Ion‐Binding to Phospholipids

Abstract

The binding of Ca2+ to monolayers and bilayers of phosphatidylserine has been investigated as a function of pH, ionic strength (NaCl concentration) and Ca2+ concentration using surface and colloid chemical techniques. The molar ratio of lipid to bound calcium decreases to 2 as the Ca2+ concentration is increased to about 0.1 mM. At [Ca2+] > 0.1 mM a 1:1 complex is formed. The apparent binding constant Ka ranges from about ∼ 106‐−104 l/mol depending on the Ca2+ concentration. After allowing for electrostatic effects and neighbour group interactions, the intrinsic binding constant Ki of the phosphorylserine polar group at pH 7 (I= 0.01 M), where it carries a net negative charge of one, is ∼ 104 l/mol; consistent values for Ki were obtained using several independent approaches. Ka for Ca2+ binding decreases with increasing NaCl concentration because the monovalent cations compete with Ca2++ for the same binding site. Na+ and K+ are equally effective in displacing 45Ca2+ adsorbed to monolayers of phosphatidylserine, both with respect to the kinetics and the equilibrium of the displacement. Ka for the reaction between phosphatidylserine and monovalent cations is about 103‐fold smaller than that of Ca2+. An investigation of the binding of Mn2+ to phosphatidylserine by both surface chemical and nuclear magnetic resonance methods shows that this cation has a similar binding constant to that of Ca2+. The Ca2+ ‐binding capabilities of monolayers containing only carboxyl groups (i.e. arachidic acid) and phosphodiester groups (i.e. dicetyl phosphate) have also been determined; the apparent pK for the –COOH group in monolayers is ⋧ 9 and that for the phosphodiester group is < 4. Since these groups do not retain the same pK values when they are in close proximity in the phosphorylserine group, the relative contributions of the two groups to the binding of Ca2+ to phosphatidylserine, is not obvious.

Lipid Spontaneous Curvatures Estimated from Temperature-Dependent Changes in Inverse Hexagonal Phase Lattice Parameters: Effects of Metal Cations

“…While there is some evidence of this trend in the literature 42 , the presence of metal cations can confound this effect. The presence of monovalent cations will act to screen headgroup charge 43 and divalent cations that associate strongly with the headgroups will promote the formation of DOPS dimers, bridged by the divalent metal, which are strongly type II lipids 44 . These competing mechanisms, underpinned by association constants of different magnitudes, generic to anionic lipids, have been reported for systems like DOPA 20 and 1,2-dioleoyl-sn-glycero-3-phosphoglycerol (DOPG) 45 .…”

Interaction of polysilicic acid with monolayers of substances containing quaternary ammonium groups

“…Further evidence that the interaction between silicic acid and the components of the lysosome membrane of lung macrophage is ionic in nature is provided by the fact that dipalmitoyl phosphatidyl serine fails to interact with silicic acid at pH either above or below the dissociation point of its carboxyl group, pH 4.2 [26][27][28]. The lack of interaction at pH < 4.2 shows that the -COOH group is not involved in hydrogen bonds with silicic acid; while at pH > 4.2 comparison with lecithin (which does interact [29]) suggests that the difference in behaviour between the two phospholipids must be due to the -CO0-group preventing electrostatic interaction of silica with the -NH~-group of phosphatidyl serine, as was suggested by Depasse [15].…”

Water relations of tetrapod integument

“…Driven by the transmembrane gradient of water concentration energy, water then diffuses (exchanges) between the various hydration shells of the polar group into the hydrocarbon core (Hauser and Phillips, 1979). Divalent cations potentially displace water molecules by competing for interaction with the negatively charged phospholipid groups (Hauser et al, 1976).…”