Solution conformations of oligomers of α ‐aminoisobutyric acid°

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Abstract
The N-acetyl(Aib)nN''-methylamides (with n = 1, 2 and 3) [Aib = .alpha.-aminoisobutyric acid] and the N-acetyl(Aib)3 methyl ester were synthesized using an oxazolone procedure. An experimental conformational analysis of this series of oligomers was carried out in water, DMSO-d6 [dimethylsulfoxide] and CDCl3 using NMR techniques, and in chloroform using IR spectroscopy. Deuterium exchange rates of amide protons in DMSO-d6 and the rates of change of these proton chemical shifts with temperature in water, DMSO-d6 and CDCl3 indicate that the oligomeric N''-methylamides adopt conformations that have no H- bonds when n = 1, one H- bond when n = 2, and two H- bonds when n = 3, and that Ac(Aib)3OMe has a conformation with 1 H- bond. An analysis of the N-H stretching region of the IR spectra of these compounds in CHCl3 also suggests the existence of these conformational states. The peptides probably adopt the 310-helical and not the .alpha.-helical conformation in solution. The Aib residue may have asymmetric geometry at the C.alpha. atom in solution, similar to that reported in the literature for the crystalline state.