Conformational study of an Aib-rich peptide in DMSO by NMR.

The strong propensity of 2-amino-2-methyl propanoic acid (Aib)-rich peptides to form stable helical structures is well documented. NMR analysis of the short peptide Z-(Aib)5-L-Leu-(Aib)2-OMe indicates the presence of a well-characterized 3(10)-helix even in dimethylsulfoxide (DMSO), a solvent known to disrupt helical structures. The structure remains stable at least up to 348 K. Stereospecific assignment of the diastereotopic methyls of Aib was achieved, with the assumption of a specific helical screw sense. The methyl more eclipsed with respect to the CO vector resonates at a higher field in the carbon dimension. Molecular dynamics simulations successfully predict the 3J(CHNH) coupling constant of Leu6 and most of the H-bonding pattern. Discrepancies were found for Aib3 and Aib7 amide protons which can be explained by a higher sensitivity of the simulations to the helix fraying at the end of the peptide and by the presence of extended conformations for Leu6 during most of the simulations.