Protein diffusion into thermally evaporated lipid films: role of protein charge/mass ratio

The kinetics of entrapment of proteins of different charge:mass ratios in thermally evaporated fatty lipid films (cationic, anionic and neutral) has been studied by quartz crystal microgravimetry (QCM) and analyzed in terms of a 1-D diffusion model. Under conditions where the proteins and immobilizing lipid matrix are oppositely charged, it is observed that the protein diffusivity increases with increasing charge:mass ratio. A notable exception to this rule was the membrane protein, Cyt c, indicating that interactions other than electrostatic dominate the immobilization of such proteins in the lipids. While entrapment of proteins also occurs in non-ionizable lipid films, the protein diffusivity did not show a meaningful correlation with the protein charge:mass ratio. Taken together with the observation that the extent of protein loading was directly related to the protein:lipid equilibrium molar ratio, the above findings support a predominantly electrostatic picture of protein immobilization in thermally evaporated cationic/anionic lipid films.