EFFECT OF INORGANIC CATIONS ON PHASE-TRANSITIONS

The effect of protons and cations on the crystal (gel)-to-liquid crystal transition temperature T(m) of isoelectric and negatively charged phospholipids are summarized. The general trends emerging are as follows: T(m) depends on the state of ionization of the phospholipid in that T(m)-vs-pH-curves parallel the titration curve of the phospholipid. Protonation of phospholipids causes T(m) to increase, deprotonation or ionization has the opposite effect. The effects of cations on the T(m) of phospholipids may be grouped into non-specific and specific effects. Unspecific effects of cations such as the screening of negative charges of the phospholipid polar group are qualitatively similar to protonation: T(m) increases, in the order monovalent < divalent < trivalent cations and the effects on negatively charged phospholipids are larger than those on isoelectric phospholipids. Unspecific, electrostatic effects on T(m) are reasonably well accounted for by the Gouy-Chapman theory. If, however, specific binding comes into play and/or electrostatic effects are accompanied by changes in phospholipid structure, simple, electrostatic theories fail to explain the observed changes in T(m). The crystal (gel)-to-liquid crystal transition is also a function of the degree of hydration: T(m) generally decreases with increasing hydration reaching a plateau in excess H2O. In addition to screening of electric charges, ions may exert yet another non-specific effect: ions may affect T(m) indirectly by competing with the phospholipid polar group for water of hydration. This indirect effect plays a role at high ionic strength and/or at low hydration of the phospholipid. Specific binding of cations to negatively charged phospholipids can lead to tight associations of the metal ion with the lipid polar group. Isothermal crystallization of the phospholipid bilayer is induced that is accompanied by a total or partial loss of water of hydration resulting in a marked increase in T(m). For instance, in crystalline Ca2+ -phosphatidylserine complexes T(m) is increased by more than 100-degrees-C.