Motions and interactions of phospholipid head groups at the membrane surface. 3. Dynamic properties of amine-containing head groups.

The dynamic properties of the amine-containing head groups of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and a new phospholipid, phosphatidylserine methyl ester, were studied. Deuterium NMR spin-lattice (T1) relaxation times of deuterium labels specifically incorporated in the head groups were measured in multilamellar dispersions of these phospholipids. As a reference point, the data were compared with T1 values from a phospholipid with a simple phosphopropanol head group. In the liquid-crystalline state at both the alpha (P--O--CD2--CH) and beta (P--O--CH2--CD) head-group segments, the values of the T1 relaxation times decreased in the order propyl greater than choline greater than ethanolamine greater than serine = serine methyl ester. In the propyl and choline head groups, the beta-segment T1 values were longer than those of the alpha segment, indicating increasing flexibility as one progressed toward the free end of the head group. The ethanolamine and serine head groups had essentially identical T1 values at all positions. Phosphorus-31 NMR spin-lattice relaxation times were found to parallel the deuterium results for the neighboring alpha segment. These data indicate that the phosphatidylserine head group is less flexible than that of phosphatidylethanolamine which in turn was more rigid than the phosphatidylcholine head group. The phosphatidylserine head-group T1 values were as short as those of the glycerol backbone moiety of phosphatidylcholine which is known to be a relatively rigid section of the phospholipid molecule. The relaxation time data for liquid-crystalline phase phosphatidylserine and gel phase phosphatidylglycerol were quantitatively similar, indicating that for those motions which contribute to T1 relaxation, the two head groups are in similar states. These differences in head-group rigidity are discussed in terms of the capacity of the various head groups to bind noncovalently to their neighbors in the plane of the membrane surface.