Interactions between phospholipid head groups at membrane interfaces: a deuterium and phosphorus nuclear magnetic resonance and spin-label electron spin resonance study.

The head group interactions in fully hydrated, mixed bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine and 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol, specifically deuterated in the head groups at the α- and β-methylene and N(CD3)3 positions, have been investigated by deuterium and phosphorus-31 nuclear magnetic resonance (NMR) and 2,2,6,6-tetramethylpiperidinyl-1-oxy (Tempo) spin-label electron spin resonance (ESR) studies at pH 7.5. In binary lipid bilayers, the two lipids were found to be completely miscible by spin-label partitioning and phosphorus-31 NMR experiments. Also the phosphorus-31 NMR studies showed no evidence that the mean orientation of the phosphate group of either lipid was significantly altered in the binary system. In contrast, large changes in the deuterium residual quadrupole splittings for the deuterated head group segments were observed by deuterium NMR for both phosphatidylcholine and phosphatidylglycerol when one lipid was titrated against the other. The quadrupole splittings for the choline methyls decreased from 1.4 kHz in pure phosphatidylcholine bilayers to 0.4 kHz for bilayers containing 75 mol % of phosphatidylglycerol at 28°C. A similar behavior was found for the head group β-CD2 segment; the quadrupole splittings were reduced from 5.8 to 1.1 kHz in the same range of concentration and temperature. A converse observation was made for the α-CD2 group in that the quadrupole splittings increased from 6.3 to about 10 kHz, and in mixed bilayers containing less than 50 mol % of phosphatidylcholine; magnetic inequivalence of the two deuterons gave two very well resolved NMR lines. In bilayers containing head group deuterated dimyristoylphosphatidylglycerol, the two α-CD2 deuterons showed quadrupole splittings of 9.0 and 10.8 kHz, which increased to 10.2 and 11.3 kHz, respectively, on adding 75 mol % of phosphatidylcholine at 28°C; for the β-CD the quadrupole splittings increased from 1.8 to 6.6 kHz for the same mixture ratio and temperature. The deuterium spin-lattice relaxation times T1 remained essentially unchanged for the α-CD2 and β-CD2 groups of dimyristoylphosphatidylcholine but increased for the choline methyls by about 20% when dimyristoylphosphatidylglycerol and dimyristoylphosphatidylcholine were mixed in a 1:1 mole ratio. The results indicate that some reorientations in the lipid head groups and changes in their amplitudes of motion are induced in the two-component bilayers by the presence of one lipid on the other but their rates of motion remain rather similar. © 1982 American Chemical Society