The Hofmeister effect in relation to membrane lipid phase stability.

The phase behaviour of L-alpha-1-palmitoyl-2-oleoyl-phosphatidylethanolamine (POPE) was examined in aqueous dispersions containing a range of sodium salts. The phase properties of the lipid exhibited a graded response to the presence of simple anions analogous to that of the Hofmeister series encountered in the study of the solution properties of proteins. Salts early in the series (such as Na2SO4 and NaCl) gave rise to substantial decreases in the temperature (Th) of the transition from the lamellar liquid-crystal to inverted hexagonal phase (L alpha----HII) and small increases in the temperature (Tm) of the transition from the lamellar gel to lamellar liquid-crystal phase (L beta----L alpha). Salts towards the end of this series (such as NaI and NaSCN) led to increases in Th and decreases in Tm. Similar effects were seen in lipid dispersions containing a series of non-ionic co-solutes. In both cases, the relative efficiency of perturbation of the lipid phase properties reflected the relative ability of the anion or co-solute to influence the structure of the bulk water. X-ray diffraction measurements indicated that these effects were probably mediated through the ability of the co-solutes to bring about changes in the extent of the lipid/water interface.