Observation of Backbone Cα-Deuteron Signals in Solid-State NMR Spectra of Labeled Alanines in Oriented Transmembrane Peptides

Solid-state deuterium NMR spectra combined with the Geometric Analysis of Labeled Alanines (GALA) method provide information about the average apparent tilt and dynamics of membrane-spanning peptides in oriented, hydrated lipid bilayer membranes. When Ala-d4 labels are used, the side-chain methyl (CD3) signals are readily detected, but the weaker backbone Cα-D resonances are often problematic to observe (van der Wel, Biophys J, 83, 1479) for peptides of the WALP family [GWW(LA)nLWWA]. In spite of this difficulty, we recently have begun to observe surprisingly intense backbone Cα-D resonances in the 2H NMR spectra for some oriented transmembrane peptides, particularly in cases where the (LA)n core sequence of WALP is interrupted by a “guest” residue such as Pro, Lys or Arg. Examples will be discussed. In some cases, the Cα-D signal intensities appear to depend upon the macroscopic orientation of the lipid bilayers with respect to the external magnetic field. The sequence and orientation dependence of the backbone Cα-D signals may therefore reflect the local and global peptide dynamics in ways that are incompletely understood. A selection of spectral examples will be presented and interpreted.