Structural principles of B-DNA grooves hydration in fibers as revealed by Monte Carlo simulations and X-ray diffraction

Being interested in possible effects of sequence-dependent hydration of B-DNA with mixed sequence in fibers, we performed a series of Monte Carlo calculations of hydration of polydeoxyribonucleotides in B form, considering all sequences with dinucleotide repeat. The computational results allow the ten base-stacking types to be classified in accordance with their primary hydration in the minor groove. As a rule, the minor groove is occupied by two water molecules per base pair in the depth of the groove, which are located nearly midway between the planes of successive base pairs and symmetrically according to the dyad there. The primary hydration of the major groove depends on the type of the given base pair. The coordinates of 3 water molecules per base pair in the depth of the major groove are determined by the type of this pair together with its position and orientation in the helix, and are practically independent on the adjacent base pairs. A/T-homopolymer tracts do not fit into this hydration pattern; the base pair edges are hydrated autonomously in both grooves. Analysis of the Li-B-DNA x-ray diffraction intensities reveals those two water positions in the minor groove. In the major groove, no electronic density peaks in sufficient distance from the base edges were found, thus confirming the absence of any helical invariance of primary hydration in this region. With the help of the rules proposed in this paper it is possible to position the water molecules of the first hydration shell in the grooves of canonical B-DNA for any given sequence.