Reversible Disruption by Cupric Ions of a Helical Conformation of a Polypeptide Derived from Ribonuclease

The tryptic peptide comprising Residues 38 to 61 of oxidized bovine pancreatic ribonuclease A was isolated and shown to be convertible in part to a helical conformation in 2-chloroethanol- or 2,2,2-trifluoroethanol-water mixtures. The characteristic trough in the optical rotatory dispersion spectrum near 233 mµ associated with the helical conformation of the peptide was progressively reduced by addition of each of 3 eq of cupric ion. The depth of the trough was regained quantitatively by addition of ethylenediaminetetraacetate equivalent to the cupric ion present. The absorption and optical rotatory spectra of the cupricpeptide complexes are qualitatively similar to those of some smaller peptides in which chelation by amide nitrogen atoms of peptide bonds has been demonstrated. The points of similarity include absorption maxima in the range of 525 to 550 mµ and optically active bands near 330 mµ and 530 mµ. A Cotton effect centered near 260 mµ in the complexes of the ribonuclease peptide is more clearly resolved than in the previous studies with smaller peptides. These results are in keeping with contributions from binding sites at the NH 2 terminus at the region of the histidine residue. Tentatively, the location of the third site is indicated from a molecular model to comprise members of the last 4, COOH-terminal residues. The effect of cupric ion binding on the helix content of the peptide is interpreted in terms of the conformational incompatibility of the chelate complexes with the α-helix. The observations do not support a strictly all-or-none metal-peptide interaction.