Lipid chain motion in an interdigitated gel phase: conventional and saturation transfer ESR of spin-labeled lipids in dipalmitoylphosphatidylcholine-glycerol dispersions
The lipid chain dynamics in the interdigitated gel phase of dipalmitoylphosphatidylcholine (DPPC) dispersed in glycerol and in the fully hydrated noninterdigitated gel phase in aqueous buffer were compared by using conventional and saturation transfer electron spin resonance (ESR) spectroscopy. Twelve different positional isomers of phosphatidylcholine spin-labeled in the sn-2 chain were used to characterize the chain motion. The outer hyperfine splittings of the conventional ESR spectra and the line height ratios at the diagnostic spectral positions in the saturation transfer ESR spectra were taken as indices of the rotational mobility of the labeled chain segments in the gel phase (0-40-degrees-C). The conventional spin label ESR spectra revealed a gradient of increasing mobility on proceeding down the chain toward the terminal methyl end in the fully hydrated DPPC gel phase bilayer structure. This gradient was absent in the interdigitated gel phase, i.e., the rotational mobility throughout the length of the lipid chain was comparable to that near the polar interface, on the conventional ESR time scale. Values of the outer hyperfine splitting for spin labels at the 5- and 14-C atom positions in the chain were 65.5 and 61.0 G in buffer, respectively, and 67.0 G for both positions in glycerol, at 0-degrees-C. At 35-degrees-C, still in the gel phase, these differences between the two systems were much greater. Saturation transfer ESR measurements revealed that the motion throughout the chain was restricted on the microsecond time scale in the interdigitated phase. The motional anisotropy was less than in the normal gel phase, and the onset of rapid long axis rotation at the pretransition of bilayers in water was absent in the presence of glycerol. Effective correlation times for long axis rotation recorded by the 5-position spin label were approximately 20-25 As in both systems at 5-degrees-C and 10 and 2 mus in glycerol and water, respectively, at 35-degrees-C. The effective rotational correlation times for the off-axial chain motion were considerably longer (ca. 40-70 mus) and were much less affected by temperature. There was no evidence for chain interdigitation in the fluid phase (43-50-degrees-C) of DPPC dispersed in glycerol. The conventional spin label ESR spectra indicated a chain flexibility gradient similar to that obtained in normal liquid-crystalline lipid bilayers, although the segmental mobility was uniformly reduced throughout the chain in the presence of glycerol, relative to the DPPC bilayers in water.