Study of the chelating capacity of nucleobase analogs with biological interest: XRD structural study and ab initio molecular orbital calculations on 1-methyl and 1,6,7-trimethyllumazine

The molecular and crystal structures of 1-methyllumazine (MLM) and 1,6,7-trimethyllumazine (MLMD) (lumazine ¼ pteridine- 2,4(1H,3H)-dione) have been XRD determined. The compound MLM crystallizes in the monoclinic system (space group P21/c) with cell dimensions: a ¼ 4:8200ð2Þ; b ¼ 9:7820ð5Þ; c ¼ 15:3230ð9Þ A ° , b ¼ 93:407ð2Þ8: The structure was solved from 1947 reflections with I . 2sðIÞ: The final R½I . 2sðIÞ was 0.0756 for 136 parameters. The compound MLMD crystallizes in the monoclinic system (space group C2) with cell dimensions: a ¼ 16:1360ð4Þ; b ¼ 6:5850ð2Þ; c ¼ 10:5380ð4Þ A° , b ¼ 125:150ð1Þ8: The structure was solved from 1800 reflections with I . 4sðIÞ: The final R½I . 4sðIÞ was 0.0547 for 165 parameters. In both structures, the pteridine derivatives are H-bond dimerized (N–H· · ·O), but whereas MLM shows a N3–H3· · ·O2 H-bond, MLMD displays a N3–H3· · ·O4 one, which is in accordance with the different sequence found for the carbonyl lengths. Ab initio molecular orbital calculations at the RHF/6-31Gp and B3LYP/6-31Gp levels using the GAUSSIAN94 program package have allowed us to simulate the molecular structures. The geometrical data are in good agreement with those calculated. The contribution of the atomic orbitals of the potential donor atoms to the higher occupied molecular orbitals allows us to propose theoretical arguments to justify the well-known N5–O4-bidentate coordinative behaviour found for these compounds in several metal complexes. q 2004 Elsevier B.V. All rights reserved