MOLECULAR AND CRYSTAL-STRUCTURE OF 5,6-DIAMINO-1-METHYLURACIL AND 5,6-DIAMINO-1,3-DIMETHYLURACIL MONOHYDRATE - SEMIEMPIRICAL CALCULATIONS (AM1 AND PM3) ON 5,6-DIAMINOURACIL DERIVATIVES

The crystal and molecular structures of 5, 6-diamino-1-methyluracil an d 5, 6-diamino-1, 3-dimethyluracil monohydrate have been determined fr om X-ray diffraction. Both compounds are planar and the two amino grou ps have two different conformations. The substituent at the 5 position seems to be a true primary amino group with a strongly sp(3) nitrogen , whereas the one at the 6 position is nearly coplanar with the uracil ring, displaying a predominant sp(2) character. Semiempirical calcula tions were made on 5, 6-diaminouracil, 5, 6-diamino-2-thiouracil and t heir endocyclic N-methylated derivatives using the AM1 and PM3 hamilto nians. These indicate that the stability decreases on increasing methy lation, the 2-thio compounds always being less stable than the 2-oxo o nes. (C) 1997 Elsevier Science B.V.