High resolution proton relaxation studies of lecithins.

Abstract 1. 1. Spin-lattice ( T 1 ) proton relaxation data have been obtained for lecithins in solution in methanol, in micelles in [ 2 H]chloroform, and in bilayers in 2 H 2 O. The relaxation times are characteristic of the type of structure formed, and reflect molecular motion within the lecithin molecule in each structure. 2. 2. The data are not consistent with a single T 1 for all the protons in the lecithin bilayer in 2 H 2 O and spin diffusion is not a dominant relaxation mechanism. 3. 3. Sonicated aqueous suspensions of dipalmitoyl lecithin show a striking set of spectral changes through the transition temperature at 43° in which the fatty acid chain resonances disappear completely and the −N(CH 3 ) 3 + resonance undergoes a sharp increase in linewidth. These spectral changes through the transition are entirely reversible and do not involve a significant change in vesicle size. 4. 4. The T 1 values for the major resonances of lecithin in bilayers are consistent with a structure, based on 13 C T 1 measurements, in which the bilayer is most tightly packed at the glycerol region of the lecithin molecules, with increasing freedom of motion both towards the terminal methyl of the alkyl chain and the −N(CH 3 ) 3 + group at the surface of the bilayer. 5. 5. Cholesterol perturbs the bilayer structure and produces changes in the T 1 values of the lecithin resonances, consistent with the effect on the permeability of the lecithin vesicle.