AGGREGATION OF LIGAND-MODIFIED LIPOSOMES BY SPECIFIC INTERACTIONS WITH PROTEINS .1. BIOTINYLATED LIPOSOMES AND AVIDIN

The aggregation of biotin-modified phospholipid vesicles (liposomes) i nduced by binding the protein avidin in solution is analyzed experimen tally and theoretically. Avidin has four binding sites that can recogn ize biotin specifically, and is able to cross-link the liposomes to fo rm large aggregates. The aggregation kinetics were followed using quas i-elastic light scattering (OLS) to measure the mean particle size, an d by measuring the solution turbidity. The rate and extent of aggregat ion were-determined as a function of vesicle concentration, protein co ncentration; and the biotin density on the surface of the liposomes. A model based on Smoluchowski kinetics, fractal concepts, and Rayleigh and Mie light scattering theory was developed to analyze the experimen tal observations. Small aggregates (< 7800 Angstrom diameter) may be t reated as globular; however, the fractal nature of larger particles mu st be taken into account. Parameters in the model are taken from molec ular simulations, or fit to the experimental observations. The aggrega tion kinetics are primarily determined by the biotin density on the li posome surface, the stoichiometric ratio of avidin molecules to liposo mes, and the liposome concentration. Good agreement is found between t he model and the experimental results. (C) 1996 John Wiley & Sons, Inc .