Two-dimensional crystallization of ferritin molecules at the air-water interface

Three kinds of dialkyl compounds, dimethyl-dioctadecyl-ammonium bromide, sodium dioctadecyl-phosphate, and sodium dioctadecanoyl-sulphate, are employed as monolayers adsorbing ferritin proteins at the air-water interface. After incubating the lipid monolayer on the water surface, proteins are injected into the subphase. After an elapsed time of between 10 and 50 min, the lipid-ferritin complexes are horizontally transferred onto a carbon film and observed by transmission electron microscope. Although the dialkyl-ammonium salt has a smaller adsorption rate of proteins (3.2 x 10(9) molecules cm(-2) min(-1)) than those of phosphate and sulphate (7.7 x 10(9) and 8.6 x 10(9) molecules cm(-2) min(-1), respectively), ammonium salt has the tendency to yield the best two-dimensional crystals of protein, and constructs small crystals consisting of 10-100 proteins in a short incubation time, For the first time, it was confirmed from quantitative analysis that two-dimensional crystal growth of proteins would require both the adsorbing ability and fluidity of the lipid monolayer.