Solid state NMR analysis of peptides in membranes: Influence of dynamics and labeling scheme

The functional state of a membrane-active peptide is often defined by its conformation, molecular orientation, and its oligomeric state in the lipid bilayer. These “static” structural properties can be routinely studied by solid state NMR using isotope-labeled peptides. In the highly dynamic environment of a liquid crystalline biomembrane, however, the whole-body fluctuations of a peptide are also of paramount importance, although difficult to address and most often ignored. Yet it turns out that disregarding such motional averaging in calculating the molecular alignment from orientational NMR-constraints may give a misleading, if not false picture of the system. Here, we demonstrate that the reliability of a simplified static or an advanced dynamic data analysis depends critically on the choice of isotope labeling scheme used. Two distinctly different scenarios have to be considered. When the labels are placed on the side chains of a helical peptide (such as a CD₃- or CF₃-group attached to the C^αC^β bond), their nuclear spin interaction tensors are very sensitive to motional averaging. If this effect is not properly accounted for, the helix tilt angle tends to be severely underestimated. At the same time, the analysis of labels in the side chains allows to extract valuable dynamical information about whole-body fluctuations of the peptide helix in the membrane. On the other hand, the alternative labeling scheme where ¹⁵N-labels are accommodated within the peptide backbone, will yield nearly correct helix tilt angles, irrespective as to whether dynamics are taken into account or not.