Stereochemistry of α-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.