Polarity profiles in oriented and dispersed phosphatidylcholine bilayers are different: An electron spin resonance study

A novel method was utilized to accurately measure thez- component of the nuclear hyperfine interaction tensor, A(zz), of a chain-labeled lipid, 16PC, and a headgroup-labeled lipid, dipalmitoylphosphatidyl-tempocholine (DPPTC), in macroscopically oriented dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) membranes, which were compared with the A(zz) values of the two labels in dispersions of the same lipids in the gel phase. We found that the A(zz) values of 16PC (DPPTC) in the oriented DPPC and DMPC bilayers are similar to 1 Gauss smaller (greater) than in the corresponding dispersions. These results indicate that the headgroup region is more polar in macroscopically oriented bilayers than in dispersions, whereas in the chain region, the order in polarity is reversed. This is consistent with previous results on partial molar volumes in the liquid-crystal phase. Differences in the morphology of the macroscopically oriented and dispersed bilayers, which might be responsible, are discussed. Nonlinear least-squares fits of the electron spin resonance spectra of DPPTC in DPPC show that there is a substantial orienting potential in the headgroup region of dispersions that is lipid phase dependent. However, in oriented membrane samples hydrated in 100% relative humidity, this orienting potential is very weak.