Stereochemistry of alpha-aminoisobutyric acid peptides in solution: helical conformations of protected decapeptides with repeating Aib-L-Ala and Aib-L-Val sequences

The decapeptides Boc-(Aib-L -Ala)₅-OMe and Boc-(Aib-L -Val)₅-OMe have been studied by 270-MHz ¹H-nmr in CDCl₃ and (CD₃)₂SO solutions. Intramolecular hydrogen-bonded NH groups have been delineated using the temperature and solvent dependence of the NH chemical shifts and differential broadening of the NH resonances, induced by addition of a nitroxide radical. Both peptides have eight solvent-shielded NH groups, suggesting that 3₁₀-helical conformations are maintained in the two solvents. In alternating Aib-X sequences, the Aib residues appear to play a dominant role in determining the preferred conformations, overriding the intrinsic stereochemical preferences of the X residues.