221 - Mixed films of a structural myelin protein and an acidic phospholipid: Part II. effect of lipid/protein ratio on monolayer surface potential

Abstract

Mixed monolayers of the apoprotein A of Folch-Lees proteolipid from the central nervous system myelin and phosphatidyl-inositol monophosphate L from wheat germ have been spread at the surface of aqueous NaCl 10 −2 M and pH = 5.5 substrates at room temperature. The ratio A/L was varied and its effect on the surface pressure and potential of the films as well as on Ca 2+ binding by them were studied. The theoretical approach worked out in the preceding Part I is used to interpret the results. Numerical treatment of the Poisson-Boltzmann equation set up in Part I using the experimental results provides a small penetration thickness for the mobile ions: l 0 = 1-3 Å. To discuss electrostatic interactions between lipid and protein, an appropriate definition of the composition of the mixed monolayers is suggested: the fraction x E of net charges of A relative to the total charge due to the sum of the net charges of the constituents. It is found that for the critical value x E ~ 0.3 of its composition the mixed film A + L at the collapse point displays discontinuities in the following properties: the collapse surface pressure and potential, and the permanent dipole moment of lipid. The relative dielectric constant is independent of composition and equal to 2 for x E < 0.5 (the lipid rich films) and gradually moves to 15 in the interval range 0.5 < x E < 1. It follows that in the mixed monolayers we study at collapse point, the occuring conformational changes observed originate mainly in the lipid constituent.