Molecular electrostatic potential of the B‐DNA helix. I. Region of the guanine–cytosine base pair

The electrostatic molecular potential minima around the guanine–cytosine base pair within a B‐DNA minihelix are computed, taking into account the contributions of the sugar‐phosphate backbone and of the adjacent base‐pairs. The calculations are based on ab initio SCF wave functions of the different constituents of the nucleic acid. The results point to significant differences in the potential between the isolated nucleic acid bases or base‐pairs and those within the DNA. Altogether the minima in the G–C regions are strongly enhanced in the minihelix. They benefit from the field created by the neighboring phosphates. From the purely electrostatic viewpoint an ambiguity remains as concerns the relative affinity of N 7 and N 3 of guanine for electrophiles. On the other hand, guanine should altogether be more susceptible than cytosine to such reagents, this ordering concerning also its NH 2 group compared to that of cytosine.