Sunzmary. The crystal structure of an iodo-derivative of cyclosporin .I has bccn dctcrniined in order to elucidate the constitution of this cyclic undecapept ide. Crystals of iodocyclosporin A are monoclinic, a = 10.475(5), b = 19.60(1), c = 21.04(1) is, ,!j =: 99.35(2)", space group P21 (Cz, No. 4). The structure was solved by the heavy atom nicthod and refined by blockdiagonal least-squares analysis to a final 12-€actor of 0.135 with hydrogen atonis in calcuhtcd positions. The cyclic peptide h a s a conformation which is partly P-pleated sheet and partly open loop. The structure analysis demonstrates for the first time the rcality of a new type of dipcptide hydrogen-bonding, predicted by Pullman from MO calculations and leading to a conforination described by him as C;cl.Cyclosporin A is a cyclic undecapeptide CszHlllNllOlz wliicli niiiy IIC isolated frtm Trichoderma poZys$orurn (Link ex Pers.) Rifai 111. The compound is neutral, rich in hydrophobic amino-acids, insoluble in water and 9%-liexane, but very soluble in all other organic solvents, and exceedingly difficult to crystallise. (In fact, it was not until some three months after the completion of this structure analysis, that excellent crystals were serendipitously obtained from acetone solutioi I at -1.5O.) \Ye were, Iiowever, able to obtain a crystalline derivative from the rcac-tion of cyclosporin and iodine in the presence of tliallium acetate.The analysis of iodocyclosporin was undertaken to deterinine the constitution of the natural product. *4t the beginning of the analysis the amino-acid cornpusition and part of the sequence was known froin chemical and spectroscopic studies 121, as was the presence of a hitherto unknown P-hydroxy, singly unsaturated Cg amino acid, the site of iodine addition. (We abbreviate this amino-acid as Cg-ene.) The structure analysis showed the constitution of cyclosporin A to be 1, where a11 optically active amino-acids are L with the exception of Ala (8) which is D.
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