Grazing Incidence Diffraction and X-ray Reflectivity Studies of the Interactions of Inorganic Mercury Salts with Membrane Lipids in Langmuir Monolayers at the Air/Water Interface

Abstract

The interactions of mercury ions with the membrane phospholipids are considered to be of great importance regarding the toxicity of this metal in living organisms. To obtain deeper insight into this problem, we performed systematic studies applying the Langmuir technique complemented with synchrotron X-ray scattering methods (grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity (XR)). We focused our attention on the interactions of inorganic mercury salts dissolved in the aqueous subphase with lipid monolayers, formed by selected membrane phospholipids, namely, dipalmitoylphosphatidylglicerol (DPPG), dipalmitoylphosphatidylcholine (DPPC), 1-octadecyl 2-sn-phosphatidylcholine (lyso-PC), and sphingomyelin (SM). Two different inorganic mercury salts, one of a hydracid, HgCl(2), and the other of an oxacid, Hg(NO(3))(2), have been investigated. Our results proved that the elastic properties of phospholipid monolayers are a key factor regarding the interactions with mercury ions. Significant differences in mercury ions complexation are observed with double-chain phospholipids (such as DPPG and DPPC) forming fluid layers of low compressibility and phospholipids forming more compressible films (like SM and lyso-PC). Namely, important changes in the monolayer characteristic were observed only for the latter kind of lipids. This is an important finding taking into account the accumulation of mercury in the central nervous system and its neurotoxic effects. SM is one of the most abundant lipids in neurons shells and therefore can be considered as a target lipid complexing mercury ions.

Iron Ion and Iron Hydroxide Adsorption to Charge-Neutral Phosphatidylcholine Templates

“…that is spread as a Langmuir monolayer (ML) at the vapor/solution (100 mM KCl and sub-mM FeCl 3 ) interface at low pH values at which the phosphate group is protonated, we employ surface sensitive X-ray diffraction and spectroscopic techniques [14][15][16][17][18][19] . Similar studies have been reported on various divalent ions with PC-based lipid monolayers albeit at relatively high salt concentrations (∼ 0.1 M; much higher than those in living cells), 20,21 showing changes in π − A isotherms but insignificant structural effects in the X-ray reflectivity or grazing incidence diffraction (GIXD). This is not surprising considering a recent moleculardynamic simulations that show strong competition between cation binding to PC and to water molecules.…”

Evaluating polymeric biomaterial–environment interfaces by Langmuir monolayer techniques

“…Frequency changes provide information about the molecular structure and interactions, whereas the quantitative evaluation of the measured band intensities allows the determination of the chain and group orientations. Grazing incidence X-ray diffraction provides information only about ordered domains within the monolayer [63,64], whereas specular X-ray reflectivity (XR) supplies access to the electron density distribution across the layer, allowing speculation about the monolayer profile constitution.…”

Biomimetic Model Membrane Systems Serve as Increasingly Valuable in Vitro Tools

“…With as little as 10 -7 M for Hg 2+ and 10 -8 M for Cd 2+ , these heavy metals were not only observed to expand the monolayer but C 1/2 values calculated (the metal ion concentration giving half of the maximum pressure change) showed a linear correlation between logarithms of C 1/2 values and logarithms of the acute lethal doses of the metal chlorides in rabbits or rats (Suzuki and Matsushita, 1969). Single component lipid monolayers of DPPG, DPPC, lyso-PC, and SM have also been utilized to observe interactions of Hg ions with membrane phospholipids (Broniatowski et al, 2010). In the presence of 500 μM HgCl 2 in the aqueous subphase, mercury ions were observed to interact more strongly to SM and lyso-PC monolayers.…”