Theory of anomalous temperature dependence in spin-labelled lipid monolayers

Abstract

Certain spin-labelled monolayers, for instance 12-nitroxide stearic acid, have the remarkable property that (pressure against area) isotherms lie above those obtained at higher temperatures, due to density increasing with temperature over quite large intervals. The qualitative explanation is that each molecule has both “extended” conformations with the spin-label as well as the head-group bonded to the water surface and “upright” conformations with only the head-group bonded. In the present paper a statistical-mechanical theory embodying these ideas is developed. The constant pressure partition function for a one-dimensional lattice model is evaluated exactly by a matrix method and (pressure against density) isotherms calculated. “Isotherm reversal” proves to be a characteristic of the model and is found for a range of interaction energy parameters. The isotherms are similar in shape to the experimental ones, with a high-density “plateau” region of reduced slope at lower temperatures. However the pressure range over which anomalous behaviour is found is smaller relative to the “plateau” pressure than that found experimentally.