Rotating-frame relaxation studies of slow motions in fluorinated phospholipid model membranes

Rotating-frame relaxation experiments have been carried out on 19F-labeled dimyristoylphosphatidylcholine model membranes. The lipids are labeled with a single CF2 group in the 4-, 8-, or 12-position of the 2-acyl chain. Both oriented lipid bilayers and multilamellar liposomes have been investigated. The relaxation rate has been measured as a function of the locking-field strength, the sample orientation, the label position, and the temperature. Our results have confirmed that extensive slow motions exist in the bilayer and dominate the low-frequency relaxation. The relaxation rate is quite sensitive to the label position. However, many other features of the relaxation are very similar for all three lipid isomers. The temperature dependence of the relaxation rate for the multilamellar liposomes differs from the oriented bilayers, which may imply that the motions are also different. To fit our data, a working model consisting of a superposition of an anisotropic reorientation term and a director fluctuation term has been proposed. We have also verified that almost all of the relaxation process is caused by modulations of the intramolecular interactions. Based on this, a view of the slow motions at a molecular level is discussed in this paper.