The crystal structure of phenoxymethylpenicillin

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Abstract
Crystals of phenoxymethylpenicillin were grown from acetone by the addition of water. They were mono-clinic, space group C2, [alpha] = 12.79[plus or minus]0.02A, b = 11.236 [plus or minus] 0.007A, c = 13.39[plus or minus]0.02A, [beta] = 115[degree]44[image], Z = 4. The intensities of 1670 reflexions recorded on Weissenberg photographs were measured, partly visually, partly with a microdensitometer. Some difficulty was experienced from the deterioration of crystals during X-ray photography. The structure was solved through the calculation of a three-dimensional Patterson synthesis, with coefficients modified to correspond to those from point atoms at rest. The S-S vector peak was the highest observed. Other atomic positions were recognized by locating pairs of peaks in the Patterson distribution related by the S-S vector. The only atomic position not so found was shown in a subsequent calculated three-dimensional electron-density distribution. The structure was refined by a 3 dimensional anisotropic least-squares treatment Though the R value is now 12.6%, the precision of atom placing does not seem very high. C-C distances deviated from expected values by an average of 0.035A, maximum 0.06A. The molecule is present in the crystal structure as the free acid; hydrogen bonds between the amide oxygen atom and the carboxyl hydroxyl group link succeeding molecules in the direction of the b axis. The thiazolidine-[beta]-lactam nucleus is geometrically similar to that found in the benzylpenicillin salts; but rotations about single bonds change markedly the relative orientations of the benzene rings in the 2 molecules. There is a close contact, 2.58A, between the side-chain amide N atom and the O of the phenoxymethyl group, which may indicate the presence of a very weak internal H bond in the molecule.