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 me mbrane protein that forms a cubic phase with monooleoylglycerol (monoo lein, MO) and aqueous solutions. The unit cell dimensions of the cubic phases (space group Pn3m) in relation to the composition of the aqueo us solution and protein concentration were analyzed according to the c oncept of a lipid packing parameter in the bilayer. The interaction be tween cyt c and MO in the cubic phase was studied by FT-IR spectroscop y and differential scanning calorimetry (DSC). The FT-IR data indicate d a somewhat higher conformational order of the MO acyl chain and stru ctural rearrangements of the polar head-group region in the cubic MO-c yt c-H2O phase. These findings, together with the increase in unit cel l dimension, suggested a decrease of the MO packing parameter upon pro tein incorporation. Furthermore, a competitive interaction between the protein and buffer ions at the lipid bilayer was observed. DSC measur ements showed that incorporation of cyt c into the cubic MO-H2O phase resulted in a significant decrease in temperature during the phase tra nsitions cubic --> reversed hexagonal --> reversed micellar, and the u nfolding 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 mol ecules within the cubic phase was highly restricted. Altogether, these findings indicate that nonelectrostatic interactions between peripher al proteins and lipid molecules in membranes might also play a role in regulating biological function.