NEUTRAL LIPID BILAYERS INTERACTING WITH CHAOTROPIC ANIONS

We have investigated the influence of the chaotropic anions I-, SCN-, and ClO4- on multilayer dispersions of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylethanolamine (DMPE) by spin label el ectron 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 i nterior of the hydrophobic zone of DPPC multilayers. This effect of th e chaotropics on the DPPC lipid chain mobility is evident in the gel p hase, disappears in the fluid phase, and is interpreted as interdigita tion of the DPPC hydrocarbon chains. The chaotropic anions induce no c hanges in the lipid chain packing in DMPE multilayers. From a thermotr opic point of view, the chaotropic anions on increasing concentration first lower the DPPC pre-transition temperature and then suppress the pre-transition for concentration greater than or equal to 0.5 M. Moreo ver, high concentrations of the anions stabilize the fluid stale of bo th DPPC and DMPE multilayers in the order ClO4- > SCN- > I-, this latt er effect being stronger in DMPE.