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-aminopteri din-4(3H)-one and on 39 dihydropteridine tautomers of it leading to an assessment of their relative energies and thence the likelihood of th eir involvement in the operation of the cofactor of the oxomolybdoenzy mes, Moco. The effect of a polar environment on the tautomer energetic s has been explored using a continuum model, At the most advanced leve l 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 1 5.5 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 qui nonoid tautomer (T2) is of considerably higher energy and seems much l ess likely to be implicated.