Abstract:
It is well accepted that structural studies with model membranes are
of considerable value in understanding the structure of biological membranes.
Many studies with models of pure phospholipids have been done; but the effects
of divalent cations and protein on these models would make these studies more
applicable to intact membrane. The present study, performed with above view,
is a structural analysis of divalent io~cardio1ipin complexes using the
technique of x-ray diffraction.
Cardiolipin, precipitated from dilute solution by divalent ionscalcium,
magnesium and barium, contains little water and the structure formed
is similar to the structure of pure cardiolipin with low water content. The
calcium-cardiolipin complex forms a pure hexagonal type II phase that exists from
40 to 400 C. The molar ratio of calcium and cardiolipin in the complex is 1 : 1.
Cardiolipin, precipitated with magnesium and barium forms two co-existing
phases, lamellar and hexagonal, the relative quantity of the two phases being
dependent on temperature. The hexagonal phase type II consisting of water filled
channels formed by adding calcium to cardiolipin may have a remarkable permeability
property in intact membrane.
Pure cardiolipin and insulin at pH 3.0 and 4.0 precipitate but form
no organised structure. Lecithin/cardiolipin and insulin precipitated at pH 3.0
give a pure lamellar phase. As the lecithin/cardiolipin molar ratio changes
from 93/7 to SO/50, (a) the repeat distance of the lamellar changes from
72.8 X to 68.2 A; (b) the amount of protein bound increases in such a way
that cardiolipin/insulin molar ratio in the complex reaches a maximum constant
value at lecithin/cardiolipin molar ratio 70/30.
A structural model based on these data shows that the molecular arrangement
of lipid and protein is a lipid bilayer coated with protein molecules. The
lipid-protein interaction is chiefly electrostatic and little, if any, hydrophobic
bonding occurs in this particular system. So, the proposed model is
essentially the same as Davson-Daniellifs model of biological membrane.
