Albumin binding onto synthetic vesicles

Vesicular entities were obtained by mixing didodecyldimethylammonium bromide and sodium dodecylsulfate in non-stoichiometric ratios. The vesicles bear a positive surface charge, due to the cationic species being in excess, and adsorb significant amounts of protein, presumably by electrostatic interactions. We modulated the net charge of bovine serum albumin by pH and observed its binding onto the above vesicles. Binding is controlled by the net charge of vesicles and albumin: it is substantial when albumin has negative charges in excess and is negligible, or non-existent, below its iso-electric point. For pH values >6.0, the binding efficiency increases in proportion to protein charge. Surface coverage changes in proportion to pH when the number of charges neutralized upon binding remains the same. The size of protein-vesicle lipo-plexes was inferred by dynamic light scattering and their charge by zeta-potential. The structure of albumin was evaluated by circular dichroism spectroscopy and estimates of alpha-helix, beta-strand and random coil contents were achieved. Increasing the beta-strand and random coil contents subsequent to binding suggests a significant interaction between vesicles and albumin. Attempts to determine the binding efficiency were made by elaborating zeta-potential values. The results were interpreted in terms of a Gibbs adsorption isotherm. Accordingly, it is possible to estimate the binding energy under different pH conditions.