Vesicle surface charge and polylysine modification of ultraviolet absorption by the olefinic bonds in charged lipid

The results of electrical conductivity and ultraviolet absorption studies on bilayer vesicles, composed of various mixtures of saturated dipalmitoylphosphatidylcholine (DPPC)and unsaturated bovine brain phoaphatidylserine (PS), in the presence and absence of polylysine indicate the following. (i) The two kinds of lipid maintain a strong transbilayer segregation over a wide range of proportions. The charged lipid (PS) preferentially locates in the inner, at PS proportions of 33% or less, and in the outer layer of the membranes when its proportion is increased to 50%. (ii) At PS proportions of 33% of less, where DPPC is the major component of the outer layer, the ability of polylysine to modify ultraviolet absorption by the olefinic bonds in the PS depends on the fluidity of the phosphatidylcholine. At higher PS proportions, where PS preferentially locates in the outer layer, modification of the ultraviolet absorption spectrum of the lipid by polylysine occurs only when the surface charge of the vesicles is diminished by the addition of bivalent metal ions. (iii) The polylysine-induced change in the ultraviolet absorption spectrum of the olefinic bonds in the vesicles was found to be very similar to that induced by dispersing unsaturated fatty acids in water rather than dissolving them in a nonpolar solvent. This suggests that polylysine modification of the vesicles spectrum may be the result of deep hydrophobic penetration by the polypeptide causing hydration and(or) parallel alignment of the dipole moments of the absorbing chromophores.