A determination of the mobility gradient in lipid bilayers by 13 C nuclear magnetic resonance

Abstract

The spin-lattice relaxation times of individual $^{13}$C nuclei in phospholipid bilayers have been measured by using both natural-abundance $^{13}$C and selectively $^{13}$C-enriched phospholipids. These relaxation times can be analysed to give values for the diffusion coefficients for motion about individual carbon-carbon bonds, and for motion about the long axis of the hydrocarbon chains in the bilayer. The assumptions involved in this analysis are examined, and the values obtained compared with those for n-alkanes, and with the results of studies of molecular motion in lipid bilayers by other methods. The results indicate that motion about carbon-carbon bonds in phospholipid bilayers is relatively rapid, and emphasize the fluid nature of the hydrocarbon chain region of such bilayers. This picture of the bilayer is compared with that obtained by techniques which measure time-average conformational properties of the hydrocarbon chains in the bilayer.