Studies of Hydrogen-Bonded Systems. III. Potential-Energy Surface, Tunneling, and Tautomeric Equilibria in the N–H···N and O···H–N Bonds of the Guanine—Cytosine Base Pair

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Abstract
An earlier study of the authors on hydrogen bonding in the guanine—cytosine complex is extended so that the σ electrons of all three connecting hydrogen bonds are considered explicitly. This extension permits a discussion of possible simultaneous tautomeric rearrangements involving more than one proton. Calculations are presented for 74 protonic configurations characterizing the simultaneous rearrangement of the N–H···N and one of the N–H···O bonds. The results indicate four relative minima corresponding to different metastable tautomeric forms. The equilibrium distribution and tunneling between tautomers is examined, and leads to a predicted mechanism for the rearrangement. Equilibration among the tautomers is estimated to have a time constant of 3.8×10−5 sec.