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.