Interactions of calcium (Ca) and zinc (Zn) cations with biomimetic membranes made of dipalmitoylphosphatidylcholine (DPPC) were studied by small angle neutron diffraction (SAND). Experiments show that the structure of these lipid bilayers is differentially affected by the two divalent cations. Initially, both Ca and Zn cause DPPC bilayers to thicken, while further increases in Ca concentration result in the bilayer thinning, eventually reverting to having the same thickness as pure DPPC. The binding of Zn, on the other hand, causes the bilayers to swell to a maximum thickness, and the addition of more Zn does not result in a further thickening of the membrane. Agreement between our results obtained using oriented planar membranes and those from vesicular samples implies that the effect of cations on bilayer thickness is the result of electrostatic interactions, rather than geometrical constraints due to bilayer curvature. This notion is further reinforced by MD simulations. Finally, the radial distribution functions reveal a strong interaction between Ca and the phosphate oxygens, while Zn shows a much weaker binding specificity.

Interactions of the divalent cations Ca2+ and Mg2+ with the zwitterionic lipid bilayers prepared of a fully saturated dipalmitoyl­phosphatidyl­choline (DPPC) or a di-monounsaturated dioleoyl­phosphatidyl­choline (DOPC) were studied by using the neutron scattering methods and molecular dynamics simulations. The effect on the bilayer structural properties confirms the direct interactions in all cases studied. The changes are observed in the bilayer thickness and lateral area. The extent of these structural changes, moreover, suggests various mechanisms of the cation–lipid interactions. First, we have observed a small difference when studying DPPC bilayers in the gel and fluid phases, with somewhat larger effects in the former case. Second, the hydration proved to be a factor in the case of DOPC bilayers, with the larger effects in the case of less hydrated systems. Most importantly, however, there was a qualitative difference between the results of the fully hydrated DOPC bilayers and the others examined. These observations then prompt us to suggest an interaction model that is plausibly governed by the lateral area of lipid, though affected indirectly also by the hydration level. Namely, when the interlipid distance is small enough to allow for the multiple lipid–ion interactions, the lipid–ion–lipid bridges are formed. The bridges impose strong attractions that increase the order of lipid hydrocarbon chains, resulting in the bilayer thickening. In the other case, when the interlipid distance extends beyond a limiting length corresponding to the area per lipid of ∼65 Å2, Mg2+ and Ca2+ continue to interact with the lipid groups by forming the separate ion–lipid pairs. As the interactions proposed affect the lipid membrane structure in the lateral direction, they may prove to play their role in other mechanisms lying within the membrane multicomponent systems and regulating for example the lipid–peptide–ion interactions.

An analysis of the molecular dynamics of ethanol solvated by water molecules in the absence and presence of a Pt surface has been performed using DL_POLY_2.19 code. The structure and diffusion properties of an ethanol–water system have been studied at various temperatures from 250 to 600 K. We have measured the self-diffusion coefficients of the 50:50% ethanol–water solution; in the absence of a Pt surface our results show an excellent agreement–within an error of 7.4% – with the experimental data. An increase in the self-diffusion coefficients with the inclusion of a Pt surface has been observed. The estimation of the diffusion coefficients of both water and ethanol in the presence of a Pt surface shows that they obey the Arrhenius equation; the calculated activation energies of diffusion of ethanol and water are 2.47 and 2.98 Kcal/mole, respectively. The radial distribution function graphs and density profiles have been built; their correlations with the self-diffusion coefficients of both ethanol and water molecules are also illustrated

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