High-Affinity Interactions of Beryllium(2+) with Phosphatidylserine Result in a Cross-Linking Effect Reducing Surface Recognition of the Lipid

Electric Fields at the Lipid Membrane Interface

This review presents a comprehensive analysis of electric field distribution at the water-lipid membrane interface in the context of its relationship to various biochemical problems. The main attention is paid to the methodological aspects of bioelectrochemical techniques and quantitative analysis of electrical phenomena caused by the ionization and hydration of the membrane-water interface associated with the phase state of lipids. One of the objectives is to show the unique possibility of controlling changes in the structure of the lipid bilayer initiated by various membrane-active agents that results in electrostatic phenomena at the surface of lipid models of biomembranes-liposomes, planar lipid bilayer membranes (BLMs) and monolayers. A set of complicated experimental facts revealed in different years is analyzed here in order of increasing complexity: from the adsorption of biologically significant inorganic ions and phase rearrangements in the presence of multivalent cations to the adsorption and incorporation of pharmacologically significant compounds into the lipid bilayer, and formation of the layers of macromolecules of different types.

The dubious origin of beryllium toxicity

Four mechanisms have been proposed in the literature to explain beryllium toxicity; they can be divided in two groups of two mechanisms: (i) replacement type: models 1 and 2; (ii) addition type: models 3 and 4. At this moment is not possible to select the best model not even to establish if one of these models will be the ultimate mechanism of beryllium toxicity. However, it is important to know the still open discussion about something so important associated with one of the simplest elements of the periodic table.

Environmental chemistry response of beryllium to diverse soil-solution conditions at a waste disposal site

This study evaluated how the variation in different sorption conditions of beryllium (Be) in soil-water systems (electrolytes; ionic strengths; competing, counter, and co-existing ions; concentrations of Be and soil; and temperature) affected Be's environmental behaviour. For this reason, potentially contaminated soil was collected from a legacy waste site near Sydney, Australia. The sorption-desorption plateau for Be was found at >12.5 g L^-1 (soil/solution), considering higher sorption and limited desorption. Variable surface charges developed by different added ions (competing ions, counter ions, and co-existence of all ions) were not always correlated with Be sorption. However, effects of added ions in Be sorption (increased by counter ions and decreased by competing ions) primarily occurred at low pH, with no noticeable changes at pH > 6 due to the hydration and precipitation behaviour of Be at higher pH. Both laboratory data and modelling indicated the substantial effect of counter ions on increased sorption of Be. Relatively higher amounts of sorption under the co-existence of all added ions were suggested from synergistic actions. Sorption was favourable (K_L > 0, and 0 < R_L < 1) across all concentrations and temperatures at pH 5.5, and high retention (84-97%) occurred after four desorption cycles indicated specific sorption. The sorption process was exothermic (ΔH > -43 kJ mole^-1), while desorption was endothermic (ΔH > +78.4 kJ mole^-1). All sorption-desorption reactions were spontaneous (ΔG = -Ve), and executed without any structural deformation (ΔS = nearly zero) of soil particles. However, the effect of temperature on desorption was influenced by the concentrations of Be. Higher retention and different sorption-desorption parameters (K_d-desorption > K_d-sorption; K_f-desorption > K_f-sorption; n_desorption/n_sorption < 1) indicate limited mobility of Be and the presence of desorption hysteresis in the studied soil under the experimental conditions.

Heterogeneity in Lateral Distribution of Polycations at the Surface of Lipid Membrane: From the Experimental Data to the Theoretical Model

Natural and synthetic polycations of different kinds attract substantial attention due to an increasing number of their applications in the biomedical industry and in pharmacology. The key characteristic determining the effectiveness of the majority of these applications is the number of macromolecules adsorbed on the surface of biological cells or their lipid models. Their study is complicated by a possible heterogeneity of polymer layer adsorbed on the membrane. Experimental methods reflecting the structure of the layer include the electrokinetic measurements in liposome suspension and the boundary potential of planar bilayer lipid membranes (BLM) and lipid monolayers with a mixed composition of lipids and the ionic media. In the review, we systematically analyze the methods of experimental registration and theoretical description of the laterally heterogeneous structures in the polymer layer published in the literature and in our previous studies. In particular, we consider a model based on classical theory of the electrical double layer, used to analyze the available data of the electrokinetic measurements in liposome suspension with polylysines of varying molecular mass. This model suggests a few parameters related to the heterogeneity of the polymer layer and allows determining the conditions for its appearance at the membrane surface. A further development of this theoretical approach is discussed.

