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 t he nuclear hyperfine interaction tensor, A(zz), of a chain-labeled lip id, 16PC, and a headgroup-labeled lipid, dipalmitoylphosphatidyl-tempo choline (DPPTC), in macroscopically oriented dipalmitoylphosphatidylch oline (DPPC) and dimyristoylphosphatidylcholine (DMPC) membranes, whic h 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 o f 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 macroscopi cally oriented bilayers than in dispersions, whereas in the chain regi on, the order in polarity is reversed. This is consistent with previou s results on partial molar volumes in the liquid-crystal phase. Differ ences in the morphology of the macroscopically oriented and dispersed bilayers, which might be responsible, are discussed. Nonlinear least-s quares fits of the electron spin resonance spectra of DPPTC in DPPC sh ow that there is a substantial orienting potential in the headgroup re gion of dispersions that is lipid phase dependent. However, in oriente d membrane samples hydrated in 100% relative humidity, this orienting potential is very weak.