Carbon-13 Nuclear Magnetic Resonance Spectroscopy of Lipids: Differential Line Broadening Due to Cross-Correlation Effects as a Probe of Membrane Structure

We have obtained proton-coupled carbon-13 nuclear magnetic resonance (NMR) spectra of a variety of lipid-water and lipid-drug-water systems, at 11.7 T, as a function of temperature, using the “magic-angle” sample-spinning (MAS) NMR technique. The resulting spectra show a wide range of line shapes, due to interferences between dipole-dipole and dipole-chemical shielding anisotropy interactions. The differential line-broadening effects observed are particularly large for aromatic and olefinic (sp²) carbon atom sites. Coupled spectra of the tricyclic antidepressants desipramine and imipramine, in 1,2-dimyristoyl-sn-glycero-3-phosphocholine-water mesophases, show well-resolved doublets having different line shapes for each of the four aromatic methine groups, due to selective averaging of the four C-H dipolar interactions due to rapid motion about the director (or drug C₂) axis. ²H NMR spectra of [2,4,6,8-²H₄]desipramine (and imipramine) in the same 1,2-dimyristoyl-sn-glycero-3-phosphocholine-water mesophase exhibit quadrupole splittings of ~0-2 and ~20 kHz, indicating an approximate magic-angle orientation of the C2-²H(¹H) and C8-²H(¹H) vectors with respect to an axis of motional averaging, in accord with the ¹³C NMR results. Selective deuteration of imipramine confirms these ideas. Spectra of digalactosyl diglyceride [primarily 1,2-di[(9Z, 12Z, 15Z)-octadeca-9,12,15-trienoyl]-3-(α-D-galactopyranosyl-1-6-β-d-galactopyranosyl)-sn-glycerol]-H₂O (in the L_α phase) show a large differential line broadening for C9 but a reduced effect for C10, consistent with the results of ²H NMR of specifically ²H-labeled phospholipids [Seelig, J., & Waespe-Šaračevi[formula-omitted], N. (1978) Biochemistry 17, 3310-3315], Thus, both desipramine and imipramine and the glycolipid show magic-angle orientation effects which reduce the amount of differential line broadening observed with other C-H vector orientations. In monogalactosyl diglyceride [primarily 1,2-di[(9Z, 12Z, 15Z)-octadeca-9,12,15-trienoyl]-3-β-D-galactopyranosyl-sn-glycerol]-H₂O (in the H_II phase), similar differential line-broadening effects are found for C9,10; C12,13, and C15,16, upon cooling. The resonances of C9 and C10 broaden before those of C12,13, which in turn broaden before those of C15,16. C10 is narrower than C9, and has less differential broadening, consistent with a magic-angle orientation. Computer simulations of the low-temperature spectra of monogalactosyl diglyceride (at −30 °C) using chemical shift and intensity values from the high-temperature spectra permit determination of individual component line widths, even in spectra showing limited overall resolution. Each of the six olefinic carbons (in the mainly linolenoyl chains) exhibits differential line broadening. The good qualitative agreement between ^l3C and ²H NMR results suggests that useful orientational (²H NMR like) information can be deduced from natural-abundance ¹³C NMR spectra of a variety of mobile solids.