EDITING C-13-NMR SPECTRA OF MEMBRANES

We report the carbon-13 'magic-angle' sample-spinning nuclear magnetic resonance (NMR) spectra of several lipid-water systems, under a variety of radiofrequency excitation conditions. Our results show that complex lipid or membrane spectra can be greatly simplified by using 'spectral editing' techniques. For example, in a 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)-water mesophase, the glycerol (C-1, C-2 and C-3) carbons are readily distinguished from the headgroup C(alpha), C(beta) and C(gamma) carbons, on the basis of their mix-time behavior in a cross-polarization (CP) experiment, while in the more complex DMPC/cholesterol-water system, many of the more rigid cholesterol carbon resonances can be edited from the phospholipid peaks. In very complex systems, such as human myelin membranes, editing permits the unambiguous observation of the mobile lipid headgroup carbon resonances, as well as the much more rigid sterol ring carbons. We also report the observation of a large differential CP due to C-H vector 'magic-angle' orientational effects in the DMPC/desipramine system. Thus, both motional or orientational reduction of the C-H dipolar interaction can lead to considerable simplifications of complex membrane spectra, and are of interest from both spectral assignment and membrane dynamics aspects.