Chain-folding initiation structures in ribonuclease A: conformational analysis of trans-Ac-Asn-Pro-Tyr-NHMe and trans-Ac-Tyr-Pro-Asn-NHMe in water and in the solid state

In order to investigate the role of beta -bends with non-native trans peptide bonds preceding Pro super(114) and Pro super(93) in the mechanism of folding of bovine pancreatic ribonuclease A, the authors examined the conformations of the synthetic peptides Ac-Asn-Pro-Tyr-NHMe(I) and Ac-Tyr-Pro-Asn-NHMe(II) in water and in the solid state. These sequences occur at residues 113-115 and 92-94, respectively, in ribonuclease A. Evidence for a significant population of I with a trans-Asn-Pro peptide bond and a beta -bend at Pro-Tyr in water includes backbone/backbone and side chain/backbone NOE's, N-H bending frequencies characteristic of a beta -bend, and hydrogen bonds detected by solvent spin-saturation transfer measurements involving the TyrNH and NHMe amide protons. The relevance of the results to the mechanism(s) of chain-folding initiation in ribonuclease A is discussed.