AB-INITIO STUDY OF THE STRUCTURE OF GUANINE-CYTOSINE BASE-PAIR CONFORMERS IN GAS-PHASE AND POLAR-SOLVENTS

The structure and energetics of the base pairing and proton transfer i n the Watson-Crick type of guanine-cytosine base pair are studied by t he HF/MINI-1 method within the full gradient optimization and self-con sistent reaction field techniques. Two minima and the transition state on the connecting minimum energy path were located on the potential e nergy surface (PES) of this complex. The minima involved the canonical amino-keto base pair (GC) and the 4-iminocytosine-6-hydroxyguanine ba se pair (GC*), which the latter being formed from GC by a simultaneou s transfer of two protons in neighbouring hydrogen bonds. A polar envi ronment was found to stabilize the canonical structure by 1.3 kcal mol (-1) with respect to the rare tautomer GC*. The GC pair represents th e global minimum on the HF/MINI-1 PES in both gas phase and polar envi ronment. The polar environment was also found to decrease the interact ion enthalpy of the GC base pair and to influence significantly the ge ometry of the hydrogen bonds. The tautomeric origin of spontaneous poi nt mutations and the recent observation of the different rotation rate s about the cytosine and guanine amino bonds in the GC base pair are d iscussed.