A cubic monoolein cytochrome c water phase: X-ray diffraction, FT-IR, differential scanning calorimetric, and electrochemical studies

Cytochrome c (cyt c) has been studied as an example of a peripheral membrane protein that forms a cubic phase with monooleoylglycerol (monoolein, MO) and aqueous solutions. The unit cell dimensions of the cubic phases (space group Pn3m) in relation to the composition of the aqueous solution and protein concentration were analyzed according to the concept of a lipid packing parameter in the bilayer. The interaction between cyt c and MO in the cubic phase was studied by FT-IR spectroscopy and differential scanning calorimetry (DSC). The FT-IR data indicated a somewhat higher conformational order of the MO acyl chain and structural rearrangements of the polar head-group region in the cubic MO-cyt c-H2O phase. These findings, together with the increase in unit cell dimension, suggested a decrease of the MO packing parameter upon protein incorporation. Furthermore, a competitive interaction between the protein and buffer ions at the lipid bilayer was observed. DSC measurements showed that incorporation of cyt c into the cubic MO-H2O phase resulted in a significant decrease in temperature during the phase transitions cubic --> reversed hexagonal --> reversed micellar, and the unfolding of the protein took place simultaneously with the first phase transition. Additionally, voltammetric and chronoamperometric studies of the direct redox conversions of cyt c at the 4,4'-dithiodipyridine-modified gold electrode revealed that the mobility of the protein molecules within the cubic phase was highly restricted. Altogether, these findings indicate that nonelectrostatic interactions between peripheral proteins and lipid molecules in membranes might also play a role in regulating biological function.