Natural abundance carbon-13 nuclear magnetic resonance studies of bovine white matter and myelin.

Whole bovine white matter yields a poorly resolved natural abundance 13C nuclear magnetic resonance (NMR) spectrum. The spectrum principally reflects carbon atoms of the constituent membrane lipids: several resonances could be specifically assigned but no resonances attributable to cholesterol are detectable. Except for the methyl group at the terminus of fatty acyl chains, lipid carbons giving rise to the 13C NMR spectrum have values of spin-lattice relaxation time between 140 and 500 ms, indicating significant restrictions on segmental and rotational mibolities but consistent with a generally fluid structural organization. The 13C NMR spectrum of myelin isolated from bovine white matter is similar to that for the whole white matter itself. In both white matter and isolated meylin, the integrated intensities for several carbon atoms are considerably less than those for the same carbon atoms in total lipid extracts. The data for white matter and myelin are consistent with a model in which observed line broadening is due to restrictions in the amplitude of chain flexing rather than to severe restrictions on chain segmental motion. Failure to detect resonances of cholesterol ring system carbon atoms may reflect marked anisotropy of rotational reorientation.