TAUTOMERISM IN URACIL, CYTOSINE AND GUANINE - A COMPARISON OF ELECTRON CORRELATION PREDICTED BY AB-INITIO AND DENSITY-FUNCTIONAL THEORY METHODS

The structures and relative energies of the tautomers of uracil, cytos ine and guanine are predicted using high level ab initio methods and a Kohn-Sham density functional method employing a B-LYP functional. The structures calculated at the MP2 level are in better accord with avai lable microwave data than are those at either the SCF or the B-LYP lev el. For both uracil and guanine highly correlated ab initio wavefuncti ons (MP4) and the density functional treatment (B-LYP) give relative e nergies of the tautomers to within 4 kJ mol(-1), whilst for cytosine t he corresponding value is somewhat larger. In the case of uracil and g uanine, this difference is not sufficient to alter the ordering of the tautomers. For cytosine, where three tautomers are within about 5 kJ mol(-1), the ordering given by the two approaches is different, with t hat from the ab initio method being in agreement with experiment. Thes e results indicate that density functional theory methods, as presentl y used, may not be as accurate as traditional ab initio methods in des cribing tautomeric equilibria.