Synthesis and study of the conformational stability of sequential polypeptides containing δ-benzyloxycarbonyl-L-ornithine and benzyl esters of L-aspartic and L-glutamic acids

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The synthesis of the sequential polypeptides (N-δ-Z-L-Orn-γ-BzI-L-Glu)n and (N-δ-Z-L-Orn-β-BzI-L-Asp)n by solution methods is described. Both polypeptides are α helical in chloroform as shown by circular dichroism. However, the conformation in this solvent is rather fragile, since small additions of TFA to a dilute solution of the polypeptides in chloroform is sufficient to cause the helix-to-coil transition, as demonstrated by cd and nmr. It is postulated that the low stability of the helical conformation is attributed, in this case, to a cooperative collapse of the ordered conformation.