1H NMR studies of the solution conformations of an analogue of the C-peptide of ribonuclease A

Two-dimensional NMR experiments have been performed on a peptide, succinyl-AE-TAAAKFLRAHA-NH2, related to the amino-terminal sequence of ribonuclease A. This peptide contains 50-60% helix in 0.1 M NaCl solution, pH 5.2, 3 degrees C, as measured by circular dichroism. NOESY spectra of the peptide in aqueous solution at low temperatures show a number of NOE connectivities that are used to determine the highly populated conformations of the peptide in solution. Short-range dNN(i, i + 1) and d alpha N(i, i + 1) connectivities and medium-range d alpha beta(i, i + 3) and d alpha N(i, i + 3) connectivities are detected. The pattern of NOE connectivities unambiguously establishes the presence of helix in this peptide. The magnitudes of the 3JHN alpha coupling constants and the intensities of the dNN(i, i + 1) and d alpha N(i,i + 1) NOEs allow the evaluation of the position of the helix along the peptide backbone. These data indicate that the amino terminus of the peptide is less helical than the remainder of the peptide. The observation of several long-range NOEs that are atypical of helices indicates the presence of a high population of peptide molecules in which the first three residues are distorted out of the helical conformation. The absence of these NOEs in a related peptide, RN-31, in which Arg 10 has been changed to Ala, suggests that this distortion at the amino-terminal end of the peptide arises from the formation of a salt bridge between Glu 2 and Arg 10.(ABSTRACT TRUNCATED AT 250 WORDS)