250-GHZ ELECTRON-SPIN-RESONANCE STUDIES OF POLARITY GRADIENTS ALONG THE ALIPHATIC CHAINS IN PHOSPHOLIPID-MEMBRANES

Rigid-limit 250-GHz electron spin resonance (FIR-ESR) spectra have bee n studied for a series of phosphatidylcholine spin labels (n-PC, where n = 5, 7, 10, 12, 16) in pure lipid dispersions of dipalmitoylphospha tidylcholine (DPPC) and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) , as well as dispersions of DPPC containing the peptide gramicidin A ( GA) in a 1:1 molar ratio. The enhanced g-tensor resolution of 250-GHz ESR for these spin labels permitted a careful study of the nitroxide g -tensor as a function of spin probe location and membrane composition. In particular, as the spin label is displaced from the polar head gro up, A(zz) decreases and g(xx) increases as they assume values typical of a nonpolar environment, appropriate for the hydrophobic alkyl chain s in the case of pure lipid dispersions. The field shifts of spectral features due to changes in g(xx) are an order of magnitude larger than those from changes in A(zz). The magnetic tenser parameters measured in the presence of GA were characteristic of a polar environment and s howed only a very weak dependence of A(zz) and g(xx) on label position . These results demonstrate the significant influence of GA on the loc al polarity along the lipid molecule, and may reflect increased penetr ation of water into the alkyl chain region of the lipid in the presenc e of GA. The spectra from the pure lipid dispersions also exhibit a br oad background signal that is most significant for 7-, 10-, and 12-PC, and is more pronounced in DPPC than in POPC. It is attributed to spin probe aggregation yielding spin exchange narrowing. The addition of G A to DPPC essentially suppressed the broad background signal observed in pure DPPC dispersions.