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Experimentally observed sequence-selective binding of metal ion to DNA oligonucleotides have been compared with variations of electrostatic potential (EP) along the helix. Calculations of EP have been performed for three atomic models of the oligonucleotide duplex [d(CGCGAATTCGCG)2] using several variants of EP calculations, including a solution of non-linear Poisson-Boltzmann equation (NPBE). N7 atom of guanine adjacent to adenine base was identified as a region with the most negative electrostatic potential in the major groove. The EP value for the Me ion binding site surpasses the value for N7 of other guanines by 10-26% depending on particular duplex conformation. Qualitatively, the sequence dependent variations of EP near guanine N7 atoms are in agreement with the sequence-selective behavior of Mn(II) and Zn(II) ions as revealed by NMR experiments. But the difference in EP between the two most negative regions near guanine N7 atoms does not exceed 1.25 kT/e. Simple model suggests that metal ions are capable to form ion-hydrate complexes with G-Pu steps of DNA duplex. These complexes are formed via one Me...G and five Me...water coordination bonds with water molecules hydrogen bonded to two adjacent purine bases in the same chain. We suppose that such a stereospecific structural possibility is the main factor which control the sequence-selectivity in the metal ion binding. A combination of both mechanisms allows to explain sequence specific Mn(II) and Zn(II) binding to a set of oligonucleotides.

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