Advantages of the classical thermodynamic analysis of single—and multi-component Langmuir monolayers from molecules of biomedical importance—theory and applications

Abstract

The Langmuir monolayer technique has been successfully used for decades to model biological membranes and processes occurring at their interfaces. Classically, this method involves surface pressure measurements to study interactions within membrane components as well as between external bioactive molecules (e.g. drugs) and the membrane. In recent years, surface-sensitive techniques were developed to investigate monolayers in situ ; however, the obtained results are in many cases insufficient for a full characterization of biomolecule-membrane interactions. As result, description of systems using parameters such as mixing or excess thermodynamic functions is still relevant, valuable and irreplaceable in biophysical research. This review article summarizes the theory of thermodynamics of single- and multi-component Langmuir monolayers. In addition, recent applications of this approach to characterize surface behaviour and interactions (e.g. orientation of bipolar molecules, drug-membrane affinity, lateral membrane heterogeneity) are presented.

Insight into the Molecular Mechanism of Surface Interactions of Phosphatidylcholines─Langmuir Monolayer Study Complemented with Molecular Dynamics Simulations

“…Surface pressure measurement was performed using the Wilhelmy method using a strip of ashless filter paper (Whatman Chr1). On the basis of experimental π– A isotherms for binary monolayers, the values of excess Gibbs free energy of mixing (Δ G exc ) were calculated, reflecting the strength of mutual interactions and the affinity of its components to each other, using equation normalΔ G normale normalx normalc = N A ( prefix∫ π * π A 12 normald π − X 1 ∫ π * π A 1 normald π − X 2 ∫ π * π A 2 normald π ) where N A is Avogadro number, A 1 , A 2 , and A 12 are area per molecule values read from isotherms of pure substances (1 and 2) and their mixtures, and X 1 and X 2 are the mole fractions of substances 1 and 2 in the mixture. Δ G exc provides information on whether the particular interaction is energetically favorable (negative values) or not (positive values), while for Δ G exc = 0, ideal mixing (or immiscibility) occurs …”

“…Δ G exc provides information on whether the particular interaction is energetically favorable (negative values) or not (positive values), while for Δ G exc = 0, ideal mixing (or immiscibility) occurs. 22 …”

“…Studies of interactions between membrane components or between the membrane and the biomolecule are based on the calculation of particular qualitative and quantitative parameters , derived from the experimental surface pressure–molecular area (π– A ) isotherms. The most informative are values of the thermodynamic excess functions of mixing, among which the most frequently determined is excess Gibbs free energy of mixing (Δ G exc ), as it allows one to determine the type (attractive/repulsive) and strength of interactions and the thermodynamic stability of the investigated system.…”