C-Alpha-Methyl-L-Valine: A Preferential Choice Over Alpha-Aminoisobutyric Acid For Designing Right-Handed Alpha-Helical Scaffolds

In synthetic peptides containing Gly and coded alpha-amino acids, one of the most common practices to enhance their helical extent is to incorporate a large number of L-Ala residues along with noncoded, strongly foldameric alpha-aminoisobutyric acid (Aib) units. Earlier studies have established that Aib-based peptides, with propensity for both the 3(10)- and alpha-helices, have a tendency to form ordered three-dimensional structure that is much stronger than that exhibited by their L-Ala rich counterparts. However, the achiral nature of Aib induces an inherent, equal preference for the right- and left-handed helical conformations as found in Aib homopeptide stretches. This property poses challenges in the analysis of a model peptide helical conformation based on chirospectroscopic techniques like electronic circular dichroism (ECD), a very important tool for assigning secondary structures. To overcome such ambiguity, we have synthesized and investigated a thermally stable 14-mer peptide in which each of the Aib residues of our previously designed and reported analogue ABGY (where B stands for Aib) is replaced by C-alpha-methyl-L-valine (L-AMV). Analysis of the results described here from complementary ECD and H-1 nuclear magnetic resonance spectroscopic techniques in a variety of environments firmly establishes that the L-AMV-containing peptide exhibits a significantly stronger preference compared to that of its Aib parent in terms of conferring alpha-helical character. Furthermore, being a chiral alpha-amino acid, L-AMV shows an intrinsic, extremely strong bias for a quite specific (right-handed) screw sense. These findings emphasize the relevance of L-AMV as a more appropriate unit for the design of right-handed alpha-helical peptide models that may be utilized as conformationally constrained scaffolds.