Solid-State NMR Analysis of the PGLa Peptide Orientation in DMPC Bilayers: Structural Fidelity of 2H-Labels versus High Sensitivity of 19F-NMR

The structure and alignment of the amphipathic α-helical antimicrobial peptide PGLa in a lipid membrane is determined with high accuracy by solid-state 2H-NMR. Orientational constraints are derived from a series of eight alanine-3,3,3-d3-labeled peptides, in which either a native alanine is nonperturbingly labeled (4×), or a glycine (2×) or isoleucine (2×) is selectively replaced. The concentration dependent realignment of the α-helix from the surface-bound “S-state” to a tilted “T-state” by 30° is precisely calculated using the quadrupole splittings of the four nonperturbing labels as constraints. The remaining, potentially perturbing alanine-3,3,3-d3 labels show only minor deviations from the unperturbed peptide structure and help to single out the unique solution. Comparison with previous 19F-NMR constraints from 4-CF3-phenylglycine labels shows that the structure and orientation of the PGLa peptide is not much disturbed even by these bulky nonnatural side chains, which contain CF3 groups that offer a 20-fold better NMR sensitivity than CD3 groups.