A MOLECULAR-DYNAMICS STUDY OF CONFORMATIONAL-CHANGES AND HYDRATION OF LEFT-HANDED D (CGCGCGCGCGCG)2 IN A NONSALT SOLUTION

Twelve dinucleotides (one complete turn) of left-handed, flexible, double-helix poly(dG-dC) Z-DNA have been simulated in aqueous solution with K+ counterions for 70 ps. Most of the d(GpC) phosphates have rotated in accordance with a Z(I) --> Z(II) transition. The Z(II) conformation was probably partly stabilized by counterions, which coordinate one of the anionic oxygens and the guanine-N7 of the next (5'--> 3' direction) base. The presence of base-coordinating ions close to the helical axis rotated and pulled about half of the d (CpG) phosphates further into the groove. These ions also gave rise to rather large deviations from the crystal structure (Z(I)) with their tendency of pulling the bases closer toward the helical axis. A flipping of the orientation about the glycosyl bond from the +sc to the -sc region was observed for one guanosine, also leading to deviations from the crystal structure. Many bridges containing one or two water molecules were found, with a dominance for the latter. They essentially formed a network of intra- and interstrand bridges between anionic and esterified phosphate oxygens. A "spine" of water molecules could be distinguished as a dark zig-zag pattern in the water density map. The lifetime of a bridge containing one water was about twice as long as that of a two-water bridge and it lasted 5-15 times longer than a hydrogen bond in water. The lifetimes were also calculated for a selection of bridge types, in order of decreasing stability: O1P/O2P ... W ... O4' much greater than O1P/O2P ... W ... guanine-N2 > O1P/O2P ... W ... O1P/O2P. The reorientational motion of water molecules in the first hydration shell around selected groups was slowed down considerably compared to bulk water and the decreasing order of correlation times was guanine-N2 > O4' > O3'/O5' > O1P/O2P.