Deprotonation of Glu9 destabilizes the alpha-helix in C-peptide of RNase A

pK values of Glu2, Glu9, and His12 in the lactone and carboxylate forms of C-peptide at 4.5 and 21 degrees, 0.1 M NACl, have been determined from pH-tritration shifts of n.m.r. signals of the glutamate gamma and histidine ring protons. These have been used in an analysis of the well known pH-induced helix-coil transition in the C-peptide lactone. It has been shown that Glu12 deprotonation results in a twofold increase of the helix content, whereas deprotonation of Glu9 leads to a 28% drop. This latter effect is probably related to Glu-9-His+12 salt bridge formation. The transition within the pH range 5.5-7.5 follows exactly the deprotonation curve of His+12 residue.