The proposed research is concerned with the determination of X-ray structure of biologically interesting small molecules. This research is largely aimed at the elucidation of the specific natures of inter- and intramolecular interactions among such molecules. Thus, for example, one of the principle projects being undertaken is to determine the interactions between adenine and nicotinamide or flavin derivatives as models for the intramolecular interactions in the redox coenzymes NAD ion and FAD, respectively, in both their oxidized and their reduced forms. We are also concerned with elucidating the differences between certain chemotherapeutic agents and the naturally occurring compounds that they mimic. We are undertaking a program to determine the nature of the molecular interactions between nucleic acids and amino acids through the structural study of crystalline model compounds incorporating the major structural features of these molecules.