Motions and interactions of phospholipid head groups at the membrane surface. 2. Head groups with hydroxyl groups

A set of phospholipids with zero, one, or two hydroxyl groups at various positions in the head group were synthesized to approach the question of hydrogen bonding at the surface of phospholipid membranes. These lipids had as head groups the alcohols propanol, ethanediol, 1,3-propanediol, and glycerol esterified to the phosphate. The four different properties of these lipids that were studied were the following: phase transition temperatures, packing properties in monolayers, the relative rates of head-group motions, and the "ordering" of the head group. The gel-to-liquid-crystalline phase transition temperatures of these derivatives were measured by differential scanning calorimetry, and the effect of hydroxyl group addition was found to be small. The phase properties were examined with phosphorus-31 NMR, and all lipids formed normal bilayer phases in aqueous mixtures. Measurement of the surface pressure-area diagrams for monolayers of these lipids showed that incorporation of a hydroxyl group into the head group had a "condensing" effect. This effect was dramatic and was attributed to the formation of hydrogen bonds within the plane of the lipid surface. These lipids were synthesized with deuterium labels on essentially every carbon segment in the head group. Measuring the 2H NMR spin-lattice (T1) relaxation time allowed a determination of the relative rates of head-group segmental motions. The addition of hydroxyl groups to the propyl head-group "skeleton" substantially reduced the rates of motion in both the liquid-crystalline and gel states. The head groups could be ordered in terms of increasing rigidity as propanol (0 -OH) less than 1,3-propanediol (1 -OH) less than ethanediol (1 -(OH) less than glycerol (2 -OH). This trend was observed for labels attached to all of the head-group carbon segments of the lipids in this set. The rates of motion of the different segments within the ethanediol or glycerol head groups were almost identical, whereas those of the propyl head group increased as one progressed toward the free end of the head group. The average activation energy for the motions involved in T1 relaxation increased upon introduction of a hydroxyl group. Measurement of the deuterium residual quadrupole splittings showed that the initial PO2--O--CD2-- segments of the hydroxyl-containing head groups have very similar ordering properties which differed distinctly from those of the propyl head group. Thus, the introduction of one hydroxyl group appears to alter the head-group conformation to a specific conformation which is shared by all of these hydroxyl group containing head groups.