N-phenyl-1-naphthylamine Research Articles

The structure of Escherichia coli membranes studied by fluorescence measurements of lipid phase transitions

Lipid phase transitions in Escherichia coli membranes and in dispersions of the extracted lipids were studied using the negatively charged fluorescence probe 1-anilinonaphthalene-8-sulfonate (ANS −) and the hydrophobic fluorescence probe N-phenyl-1-naphthylamine (NPN). The fluorescence change, ΔI, at the phase transition approaches a limiting value ( ΔI) lim with increasing dye concentration. A comparison of the limiting values ( Δ) lim NPN obtained for membranes and the lipid standard allows us to estimate the lipid fraction, ρ, in the membrane that takes part in the phase transition ( ρ = 80%). The same procedure carried out with ANS − yields a value of 42.5% for the lipid fraction that is accessible from the aqueous phase. These values, combined with published freeze-etching data for the particle density within the fracture plane of membranes are used to quantify the Davson-Danielli-Robertson-Benson-Singer membrane model which assumes a fluid lipid bilayer with “integral” proteins embedded in the lipid matrix and surface proteins attached to the lipid head groups. It appears that on the average one “integral” membrane protein is surrounded by about 600 lipid molecules and that about 130 of these molecules are closely coupled to the protein molecule, forming an halo in which the chain-chain interaction between the lipids is disturbed. About half of the bilayer surface is covered with proteins; part of these seem to be stacked.