An Empirical Relation for the Surface Potential of Phosphatidic Acid Monolayers: Its Dependence on Calcium and the Role of Double Layer Theory
The physical chemistry of the phospholipid-aqueous interface has been a subject of interest for many years. The comparative ease with which phospholipid monolayers, bilayers, and vesicles can be prepared, along with their unique properties, has made them attractive as models for biomembranes. The actual “work” that biomembranes do depends invariably on the specific nature of their integral proteins. However, these are thought to be modulated in function, to some extent, by the phospholipid backbone comprising the basic bilayer structure. The relative fluidity of the phospholipid matrix is regarded as important in this respect. Accordingly the crystal-liquid crystal phase transition of phospholipids has been much studied (e.g. Trauble and Eibl, 1974; Jacobson and Papahadjopoulos, 1975; Galla and Sackmann, 1975; MacDonald et al., 1976; Träuble et al, 1976; Disalvo, 1983). In the case of acidic phospholipids, an isothermal phase transition from the crystal to the liquid-crystal state can be induced by increasing the monovalent cation concentration, or by increasing the pH. An increase in the concentration of divalent ions has the opposite effect. Gouy-Chapman double layer theory has often been invoked to advance an explanation of these phenomena (e.g. the fluidizing effect of monovalent cations on the acidic phospholipids in the protonated state, c.f. Trauble and Eibl, 1974).
KeywordsDivalent Cation Surface Potential Surface Charge Density Film Condensation Acidic Phospholipid
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