13C-13C rotational resonance in a transmembrane peptide: a comparison of the fluid and gel phases

(19)F-Labeling of Peptides Revealing Long-Range NMR Distances in Fluid Membranes

NMR distance measurements lie at the heart of structural biology. However, long-range distances could not yet be detected in liquid-crystalline biomembranes, because dipolar couplings are partially averaged by the intrinsic molecular mobility. Using conformationally constrained (19)F-labeled amino acids as reporter groups, we could more than double the accessible interatomic distance range by combining a highly sensitive solid-state multipulse (19)F-NMR scheme with a favorable sample geometry. Two rigid 4F-phenylglycine labels were placed into the helical antimicrobial peptide PGLa embedded in fluid oriented membrane samples. A modified Carr-Purcell-Meiboom-Gill sequence yielded an intramolecular distance of 6.6 Å for the labels spanning one helix turn, and 11.0 Å was obtained when the labels spanned two turns. This approach should now also allow the characterization of conformational changes in membrane-active peptides and of oligomeric assemblies in a biologically relevant lipid environment.

Solid-state NMR structure determination of melittin in a lipid environment

Solid-state (13)C NMR spectroscopy was used to investigate the three-dimensional structure of melittin as lyophilized powder and in ditetradecylphosphatidylcholine (DTPC) membranes. The distance between specifically labeled carbons in analogs [1-(13)C]Gly3-[2-(13)C]Ala4, [1-(13)C]Gly3-[2-(13)C]Leu6, [1-(13)C]Leu13-[2-(13)C]Ala15, [2-(13)C]Leu13-[1-(13)C]Ala15, and [1-(13)C]Leu13-[2-(13)C]Leu16 was measured by rotational resonance. As expected, the internuclear distances measured in [1-(13)C]Gly3-[2-(13)C]Ala4 and [1-(13)C]Gly3-[2-(13)C]Leu6 were consistent with alpha-helical structure in the N-terminus irrespective of environment. The internuclear distances measured in [1-(13)C]Leu13-[2-(13)C]Ala15, [2-(13)C]Leu13-[1-(13)C]Ala15, and [1-(13)C]Leu13-[2-(13)C]Leu16 revealed, via molecular modeling, some dependence upon environment for conformation in the region of the bend in helical structure induced by Pro14. A slightly larger interhelical angle between the N- and C-terminal helices was indicated for peptide in dry or hydrated gel state DTPC (139 degrees -145 degrees ) than in lyophilized powder (121 degrees -139 degrees ) or crystals (129 degrees ). The angle, however, is not as great as deduced for melittin in aligned bilayers of DTPC in the liquid-crystalline state (approximately 160 degrees ). The study illustrates the utility of rotational resonance in determining local structure within peptide-lipid complexes.