Abstract
We have investigated the influence of the chaotropic anions I −, SCN −, and Cl0 4 − on multilayer dispersions of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylethanolamine (DMPE) by spin label electron spin resonance (ESR) spectroscopy and by spectrophotometry. We find that on increasing the concentration of the anions up to 1 M, the spectral anisotropy of the nitroxide stearic acid spin labels spectra is decreased in the interfacial region and is increased in the deep interior of the hydrophobic zone of DPPC multilayers. This effect of the chaotropics on the DPPC lipid chain mobility is evident in the gel phase, disappears in the fluid phase, and is interpreted as interdigitation of the DPPC hydrocarbon chains. The chaotropic anions induce no changes in the lipid chain packing in DMPE multilayers. From a thermotropic point of view, the chaotropic anions on increasing concentration first lower the DPPC pre-transition temperature and then suppress the pre-transition for concentration ≥ 0.5 M. Moreover, high concentrations of the anions stabilize the fluid state of both DPPC and DMPE multilayers in the order C10 4 − > SCN −> I −, this latter effect being stronger in DMPE.
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