V. Razumas et al., A CUBIC MONOOLEIN CYTOCHROME-C WATER PHASE - X-RAY-DIFFRACTION, FT-IR, DIFFERENTIAL SCANNING CALORIMETRIC, AND ELECTROCHEMICAL STUDIES, Journal of physical chemistry, 100(28), 1996, pp. 11766-11774
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.