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 1 H‐nmr in CDCl 3 and (CD 3 ) 2 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 3 10 ‐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.