) and 6-31G(d,p) basis sets have been used for B97-D3 and M06-2× calculations, correspondently. To estimate basis set superposition error, the recently developed semi-empirical procedure that calls geometrical counterpoise type correction for inter- and intra-molecular basis set superposition error (gcp) has been used in the case of def2-SV(P) basis set. We found that both considered DFT functionals predict very similar results for geometrical ad energetic characteristics. We also found that in contrast to average classical molecular dynamics and data of simple geometrical models, both considered DFT functionals predict the existence of duplex specific geometries. A prediction of interaction energies of d(A)3d(T)3 and d(G)3d(C)3 duplexes accomplished in this study also verifies the applied models and confirms reliability of the new computational gcp technique.