Metal ion effects on cis/trans isomerization of proline residues in short chain peptides in solution

The-effect of capper(II) ions on the probabilities of existence of the four detectable conformers of the tetrapeptide Tyr-Pro-Phe-Pro (beta -casomorphin 4) in [H-2(6)]DMSO was investigated by H-1 NMR Spectroscopy. Integration of the Phe-NH signals provided the relative populations in the free state as tt/tc/ct/cc = 28.34.29:9 at 293 K (c = cis, t = trans). Copper(ii) was shown to bind to all four isomers, yielding complexes with two different structures, depending on the conformation of Pro(2). The interpretation of paramagnetic relaxation rates of Pro(2)-H alpha signals provided the corresponding isomeric probabilities in the metal-bound state as 13:36:20:31. The observed stabilization of the conformation with the lowest probability of-existence (cc) may be relevant for the biological role of copper and other metal ions.