Position effect on apparent helical propensities in the C-peptide helix.

A search has been made for position effects on apparent helix propensities when another amino acid is substituted for alanine in the C-peptide helix of ribonuclease A. Three internal alanine residues (Ala4, Ala5, Ala6) are used as sites for substitution. Five amino acids, Glu, His, Arg, Lys and Phe, are substituted singly in individual peptides at each of these three positions, and the pH profiles of helix content for the substituted peptides have been determined. The effect of using an acetyl or a succinyl amino-terminal-blocking group has also been determined for each substitution. A strong position effect is found at Ala5: the helix content of the substituted peptide is significantly higher for substitution at position 5 than at positions 4 or 6 in almost all cases. The reason for the position 5 effect is unknown. The results also show that electrostatic interactions often influence substitution experiments, and they provide data on the variability of substitution experiments made with a natural sequence peptide.