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 a-helical antimicrobial peptide PGLa in a lipid membrane is determined with high accuracy by solid-state H-2-NMR. Orientational constraints are derived from a series of eight alanine-3,3,3-d(3)-labeled peptides, in which either a native alanine is nonperturbingly labeled (43), or a glycine (23) or isoleucine (23) is selectively replaced. The concentration dependent realignment of the alpha-helix from the surface-bound S-state to a tilted T-state by 30 degrees is precisely calculated using the quadrupole splittings of the four nonperturbing labels as constraints. The remaining, potentially perturbing alanine-3,3,3-d(3) labels show only minor deviations from the unperturbed peptide structure and help to single out the unique solution. Comparison with previous F-19-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.