Crystal and Molecular Structure of an Iodo‐derivative of the Cyclic Undecapeptide Cyclosporin A

The crystal structure of an iodo‐derivative of cyclosporin A has been determined in order to elucidate the constitution of this cyclic undecapeptide. Crystals of iodocyclosporin A are monoclinic, a = 10.475(5), b = 19.60(1), c = 21.04(1) Å, β 99.35(2)°, space group P21 (C , No. 4). The structure was solved by the heavy atom method and refined by block‐diagonal least‐squares analysis to a final R‐factor of 0.135 with hydrogen atoms in calculated positions. The cyclic peptide has a conformation which is partly β‐pleated sheet and partly open loop. The structure analysis demonstrates for the first time the reality of a new type of dipeptide hydrogen‐bonding, predicted by Pullman from MO calculations and leading to a conformation described by him as C .