Raft-Like Phase Coexistence Revealed by Solid-State Carbon-13 Separated Local Field Mas NMR

Abstract

Molecular interactions such as hydrogen-bonding and hydrophobic matching are observed for raft-like membranes using solid-state NMR [1]. We applied natural abundance separated-local field magic angle spinning 13C NMR to study the components of a canonical raft-like membrane system. This method probes lipid structure at the headgroup, glycerol backbone, and acyl chains simultaneously for each membrane component. We resolved isotropic 13C NMR chemical shifts and 13C-1H residual dipolar couplings (RDCs) for single-component, binary, and ternary membranes. These measurements are interpreted using a mean-torque model providing cross-sectional areas per lipid and volumetric hydrocarbon thicknesses [2]. The structural changes are related to mixing or de-mixing of lipids and cholesterol. In the single-component membranes, EYSM is highly ordered from hydrogen-bonding at the backbone, while POPC is highly disordered due to acyl chain unsaturation. The combination of these lipids leads to a reduction of EYSM order and an increase in POPC order due to inter-chain contacts. Addition of cholesterol yields a significant increase in hydrophobic matching in POPC and EYSM membranes [3]. From these structural results for single-component and binary systems, we evaluated the interactions contributing to lipid mixing in the ternary membrane system. EYSM exhibits a cross-sectional area and hydrocarbon thickness resembling the single-component system. In contrast, POPC exhibits a cross-sectional area and hydrocarbon thickness characteristic of the binary lipid-cholesterol mixture. This suggests a liquid-ordered phase is present in an ideally mixed ternary system. Our experimental results contribute to our understanding of raft-like membrane self-assembly, and can be used to interpret changes in membrane structure that occur during protein recognition events and membrane fusion. [1] T. Bartels et al. (2008) JACS 130 14521-14532. [2] H.I. Petrache et al. (2000) Biophys. J. 79, 3172-3192. [3] G.M. Martinez et al. (2004) Langmuir 20 1043-1046.