Octapeptides to Pentadecapeptides

As the number of residues increases in a cyclic peptide, the greater is the variety of possible conformations for the backbone. For eight to fifteen residues in cyclic peptides, conformations have been found with folds in the form of bracelets and saddles, whereas others are in an extended form in elongated loops like rubber bands. The interior of the rings may contain solvent molecules or alkali metal ions. A variety of NH 0=C internal hydrogen bonds, from 3- l to 5- l, have been found. Most of the peptide units occur in the trans conformation, but occasionally peptide units with substituted nitrogen atoms have the cis conformation. 

CH3OH molecules located outside the peptide ring. The conformational sequence is trans,trans,cis,cis,cis,trans,trans,cis,cis,cis with a true center of symmetry. 

The cyclic octadepsipeptide cyc/o(Dlle-LLac-Llle-DHyv)2 (Duax et al, 1980) contains an approximate twofold rotation axis, and its backbone is elongated. All amide and ester linkages are trans. The unusual feature of this molecule is a new type of intramolecular NH 0=C bond with 8 atoms in 

The extremely poisonous green mushroom Amanita phalloides contains a number of cyclic peptides classified as phallotoxins (heptapeptides), ama-toxins (octapeptides), and a decapeptide antagonist called antamanide (Wieland and Wieland, 1972). Crystal structure analyses have been completed on jS-amanitin, on antamanide, and on a biologically active analog, both com-plexed and uncomplexed. 

The conformation of uncomplexed antamanide is entirely different from that of the alkali metal ion complexes. Not only is the cyclic backbone elongated and flattened, but sequences 4,5,6 and 9,10,1 turn inside-out so that the 

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