TheGlu2- l l l Arg 10 + side-chain interaction in the C-peptide helix of ribonuclease A

Helix formation; Salt bridge; Side-chain interaction Previous tudies have identified Lys 1, Glu 2, and His 12 as the charged residues responsible for the ps-dependent stability of the helix formed by the isolated C-peptide (residues 1-13 of ribonuclease A). Here we examine whether the helix-stabilizing behavior of Glu 2- results from a Glu 2-. * a Arg lO+ interaction, which is known to be present in the crystal structure of ribonuclcasc A. The general approach is to measure the helix content of C-peptide analogs as a function of three variables: pH (titration of ionizing groups), amino acid identity (substitution test), and NaCl concentration (ion screening test). In order to interpret the results of residue replacement, several factors in addition to the putative Glu 2-... Arg 10 + interaction have been studied: intrinsic helix-forming tendencies of amino acids; interactions of charged residues with the a-helix macrodipole; and helix-lengthening effects. The results provide strong evidence that the Glu 2-.. Arg 10c interaction is linked to helix formation and contributes to the stability of the isolated C-peptide helix. NMR evidence supports these conclusions and suggests that this interaction also acts as the N-terminal helix stop signal. The implications of this work for protein folding and stability are discussed. 1. lntrodtletioll The C-peptide (residues 1-13) from RNase A provided the first example of a peptide able to form a short monomolecular helix in aqueou