NMR‐Spectroscopic Assignment of the Aib‐Methyl Groups in α‐Helical and β‐Turn Environment with the Use of Selectively Deuterated Aib The Solution Conformations of Boc‐Ala‐Aib‐Ala‐OMe

Boc‐Ala‐Aib‐Ala‐OMe (1a) adopts in the crystal a conformation similar to a β‐turn type II. From 13C‐CP‐MAS spectra the magnetic nonequivalence of the Aib‐Cβ atoms amounts to 6.1 ppm. The pro‐S‐Cβ carbon signal is shifted upfield as can be shown by comparison with the spectra of Boc‐Ala‐(R)‐Aib[D1]‐Ala‐OMe (1b). This assignment corresponds to a downfield shift of the Aib pro‐S‐Cβ carbon in the helix of Boc‐[L‐Glu(OBzl)]4‐Aib‐[L‐Glu(OBzl)]7‐O‐PEG‐M (2a), where it was established with the aid of the (R)‐Aib[D3]‐containing analog (2b), From molecular dynamics simulations of Boc‐Ala‐Aib‐Ala‐OMe we propose two major conformational species in solution that account for the observed MNE of 2.1 ppm, the temperature gradients of the amide protons, the coupling constants, and the distances derived from ROESY spectra.