Solution Conformations of Penta and Heptapeptides Containing Repetitive \alpha-Aminoisobutyryl-L-Alanyl and \alpha-Aminoisobutyryl-L-Valyl Sequences

The presence of folded solution conformations in the peptides $Boc-Ala-{(Aib-Ala)}_2-OMe$, $Boc-Val-{(Aib-Val)}_2-OMe$, $Boc-Ala-{(Aib-Ala)}_3-OMe$ and $Boc-Val-{(Aib-Val)}_3-OMe$ has been established by 270MHz H NMR. Intramolecularly H-bonded NH groups have been identified using temperature and solvent dependence of NH chemical shifts and paramagnetic radical induced broadening of NH resonances. Both pentapeptides adopt $3_{10}$ helical conformations possessing 3 intramolecular H-bonds in $CD{Cl}_3$ and ${({CD}_3)}_2SO$. The heptapeptides favour helical structures with 5 H-bonds in $CD{Cl}_3$. In ${({CD}_3)}_2SO$ only 4 H-bonds are readily detected