The Equilibria in Lipid–Lipoic Acid Systems: Monolayers, Microelectrophoretic and Interfacial Tension Studies

Abstract

In this examination, we investigated the effect of lipoic acid (LA) on the properties of biological membrane models (monolayers, bilayers, and liposomes) formed from phosphatidylcholine (PC) or phosphatidylserine (PS) using the Langmuir, microelectrophoresis, and interfacial tension methods. The Langmuir technique allowed us to calculate the π–A isotherms and determine the molecular surface areas of pure and mixed monolayers. Using mathematical equations, we established that LA and the lipids formed complexes at a 1:1 ratio. The interfacial tension method was based on Young and Laplace’s equation. We assumed the formation of a 1:1 complex in the PC–LA system. Using the mathematical relationships, we derived the parameters characterizing the resulting complex, i.e., the surface occupied by the complex and the interfacial tension and stability constant of the formed complex. The microelectrophoretic method was used to determine the dependence of the zeta potential of the lipid membranes as a function of the pH (pH 2 to 10) of the electrolyte solution. The results indicate that modification of PC or PS membranes with LA affects changes in the zeta potential and the isoelectric point values.

Electrophoretic Light Scattering and Electrochemical Impedance Spectroscopy Studies of Lipid Bilayers Modified by Cinnamic Acid and Its Hydroxyl Derivatives

“…Since the isoelectric point of the membrane is one of the most important parameters describing its variable-charge surfaces, it is important to point out that it has shifted towards lower pH values for DOPC as compared to PS (decrease from pH ∼3.5 to pH ∼1.7, respectively). This is coherent with other investigations demonstrating that the isoelectric point of PC lipids fits into the pH values ranging between 3.2 and 4.0 [41,46,47], while reaching 1.4-1.7 values for PS lipids [41,46]. The surface charge densities of the DOPC, DOPC/PS 9:1, DOPC/PS 8:2, and PS liposomal membrane, plain and modified with ferulic acid, are shown as a function of pH in Figure 4.…”

“…Since the isoelectric point of the membrane is one of the most important parameters describing its variable-charge surfaces, it is important to point out that it has shifted towards lower pH values for DOPC as compared to PS (decrease from pH ∼3.5 to pH ∼1.7, respectively). This is coherent with other investigations demonstrating that the isoelectric point of PC lipids fits into the pH values ranging between 3.2 and 4.0 [41,46,47], while reaching 1.4-1.7 values for PS lipids [41,46]. Considering the ELS data presented in Figures 2-4 and comparing membranes with the same phospholipid composition, it can be observed that the smallest changes in the surface charge density values were obtained in the presence of the most hydrophobic of all, cinnamic acid, whereas the most prominent alterations were noticed in the presence of the most hydrophilic compound, ferulic acid.…”

Interactions between Phosphatidylcholine and Kaempferol or Myristicin: Langmuir Monolayers and Microelectrophoretic Studies

“…To validate the results of the stability constant and the area occupied by a molecule of the complex, the slopes of the tangents at the points x F → 0.00 and x F → 1.00 should be determined using the 9 and 10 equations [ 24 , 30 ]: …”

“…Six measurements were made (each covering 100–200 series, duration 5 s) for each pH value for each sample. The experiments were carried out three times [ 23 , 24 , 32 , 33 , 34 ]. The value of electrophoretic mobility enables the determination of the surface charge density according to the equation [ 50 ]: where u is the electrophoretic mobility, η is the viscosity of the solution, and d is the thickness of the diffuse layer.…”

“…These parameters are very important in characterizing both natural and artificial lipid membranes. Previously, we have described changes in the surface pressure, the area occupied by the particles, and surface charge density, which affects, for example, the composition of membrane phosphatidylcholine-diosgenin [ 22 , 23 ] and phosphatidylcholine-lipoic acid [ 24 ].…”