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.

Keywords: Langmuir monolayers; air/water interface; excess thermodynamic functions; interactions; phase transitions.

Figure 1.

Schematic representation of Langmuir monolayer formation: (a) spreading; (b), (c) compression on a Langmuir trough; (d) the resultant surface pressure-area (π–A) isotherm on the example of DPPC. Inset- compression modulus ($C_{\mathrm{s}}^{\text{–}1}$) versus surface pressure dependence.

Figure 2.

ΔG^exc plots for: (a) cholesterol/sphingomyelin and (b) cholesterol/ganglioside mixtures (modified from [78,79] with permission from Elsevier and the Royal Society Publishing.

Figure 3.

Comparison of (a) A^exc and (b) ΔG^exc calculated at selected surface pressure values for DOPC/Chol/SM systems mixed in different proportions. Adapted under Creative Commons Attribution 3.0 License from [87].

Figure 4.

Excess area of mixing A^exc for ergosterol- or cholesterol-containing monolayers in function of AmB content. Adapted from [66] with permission from Elsevier.

Figure 5.

ΔG^exc-X_edelfosine plots for mixtures of edelfosine with chosen lipids at 30 mN m⁻¹. Modified from [104] with permission from ICE Publishing.

Figure 6.

ΔG^exc–XAPCs plots for systems mimicking normal and tumour membrane treated with APCs at 30 mN m⁻¹. Adapted under Creative Commons Attribution License from [112].