The saturation transfer electron spin resonance (STESR) spectra of spin-labeled phosphatidylcholines in gel phase lipid bilayers are shown to be sensitive to dipolar spin-spin interactions with paramagnetic ions in the aqueous phase. The reciprocal integrated intensity of the STESR spectrum is linearly dependent on aqueous Ni2+ ion concentration, hence, confirming the expectation that the STESR intensity is directly proportional to the spin-lattice relaxation time of the spin label. The gradient of the relaxation rate with respect to Ni2+ ion concentration decreases strongly with the position of the nitroxide group down the sn-2 chain of the spin-labeled lipid and is consistent with a 1/R3 dependence on the distance, R, from the bilayer surface. The values derived for the dimensions of the bilayer and lipid molecules in the case of dipalmitoyl phosphatidylcholine (DPPC) are in good agreement with those available from x-ray diffraction studies. Allowance for the multibilayer nature of the DPPC dispersions gives an estimate of the water layer thickness that is also consistent with results from x-ray diffraction. The profile of the paramagnetic ion-induced relaxation is drastically changed with DPPC dispersions in glycerol for which the lipid chains are known to be interdigitated in the gel phase. The terminal methyl groups of the lipid chains are located approximately in register with the C-3 atoms of the sn-2 chain of the oppositely oriented lipid molecules in the interdigitated phase. The thickness of the lipid layer and the effective thickness of the lipid polar group are reduced by ∼40% in the interdigitated phase as compared with the bilayer phase. The calibrations of the distance dependence established by use of spin labels at defined chain positions should be applicable to STESR measurements on other biological systems.
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