We have obtained proton (1H) “magic-angle” sample-spinning (MASS) nuclear magnetic resonance (NMR) spectra of a variety of smectic liquid crystalline phases, including sodium decanoate (30.1 wt %)-decanol (38.9 wt %)-water, potassium oleate (72 wt %)-water, and 1, 2-dimyristoyl-sn-glycero-3-phosphocholine (lecithin)(50 wt %)-water, in addition to investigating the effects of temperature and cholesterol (CHOL) addition on the lecithin spectrum. Our results indicate that even relatively slow (~3 kHz) MASS causes averaging of the dipolar interactions causing line broadening in the static NMR spectra, at least for the non-CHOL species. All of the major proton-containing groups are well resolved, the resolution being at least as good as obtained in previous studies of oriented samples or sonicated systems. The methylene chain protons in each liquid crystalline lipid bilayer system give rise to intense, sharp, spinning sidebands (SSBs) due to the special form of the dipolar Hamiltonian. The methyl groups of the lipids, and the trimethylammonium group in lecithin, do not yield intense SSB patterns. Addition of CHOL causes attenuation of the center-band methylene peak of the lecithin, and a corresponding increase in SSB intensity. All or nearly all of the non-CHOL protons present in the samples appear to contribute to the high-resolution spectra, within our experimental error of ~ 10-20%. Use of a chain-deuterated lecithin allows peaks arising from the side chain of CHOL to be observed. In the gel phase of lecithin, only the trimethylammonium peak is apparent. The high-resolution afforded by MASS of the liquid crystalline phases permits rapid determination of the spin-lattice relaxation times (T1) of all resolved resonances. In addition, the observation of numerous chemically shifted peaks permits the use of two-dimensional (2-D) NMR techniques, which can give information on the spatial proximity of the various groups in the bilayer. Taken together, our results indicate a very promising future for high-field 1H MASS NMR studies of other lipid and membrane systems because of the extremely high sensitivity of the 1H nucleus and the unique ability to obtain chemical shift, T1 and 2-D information from a single sample, without recourse to isotopic labeling, macroscopic sample orientation, or ultrasonic irradiation.
ASJC Scopus subject areas
- Colloid and Surface Chemistry