Molecular structure and intermolecular interactions of N1'-methoxycarbonylbiotin methyl ester: a model for carboxybiotin.

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Abstract
The crystal structure of N1''-methoxycarbonylbiotin methyl ester, a model for N1''-carboxybiotin, was determined [by X-ray diffraction]. The ureido carbonyl bond has more double bond (keto) character than does the corresponding bond in free biotin, which has single bond (enolate) character. There is an interesting intermolecular interaction between the ureido carbonyl oxygen and a methyl group. Comparison of the molecular structure and crystal packing with those of free biotin suggests that the coenzyme may have evolved with the incorporation of the ureido moiety because the electronic configuration of this region of the molecule is sensitive to N1'' carboxylation. On decarboxylation, the ureido carbonyl bond becomes more polarized (C.sbd.O-), thereby facilitating the deprotonation of N1'' and increasing its nucleophilicity. As a result, carboxylation can occur readily. On carboxylation, the carbonyl bond is depolarized (C.dbd.O), allowing the carboxylated coenzyme to interact with nonpolar groups and carboxylate them. The carboxylation and decarboxylation of biotin appear to act as a mechanistic switch, turning off and on the polarization of the ureido carbonyl bond as well as modulating the nucleophilicity of N1''.