Modulation of cytochrome C coupling to anionic lipid monolayers by a change of the phase state: a combined neutron and infrared reflection study

The effect of monolayer domain formation on the electrostatic coupling of cytochrome c from the subphase to a monolayer at the air/water interface was studied using a combination of neutron reflection (NR) and infrared reflection absorption spectroscopy (IRRAS) techniques. The monolayers consisted of a binary mixture of the zwitterionic phosphatidylcholine and the anionic phosphatidylglycerol. For a monolayer of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylglycerol (DMPG, 30 mol%), which exhibits a non-ideal mixing of the two lipid components, we observed a significantly higher protein coupling to the liquid-condensed phase compared to the liquid-expanded state. In contrast, this higher protein binding was not observed when the two lipids had identical chain lengths (nearly ideal mixing). Similarly, for an equimolar mixture of DPPC and DMPG, we did not observe significant differences in the protein binding for the two phase states. The results strongly suggest that the domain formation in a condensed monolayer under non-ideal lipid mixing conditions is crucial for the cytochrome c binding strength. Furthermore, this study demonstrates the significant advantages of gathering information on protein-monolayer coupling by the combined use of a dedicated IRRAS set-up with the NR technique.