Computer simulation of aggregation in adsorbed protein layers

Abstract

Some theoretical models describing the adsorption of proteins from solution are investiated. Computer simulations are carried out on a hexagonal lattice that allows lateral interactions between adsorbed protein molecules to be incorporated in the model. Diffusion limited adsorption determines the amount of particles at the surface and aggregation is allowed by lateral diffusion along the surface. The computer model can be characterized as a two-dimensional, stochastic, cellular automation where the surface sites are arranged on a hexagonal lattice. The fate of every lattice site, or cell, is updated in discrete time steps. Different rules concerning nearestneighbor interactions were tested, giving both irreversible and reversible diffusion-limited aggregation (DLA). — The fractal dimensions of individual clusters were extracted from simulated aggregates and found to be consistent with literature values on irreversible and reversible DLA in two dimensions. By comparing computer simulated aggregation to experimental electron micrographs of adsorbed ferritin layers some conclusions can be drawn concerning the adsorption of ferritin to hydrophobic quartz surfaces. The adsorbed protein molecules must, most probably, the mobile along the surface with a diffusion constant considerably lower (5 · 10⁴) than in the bulk solution. The initial aggregation could be well described by reversible DLA-clustering (at θ=0.015), but at higher surface coverages of proteins (at θ=0.1) the present model could not account for the low experimentally observed fractal dimension (d _f =1.05).