Multinuclear Solid‐State‐NMR and FT‐IR‐Absorption Investigations on Lipid/Trichogin Bilayers

Abstract

Solid-state NMR and FT-IR absorption spectroscopy are employed to study the molecular properties of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipids as a function of trichogin-OMe content, a membrane-active analogue of the peptaibol trichogin GA IV. Variable-temperature NMR studies are performed, comprising (13)C-, (31)P-, and (14)N-NMR line-shape and relaxation experiments, to provide information about the mobility and ordering of the phospholipid head group and the acyl-chain region in the absence and presence of trichogin-OMe. Likewise, variable-temperature FT-IR-absorption studies are performed, and the conformation-sensitive CH(2) stretching bands are analyzed to examine the conformational state of the acyl chain. At lower trichogin-OMe concentrations, the peptide exhibits no remarkable influence on the dynamics and ordering features of the phospholipid molecules. It is concluded that, in this case, trichogin-OMe is embedded in the lipid bilayer, with its helix axis laying parallel to the bilayer plane, the more hydrophobic part pointing towards the inner part of the bilayer ('carpet-like' superstructure). The lipid dynamics are probed by rotating-frame spin-lattice-relaxation (T(1rho)) experiments for the (13)C and (31)P nuclei, which are assumed to be dominated by collective-order fluctuations. Variation of T(1rho) with sample composition is attributed to changes of the membrane stiffness. For the sample with the highest lipid/peptide (L/P) molar ratio, i.e., L/P 5 : 1, phase separation as a result of membrane disruption occurs. In this case, a second spectroscopic component can be separated in the (31)P-NMR spectra. In addition, the (motionally averaged) magnetic interactions are greatly reduced, the actual values differing for both components. The second spectroscopic component refers to membrane components with high trichogin-OMe concentration and to strong lipid-trichogin-OMe interactions, as reflected by significant changes of the head-group orientation (toroidal model). At the same time, DMPC molecules exist with minor lipid-trichogin-OMe interactions, most probably in smaller liposomes, trichogin-OMe being embedded in a 'carpet-like' manner. Moreover, lipid ordering is generally reduced for the highly concentrated sample, which may result from fast lateral lipid motion along the curved bilayer surface.