Phase Transitions in Monolayers of Human Apolipoprotein C-I

Abstract

Human apolipoprotein C-I was deposited onto a highly ionic water subphase to form a monolayer in a Langmuir trough. Pressure−area isotherms of the human apolipoprotein C-I monolayer, as well as direct observations with Brewster angle microscopy, were carried out. Two first-order phase transitions were found. The first phase transition involves the coexistence of a fluid condensed phase and a gas phase, whereas the second one is a transition between two condensed phases. A protein model is proposed, where the apolipoprotein C-I is made of two amphiphilic α-helices bonded with a poorly structured polypeptide fragment. With this model, we have been able to explain our results, in particular the second phase transition, which seems to correspond to a conformational change in the protein. This conformational change could be due to the desorption of one amphiphilic α-helix from the subphase, when lateral pressure is applied to the protein monolayer. The relevance of this conformational change for lipid binding activity is discussed.