A monolayer phase behaviour study of phosphatidylinositol, phosphatidylinositol 4-monophosphate and their binary mixtures with distearoylphosphatidylethanolamine

Abstract

The monolayer phase behaviour of phosphatidylinositol (PI), phosphatidylinositol 4-monophosphate (4-PIP) and their binary mixtures with distearoylphosphatidylethanolamine (DSPE) has been studied at the air/water interface. PI and 4-PIP exhibited a fluid phase at all surface pressures above 0 mN m⁻¹. At a given surface pressure, 4-PIP monolayers yielded higher molecular areas than PI which was attributed to the increased repulsion due to the more highly charged headgroup. As expected, mixtures of PI with DSPE, which are known to have a condensed phase structure at all surface pressures above 0 mN m⁻¹, exhibited an expansion of the film which increased with increasing proportions of PI. However, this was not observed for DSPE/4-PIP mixtures for which greater 4-PIP concentrations did not always induce a corresponding increase in molecular area. Both PI/DSPE and 4-PIP/DSPE mixed monolayers were found to deviate from either ideal mixing or complete immiscibility likely due to either an additional charge repulsion between headgroups or, less likely, to a partial miscibility of the inositols and DSPE in the fluid phase.