Comparison of the structures and vibrational modes of carboxybiotin and N-carboxy-2-imidazolidone

The spontaneous decarboxylation of N-carboxy-2-imidazolidone (a model for carboxybiotin) and N(1')carboxybiotin can be followed at high pH by Raman and FTIR spectroscopies, The major bands associated with vibrations of the carboxylate group have been assigned on the basis of quantum mechanical calculations of N-carboxy-2-imidazolidone and N(1')-carboxy-2-methylbiotin. The carboxylate modes are the asymmetric stretch, coupled to the ureido carbonyl stretch, near 1710 cm(-1), the symmetric stretch near 1340 cm(-1), and the -CO2- scissoring motion near 830 cm(-1). In the case of carboxybiotin, the last two modes are strongly coupled with biotin ring modes. All three carboxylate modes disappear as spontaneous decarboxylation occurs, to be replaced by features attributable to the noncarboxylated ring structures. The HF/6-31G* optimized structure of 2-methylbiotin revealed that the ureido ring portion is essentially planar, in accord with a number of X-ray crystallographic structures of biotin compounds. However, calculations at this level and at the B3LYP/631+G(d) level (using density functional theory) predict that the ureido ring in biotin puckers upon carboxylation. Comparison of the structures of carboxybiotin and carboxyimidazolidone, derived at the HF/6-31G* level, indicates that lengths of the ring-nitrogen-to-carboxylate bonds are equal and that the torsional angles about this linkage are very similar. This strong structural similarity provides a rationale for the observation that, at high pH, the spontaneous rates of decarboxylation of these two molecules are very similar.