Conformational study of two synthetic peptides with sequence analogies to the N-terminal fragment of RNase A

The conformational properties of two synthetic model peptides, AEAAHAAEAAHMG (PA) and AEAAHAFEAAHMG (PF), have been studied using CD and 1H-NMR methods. In both peptides, glutamate and histidine residues are situated in such a way that two salt bridges between Glu- (i) and His+ (i + 3) can be formed. A salt bridge of this type (Glu- 9-His+ 12) was postulated previously to stabilize, to a great extent, the alpha-helical conformation of isolated N-terminal fragments of RNase A: C-peptide and S-peptide (A. Bierzyński, P.S. Kim and R.L. Baldwin, Proc. Natl. Acad. Sci. U.S.A. 79 (1982) 2470). Although in both PA and PF salt bridges between glutamates and histidines are formed, as demonstrated by the pH-titration curves of the glutamate gamma-proton signals, no traces of helical conformation have been detected. Evidently, the Glu- (i)-His+ (i + 3) salt bridges do not stabilize the alpha-helical conformation. A comparative analysis of PA and PF NMR spectra provides strong evidence that the phenylalanyl ring in PF interacts not only with the hydrophobic methyl groups of almost all alanine residues but also with the histidine rings and the glutamate side chains in their protonated as well as deprotonated forms. Similar interactions, involving Phe 8, can be expected in the N-terminal fragments of RNase and should be taken into account as an important factor determining the conformational properties of C- and S-peptides.