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

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Abstract
PK values of Glu2, Glu9, and His12 in the lactone and carboxylate forms of C-peptide at 4.5 and 21.degree., 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 ben used in an analysis of the well known pH-induced helix-coil transition in the C-peptide lactone. It has been shown that Glu 12 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. Ther transition within the pH range 5.5-7.5 follows exactly the deprotonation curve of His+ 12 residue.