Brewster angle microscopy studies of the morphology in dipalmitoyl phosphatidyl glycerol monolayers spread on subphases of different pH

Abstract

Dipalmitoyl phosphatidyl glycerol (DPPG) was investigated as Langmuir monolayers at the air/water interface by means of surface pressure measurements in addition to Brewster angle microscopy (BAM). A characteristic phase transition region appeared in the course of surface pressure-area (π-A) isotherms for monolayers spread on alkaline water or on buffer subphases, while on neutral or acidic water the plateau region was found to disappear. This phase transition region was attributed to the ionisation of DPPG monolayer. Quantitative Brewster angle microscopy measurements reveal that at the transition the thickness of ionised DPPG monolayer increases by 4.2 Å as a result of conformational changes of the ionised polar groups, which tend to emerge from the bulk subphase up to the surface due to their dehydration upon compression.