Using a Sterically Restrictive Amino Acid as a 19F NMR label To Monitor and To Control Peptide Aggregation in Membranes

Aggregation of peptides into amyloid fibrils or other β-sheet structures is usually related to malfunction. This process is often induced by lipid bilayers but is difficult to monitor in the membrane-bound state. Here, we show how aggregation is readily detected by solid state 19 F NMR using a fluorine-labeled amino acid and how the same sterically rigid side chain can be also be employed to prevent aggregation. Selective CF 3 -phenylglycine labels were incorporated either as the L - or D -enantiomer into the membrane-active model amphiphilic peptide (MAP). Oriented circular dichroism showed that the D -epimeric peptides maintained an α-helical conformation in DMPC at all peptide-to-lipid ratios, while the L -epimers turned into β-sheet structures above 1:200, just like the wild-type. Aggregation was clearly revealed by the appearance of powder lineshapes in the 19 F NMR spectra of oriented samples. The 19 F NMR dipolar couplings from four D -epimers were analyzed as orientational constraints, showing that the helix of MAP undergoes a concentration-dependent realignment in DMPC from a surface-bound to a tilted state.