Multivalent ions and biomolecules: Attempting a comprehensive perspective

Ions are ubiquitous in nature. They play a key role for many biological processes on the molecular scale, from molecular interactions, to mechanical properties, to folding, to self-organisation and assembly, to reaction equilibria, to signalling, to energy and material transport, to recognition etc. Going beyond monovalent ions to multivalent ions, the effects of the ions are frequently not only stronger (due to the obviously higher charge), but qualitatively different. A typical example is the process of binding of multivalent ions, such as Ca2+ , to a macromolecule and the consequences of this ion binding such as compaction, collapse, potential charge inversion and precipitation of the macromolecule. Here we review these effects and phenomena induced by multivalent ions for biological (macro)molecules, from the "atomistic/molecular" local picture of (potentially specific) interactions to the more global picture of phase behaviour including, e. g., crystallisation, phase separation, oligomerisation etc. Rather than attempting an encyclopedic list of systems, we rather aim for an embracing discussion using typical case studies. We try to cover predominantly three main classes: proteins, nucleic acids, and amphiphilic molecules including interface effects. We do not cover in detail, but make some comparisons to, ion channels, colloidal systems, and synthetic polymers. While there are obvious differences in the behaviour of, and the relevance of multivalent ions for, the three main classes of systems, we also point out analogies. Our attempt of a comprehensive discussion is guided by the idea that there are not only important differences and specific phenomena with regard to the effects of multivalent ions on the main systems, but also important similarities. We hope to bridge physico-chemical mechanisms, concepts of soft matter, and biological observations and connect the different communities further.

Calcium-Lipid Interactions Observed with Isotope-Edited Infrared Spectroscopy

Calcium ions bind to lipid membranes containing anionic lipids; however, characterizing the specific ion-lipid interactions in multicomponent membranes has remained challenging because it requires nonperturbative lipid-specific probes. Here, using a combination of isotope-edited infrared spectroscopy and molecular dynamics simulations, we characterize the effects of a physiologically relevant (2 mM) Ca²⁺ concentration on zwitterionic phosphatidylcholine and anionic phosphatidylserine lipids in mixed lipid membranes. We show that Ca²⁺ alters hydrogen bonding between water and lipid headgroups by forming a coordination complex involving the lipid headgroups and water. These interactions distort interfacial water orientations and prevent hydrogen bonding with lipid ester carbonyls. We demonstrate, experimentally, that these effects are more pronounced for the anionic phosphatidylserine lipids than for zwitterionic phosphatidylcholine lipids in the same membrane.

Isothermal Titration Calorimetry of Be2+ with Phosphatidylserine Models Guides All-Atom Force-Field Development for Lipid-Ion Interactions

Beryllium has multiple industrial applications but exposure to its dust during manufacturing is associated with developing chronic inflammation in lungs known as berylliosis. Besides binding to specific alleles of MHC-II, Be²⁺ was recently found to compete with Ca²⁺ for binding sites on phosphatidylserine-containing membranes and inhibit recognition of this lipid by phagocytes. Computational studies of possible molecular targets for this small toxic dication are impeded by the absence of a reliable force field. This study introduces parameters for Be²⁺ for the CHARMM36 additive force field that represent interactions with water, including free energy of hydration and ion-monohydrate interaction energy and separation distance; and interaction parameters describing Be²⁺ affinity for divalent ion binding sites on lipids, namely phosphoryl and carboxylate oxygens. Results from isothermal titration calorimetry experiments for the binding affinities of Be²⁺ to dimethyl phosphate and acetate ions reveal that Be²⁺ strongly binds to phosphoryl groups. Revised interaction parameters for Be²⁺ with these types of oxygens reproduce experimental affinities in solution simulations. Surface tensions calculated from simulations of DOPS monolayers with varied concentrations of Be²⁺ are compared with prior results from Langmuir monolayer experiments, verifying the compacting effect that produces greater surface tensions (lower pressures) for Be²⁺-bound monolayers at the same surface area in comparison with K⁺. The new parameters will enable simulations that should reveal the mechanism of Be²⁺ interference with molecular recognition and signaling processes.

Calcium-Induced Lipid Nanocluster Structures: Sculpturing of the Plasma Membrane

The plasma membrane of the cell is a complex, tightly regulated, heterogeneous environment shaped by proteins, lipids, and small molecules. Ca²⁺ ions are important cellular messengers, spatially separated from anionic lipids. After cell injury, disease, or apoptotic events, anionic lipids are externalized to the outer leaflet of the plasma membrane and encounter Ca²⁺, resulting in dramatic changes in the plasma membrane structure and initiation of signaling cascades. Despite the high chemical and biological significance, the structures of lipid-Ca²⁺ nanoclusters are still not known. Previously, we demonstrated by solid-state nuclear magnetic resonance (NMR) spectroscopy that upon binding to Ca²⁺, individual phosphatidylserine lipids populate two distinct yet-to-be-characterized structural environments. Here, we concurrently employ extensive all-atom molecular dynamics (MD) simulations with our accelerated membrane mimetic and detailed NMR measurements to identify lipid-Ca²⁺ nanocluster conformations. We find that major structural characteristics of these nanoclusters, including interlipid pair distances and chemical shifts, agree with observable NMR parameters. Simulations reveal that lipid-ion nanoclusters are shaped by two characteristic, long-lived lipid structures induced by divalent Ca²⁺. Using ab initio quantum mechanical calculations of chemical shifts on MD-captured lipid-ion complexes, we show that computationally observed conformations are validated by experimental NMR data. Both NMR measurements of diluted specifically labeled lipids and MD simulations reveal that the basic structural unit that reshapes the membrane is a Ca²⁺-coordinated phosphatidylserine tetramer. Our combined computational and experimental approach presented here can be applied to other complex systems in which charged membrane-active molecular agents leave structural signatures on lipids.