The relative stabilities of dihydropterins; a comment on the structure of Moco, the cofactor of the oxomolybdoenzymes

Ab initio calculations have been performed on 7,8-dihydro-2-aminopteridin-4(3H)-one and on 39 dihydropteridine tautomers of it leading to an assessment of their relative energies and thence the likelihood of their involvement in the operation of the cofactor of the oxomolybdoenzymes, Moco. The effect of a polar environment on the tautomer energetics has been explored using a continuum model. At the most advanced level of calculation, and in the presence of water, the 7,8-dihydro-3H-(T1) and 5,6-dihydro-3H- (T4) tautomers are essentially isoenergetic, with 7,8-dihydro-6H- (T13) and 5,8-dihydro-3H- (T3) tautomers only 8.0 and 15.5 kJ mol-1 more energetic than T1. Any of these four tautomers can reasonably be implicated in the mode of action of Moco. The ortho quinonoid tautomer (T2) is of considerably higher energy and seems much less likely to be implicated.