Abstract
Peptides composed exclusively of alpha,alpha-dialkyated amino acid residues, such as the alpha, alpha-dimethylglycine residue (aib), are known to adopt 310 helices in solution. The 310 helix is characterized by an i-to-i+3 intramolecular hydrogen-bonding pattern as opposed to the i-to-i+4 hydrogen-bonding pattern of the more familiar alpha-helix. Recent work in other laboratories has concluded that the alpha-helix may be the preferred helical structure for homooligomers of some alpha, alpha-dialkylated amino acid residues, such as alpha-methyl-alpha-ethylglycine, under certain conditions of solvent and C-terminal protection. In order to understand more deeply the helical preferences of short oligomers of aib, we have performed 1H NMR-based hydrogen/deuterium exchange studies, using methanol solvent, to characterize the alpha/310 helix equilibrium in a series of short oligomers (n = 4, 6) of aib residues. We have compared oligomers of aib C-terminated in tert-butyl ester and the tert-butyl ester of beta-alanine. In methanol solvent, the βeτα-alanine-based C-protecting group appears to allow for a greater percentage of α-helix than the tert-butyl ester.
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