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 rin g of biotin, have been examined by high-resolution single-crystal X-ra y diffraction and by ab initio quantum chemical calculation. Two cryst alline forms of glycoluril were studied crystallographically, both at 105 K and at 293 K. Calculations were performed both on the isolated m olecule and on a five-molecule cluster lifted out of one of the crysta l structures. Qualitative agreement among the experiments and calculat ions was universal; quantitative agreement was observed between the lo w temperature diffraction results and the cluster calculations. The in ner pocket of the bicyclic ring compound is a basin of negative electr ostatic 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 c rystal form to crystal form extends beyond the electrostatic propertie s to include the rigid body thermal motions at a specific temperature. Anharmonic thermal motions of the oxygen and nitrogen atoms of glycol uril parallel nonrigid librations needed to model those same atoms in a rigid body formalism.