The behavior of the adsorption of cytochrome C on lipid monolayers: A study by the Langmuir-Blodgett technique and theoretical analysis

•Theoretical equations are proposed to analyze the behavior of Cyt c on lipid monolayers.•The quantitative characteristics could be concluded that the different concentrations of Cyt c influence lipid monolayers.•Theoretical models are introduced that takes into account a phase transition between the LE and LC phase of DPPC.•The adsorption capacity and conformation reorganization of Cyt c on lipid monolayers are investigated.•There is a largest adsorption quantity of Cyt c on the lipid monolayers.

Cytochrome c (Cyt c) is an essential component of the inner mitochondrial respiratory chain because of its function of transferring electrons. The feature is closely related to the interaction between Cyt c and membrane lipids. We used Langmuir-Blodgett monolayer technique combined with AFM to study the interaction of Cyt c with lipid monolayers at air-buffer interface. In our work, by comparing the mixed Cyt c-anionic (DPPS) and Cyt c-zwitterionic (DPPC/DPPE) monolayers, the adsorption capacity of Cyt c on lipid monolayers is DPPS > DPPE > DPPC, which is attributed to their different headgroup structures. π-A isothermal data show that Cyt c (v = 2.5 μL) molecules are at maximum adsorption quantity on lipid monolayer. Moreover, Cyt c molecules would form aggregations and drag some lipids with them into subphase if the protein exceeds the maximum adsorption quantity. π-T curve indicates that it takes more time for Cyt c molecular conformation to rearrange on DPPE monolayer than on DPPC. The compressibility study reveals that the adsorption or intermolecular aggregation of Cyt c molecules on lipid monolayer will change the membrane fluidization. In order to quantitatively estimate Cyt c molecular adsorption properties on lipid monolayers, we fit the experimental isotherm with a simple surface state equation. A theoretical model is also introduced to analyze the liquid expanded (LE) to liquid condensed (LC) phase transition of DPPC monolayer. The results of theoretical analysis are in good agreement with the experiment.

Graphical abstractThe molecular model of different amounts of Cyt c (0, 2.5, 5, 10, 15 μL, respectively) interaction with lipid monolayer.Download full-size image