Interaction of crystalline bacterial cell surface proteins with lipid bilayers in liposomes. A sound velocity study

Abstract

Sound velocity and density measurements have been used to study physical properties of plain unilamellar liposomes and covered by a crystalline cell surface layer (S-layer) of Bacillus coagulans, strain E38-66. The liposomes were composed of dipalmitoylphosphatidylcholine (DPPC), cholesterol and hexadecylamine. The measured parameters — velocity number [ ul and density ρ — decrease with increasing temperature for both suspensions of liposomes with S-layers (S-DPPC) and without S-layers. The values of [ u] at T<29°C are smaller, and at T>29°C larger for S-DPPC than those of plain liposomes. The density of the S-DPPC system was slightly greater than those of plain bilayer suspensions. The specific adiabatic compressibility, ϕ k/ β 0, calculated from the values of [ u] and of the specific volume, ϕ k, increases with temperature for both liposome systems. The comparison of the parameter ϕ k/ β 0 for S-DPPC and DPPC indicates that the adiabatic compressibility of S-DPPC at T<20°C was higher and at T>20°C was lower in comparison with that of plain DPPC. This is evidence of an interesting phenomenon of softening and condensing effects of S-proteins on the lipid bilayer depending on the temperature.