DFT study of geometrical and vibrational features of a 3',5'-deoxydisugar-monophosphate (dDSMP) DNA model in the presence of counterions and solvent.

The B3LYP/6-31++G* theoretical level was used to study the influence of various hexahydrated monovalent (Li+, Na+, K+) and divalent (Mg2+) metal counterions in interaction with the charged PO2- group, on the geometrical and vibrational characteristics of the DNA fragments of 3',5'-dDSMP, represented by four conformers (g+g+, g+t, g-g- and g-t). All complexes were optimized through two solvation models [the explicit model (6H2O) and the hybrid model (6H2O/Continuum)]. The results obtained established that, in the hybrid model, counterions (Li+, Na+, K+, Mg2+) always remain in the bisector plane of the O1-P-O2 angle. When these counterions are explicitly hydrated, the smallest counterions (Li+, Na+) deviate from the bisector plane, while the largest counterions (K+ and Mg2+) always remain in the same plane. On the other hand, the present calculations reveal that the g+g+ conformer is the most stable in the presence of monovalent counterions, while conformers g+t and g-t are the most stable in the presence of the divalent counterion Mg2+. Finally, the hybrid solvation model seems to be in better agreement with the available crystallographic and spectroscopic (Raman) experiments than the explicit model. Indeed, the six conformational torsions of the C4'-C3'-O3'-PO-2-O5'-C5'-C4' segment of all complexes of the g-g- conformer in 6H2O/Continuum remain similar to the available experimental data of A- and B-DNA forms. The calculated wavenumbers of the g+g+ conformer in the presence of the monovalent counterion and of g-t conformer in presence of the divalent counterion in the hybrid model are in good agreement with the Raman experimental data of A- and B-DNA forms. In addition, the maximum deviation between the calculated wavenumbers in the 6H2O/Continuum for the g+g+ conformer and experimental value measured in an aqueous solution of the DMP-Na+ complex, is <1.07% for the PO2- (asymmetric and symmetric) stretching modes and <2.03% for the O5'-C5' and O3'-C3' stretching modes. Graphical abstract dDSMP-(OO)- Mg2+/6W/Continuum.