PROTEIN ADSORPTION ON LIPID MONOLAYERS AT THEIR COEXISTENCE REGION

We investigate theoretically the behavior of proteins as well as other large macromolecules which are incorporated into amphiphilic monolaye rs at the air-water interface. We assume the monolayer to be in the co existence region of the ''main'' transition, where domains of the liqu id condensed phase coexist with the liquid expanded background. Using a simple mean-field free energy accounting for the interactions betwee n proteins and amphiphilic molecules, we obtain the spatial protein di stribution with the following characteristics. When the proteins prefe rentially interact with either the liquid condensed or liquid expanded domains, they will be dissolved in the respective phase. When the pro teins are energetically rather indifferent to the density of the amphi philes, they will be localized at the line boundary between the (two-d imensional) liquid expanded and condensed phases. In between these two limiting cases, a delocalization transition of the proteins takes pla ce. This transition is accessible by changing the temperature or the a mount of incorporated protein. These findings are in agreement with re cent fluorescence microscopy experiments. Our results also apply to li pid multicomponent membranes showing coexistence of distinct fluid pha ses.