Chapter 5 Physicochemical Insights into Equilibria in Bilayer Lipid Membranes

Abstract

This chapter concerns the equilibria of some biomolecules in bilayer lipid membranes. The presented research of biolipid interaction was concentrated on quantitative description of experiments that take part within the bilayer as well as on its surface. Assumed models of interaction between amphiphilic molecules, between transport proteins and monovalent cations and the equilibria that take place there as well as acid–base equilibria were described by mathematical equations for the systems studied. These theoretical models were verified experimentally using interfacial tension and electrochemical impedance spectroscopy techniques. For systems of two kinds of lipids, the possibility of a 1:1 complex formation was assumed what would explain the deviation from the additivity rule. Calculated values of parameters (equilibrium constants, molecular areas of complexes, interfacial tensions, capacitances and resistances of molecules and complexes) were used for calculation model curves. The comparison of model curves and experimental points verified assumed models. The complex formation between the gramicidin molecule and K + ion was investigated by the interfacial tension method. Electrochemical impedance spectroscopy was used for the study of gramicidin D dimerization and transport of monovalent cations across lipid bilayers by the dimers. Both experimental methods mentioned were used for the research ability of valinomycin to form a 1:1 potassium-ion complex on the lipid bilayer/electrolyte solution interface. Very simple methods were proposed to determine the parameters used to describe the gramicidin dimer and ionophore-K + complexes. It could be demonstrated that determined parameters values were of a similar order of magnitude to those obtained by other measuring techniques. The effect of pH of electrolyte solution on the bilayer lipid membrane built from different lipids was also determined. The obtained curves demonstrate the maximal interfacial tension values at the isoelectric point. A course of these curves was well characterized by the simplified description based on the Gibbs isotherm, but only in proximity of the isoelectric point. While, using the exact definition of surface excess within the Gibbs equation (taking into account volumes of adsorbed ions at the membrane surface) permits us to explain the run of experimental curves in the whole pH range. Also in this chapter the models derived to describe adsorption of the H + and OH − ions at lipid surfaces formed from phospholipids were proposed, which would reproduce changes in interfacial tension more correctly, particularly in the ranges distant from the isolelectric point. In models, contribution of the individual forms of lipid molecule to interfacial tension of the bilayer was assumed to be additive. This chapter concentrated especially on phospholipids because they are major fractions of lipids found in biological membranes: phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylserine (PS) were chosen. PC was the basic component of the formed bilayers because it has been widely examined and described in the literature, but also because it creates permanent monolayers and bilayers, of which one can easily build in the other components. Cholesterol, gramicidin and valinomycin were also studied since they play an important biochemical role in cell membranes. Cholesterol is an important factor in controlling physical properties of biological membranes and their functions. Gramicidin is a model of more complicated biological ionic channels. For this reason, many studies have been done using this simple channel-forming polypeptide. It has been reported that valinomycin acts as a selective carrier of K + ions in a variety of cell membranes as well as on liposomes and lipid bilayers. The interactions between membrane lipids are nowadays intensively developed. This results from the suitability of this research for understandings of phenomena proceeding in cellular membranes. However, there is still the lack of the quantitative description of the systems. It is required for a better understanding of the processes that take place in biological membranes with the aim of forming the artificial membrane that would very closely resemble the properties of the natural membrane. Therefore, the knowledge of molecular structure and organization of phospholipids is necessary. Data presented in this work, received from mathematical derivation and confirmed experimentally are of great importance for interpretation of phenomena occurring in lipid monolayers and bilayers. These results can help in a better understanding of biological membranes and in their biophysical studies. Simple and very interesting methods proposed in this chapter can be used with success for the determination of the equilibrium constant value of any 1:1 lipid–other lipid complex and any 1:1 ionophore–monovalent ion complex and for acid–base equilibria between any phospholipids and ions from electrolyte solution (H + and OH − ).