Solubilization of phospholipids by chaotropic ion solutions.

Phosphatidylcholine (PC) dissolves in solutions of concentrated neutral trichloroacetate and tribromoacetate, anions known to be extremely chaotropic. Solubilization of egg yolk PC occurs in Na-trichloroacetate between 2 and 3 M and in Na-tribromoacetate between 1.5 and 2 M. The resulting optically clear solutions were found by gel exclusion chromatography to consist of micelles containing of the order of 10(2) lipid molecules which, according to 31P NMR, are undergoing rapid, isotropic motion. Dipalmitoyl-PC is not solubilized below its normal phase transition temperature, although it remains in solution if first dissolved above that temperature; transition temperature and enthalpy change are both drastically depressed in 3 M Na-trichloroacetate. Na-trichloroacetate (3 M) also dissolves sphingomyelin but not phosphatidylethanolamine, acidic phospholipids, or egg PC-cholesterol mixtures of more than 33% cholesterol. Destabilization of the lamellar phase of egg PC relative to the micellar phase is promoted by trichloroacetate binding to and intercalating between the lipid molecules. Trichloroacetate was found to bind to egg PC liposomes with an affinity constant of about 1 M-1, so that nearly 1:1 association between egg PC and trichloroacetate is predicted at the concentration at which solubilization occurs. A surface area dilation by an amount (50+%) consistent with formation of micelles was confirmed by surface tension isotherms of egg PC on 3 M Na-trichloroacetate. The chaotropic effect may synergize binding by reducing the energy cost of the exposure of hydrophobic portions of lipid that is necessitated by the small radius of curvature of micelles. Several chaotropic agents less potent than trichloroacetate and tribromoacetate did not solubilize egg PC.