Specific Ion Effects in Cholesterol Monolayers

Abstract

The interaction of ions with interfaces and, in particular, the high specificity of these interactions to the particular ions considered, are central questions in the field of surface forces. Here we study the effect of different salts (NaI, NaCl, CaCl2 and MgCl2) on monolayers made of cholesterol molecules, both experimentally (surface area vs. lateral pressure isotherms measured by a Langmuir Film Balance) and theoretically (molecular dynamics (MD) all-atomic simulations). We found that surface isotherms depend, both quantitatively and qualitatively, on the nature of the ions by altering the shape and features of the isotherm. In line with the experiments, MD simulations show clear evidences of specific ionic effects and also provide molecular level details on ion specific interactions with cholesterol. More importantly, MD simulations show that the interaction of a particular ion with the surface depends strongly on its counterion, a feature ignored so far in most theories of specific ionic effects in surface forces.

Insights into the structure and nanomechanics of a quatsome membrane by force spectroscopy measurements and molecular simulations

“…It is also worth noting that previous experimental and simulation work on pure Chol monolayers has shown a strong interaction between Cland Chol. 49 Here the simulations indicate that the addition of NaCl produces not only an incorporation of Clinside the bilayer but also an incorporation of Brinside the bilayer.…”

“…Ions alteration of the nanomechanics of lipid bilayers is a known effect; they have an important contribution to the membrane mechanical resistance, by enhancing the lateral packing, translated into a higher F b . 41,42,[46][47][48][49] To better understand the effect of ions into SQMs, we assessed the topographical and mechanical properties of the QS_H 2 O membrane before and after replacing the liquid environment from water to PBS/NaCl. Figure 4 shows the topography, F b maps and histograms for these two cases.…”

Condensation of Model Lipid Films by Cholesterol: Specific Ion Effects

“…Moreover, the irregular behavior in the ε 0 of the film shown in Table 4 is a consequence of the opposite effects induced by CHOL and Mg 2+ which compete at the interface. The lift of the isotherms corresponds to the transition from gaseous to condensed state, with the CHOL molecules oriented with the hydroxyl group pointing towards the water phase and a condensed packing of the hydrophobic body of the CHOL molecule [23]. The specific interaction of Mg 2+ with CHOL results in this condensation occurring at higher MMAs owing to a less condensed packing state and orientation ( Figure 6).…”

“…Therein, it was proved that the average area per molecule in mixtures was much lower than what would be expected from the individual components. Since then, the condensing effect and the ability of CHOL to induce ordering within the hydrocarbon chain of many lipid films, has been extensively characterized using Langmuir films [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. For instance, this technique has been used to bring new insight into the functional role of CHOL in milk polar lipids, used in the preparation of dairy emulsions, e.g., infant milk formulas.…”

“…Moreover, in both monolayers and bilayers, the interaction of lipids with ions depends on the ion nature, on the lipid headgroups, and also on the degree of unsaturation of the lipid hydrocarbon chain [37][38][39][40][41][42][43][44]. In this sense, we recently studied the effect of different salts (NaI, NaCl, CaCl 2 , and MgCl 2 ) on monolayers made of CHOL molecules, both experimentally and theoretically [23]. In particular, we performed experiments of surface area (A) vs. lateral pressure (π) isotherms measured by a Langmuir Film Balance together with molecular dynamics (MD) all-atomic simulations.…”