Dynamic Structure of Vesicle-Bound Melittin in a Variety of Lipid Chain Lengths by Solid-State NMR

摘要

Solid-state ³¹P- and ¹³C-NMR spectra were recorded in melittin-lecithin vesicles composed of 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) or 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). Highly ordered magnetic alignments were achieved with the membrane surface parallel to the magnetic field above the gel-to-liquid crystalline phase transition temperature (Tc). Using these magnetically oriented vesicle systems, dynamic structures of melittin bound to the vesicles were investigated by analyzing the ¹³C anisotropic and isotropic chemical shifts of selectively ¹³C-labeled carbonyl carbons of melittin under the static and magic-angle spinning conditions. These results indicate that melittin molecules adopt an α-helical structure and laterally diffuse to rotate rapidly around the membrane normal with tilt angles of the N-terminal helix being −33° and −36° and those of the C-terminal helix being 21° and 25° for DLPC and DPPC vesicles, respectively. The rotational-echo double-resonance method was used to measure the interatomic distance between [1-¹³C]Val⁸ and [¹⁵N]Leu¹³ to further identify the bending α-helical structure of melittin to possess the interhelical angles of 126° and 119° in DLPC and DPPC membranes, respectively. These analyses further lead to the conclusion that the α-helices of melittin molecules penetrate the hydrophobic cores of the bilayers incompletely as a pseudo-trans-membrane structure and induce fusion and disruption of vesicles.