Stereochemistry of a-Aminoisobutyric Acid Protected Decapeptides with Repeating Aib-L-Ala

The decapeptides Boc-(Aib-L-Ala)s-OMe and Boc-(Aib-L-Val)s-OMe have been studied by 270-MHz lH-nmr in CDCl3 and (CD&SO 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 dlo-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.