Amide 1H n.m.r. study of the folding of ribonuclease C-peptide

Abstract

The folding of ribonuclease A 1–13 (C-peptide) in H 2O near 0°C has been monitored by means of the amide and side chain NH proton resonances. The C-peptide carboxylate at low temperature forms, in a significant amount, a folded structure similar to the one that the 1–19 S-peptide adopts in the same conditions (3–13 α-helix). A quantitative comparison between helix stabilities of the lactone and carboxylate forms of C-peptide and S-peptide is reported. It is concluded that the proposed His 12 + … Hse 13 (COO − salt bridge, which competes with the one-turn stabilizing salt bridge His 12 + … Glu 9 − in the C-peptide carboxylate, does not suppress helix formation as previously suggested but it merely reduces its stability. The behaviour of the N 5-H resonance of the Arg 10 + side chain provides evidence for its implication in a further stabilizing interaction, most probably with Glu 2 −.