HIGH-RESOLUTION X-RAY-DIFFRACTION AND AB-INITIO QUANTUM-CHEMICAL STUDIES OF GLYCOLURIL, A BIOTIN ANALOG

The electrostatic properties of glycoluril, a model for the ureido ring of biotin, have been examined by high-resolution single-crystal X-ray diffraction and by ab initio quantum chemical calculation. Two crystalline forms of glycoluril were studied crystallographically, both at 105 K and at 293 K. Calculations were performed both on the isolated molecule and on a five-molecule cluster lifted out of one of the crystal structures. Qualitative agreement among the experiments and calculations was universal; quantitative agreement was observed between the low temperature diffraction results and the cluster calculations. The inner pocket of the bicyclic ring compound is a basin of negative electrostatic potential, reaching a minimum near the ureido oxygen atoms of -0.2 e/Angstrom, while the back side of the molecule is everywhere of positive potential. The transferability of molecular properties from crystal form to crystal form extends beyond the electrostatic properties to include the rigid body thermal motions at a specific temperature. Anharmonic thermal motions of the oxygen and nitrogen atoms of glycoluril parallel nonrigid librations needed to model those same atoms in a rigid body formalism.