The conformations of some cholanic acids and related compounds are discussed. The intramolecular van der Waals energy surface of desoxycholic acid, computed as a function of four internal rotation .angles of the side chain, presents several minima indicating a remarkable conformational flexibility. The agreement with the experimental conformations is satisfactory. The arrangement of the carboxyl group in the crystals mainly depends on its ability to form hydrogen bonds rather than on the intramolecular van der Waals energy. Ring D shows different conformations which may chiefly arise from short-range intramolecular interactions. However, anisotropic packing forces could also influence the conformation of ring D in the crystals.
The method of calculating the potential energy in crystals to determine the molecular packing ha~ rarely been used. Since this approach is likely to be employed more frequently to solve the phase problem and for theoretical calculation of physical properties, it is believed to be useful both to clarify the strategy used in applying the packing analysis and to emphasize some misconceptions about its real power and limits on the basis of our experience. As an example the paper of Kawaguchi, Takashina, Tanaka & Watanab6 on the crystal structure of bromoform is discussed.
Two programs useful in crystallographic computation are described. The first permits the calculation of cartesian atomic coordinates from molecular interatomic distances and bond angles. The second obtains the best fit between two molecular models as a function of three rotation angles, by searching the smallest weighted sum of the distances between corresponding atoms.
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