Charge Density Study of 9-Ethyladenine/5,5-Diethylbarbituric Acid. M. Niederhut, B.M. Craven. Department of Crystallography, University of Pittsburgh, Pittsburgh, PA 15260.

We aim to obtain an experimental model for the barbital drug receptor site. This will be done by deriving the van der Waals envelope and the electrostatic potential in a cavity formed in the crystal structure of the 1:1 complex of 9-ethyladenine/5,5-diethylbarbituric acid when a single barbital molecule is removed. In the structure, molecules crystallize in layers involving both Watson-Crick and Hoogsteen type hydrogen bonding. There are also carbonyl-carbonyl intermolecular distances which are unusually short. The room temperature structure was first reported by Voet (JACS, 94:23, 8213-8222, 1972.) We have collected new diffraction data with MoKa radiation (sin theta/lambda < 1 Å-1) at 109K.