Crystallization of proteins on functionalized surfaces

Functionalized mica sheets and polystyrene films exposing ionisable groups have been used as heterogeneous nucleating surfaces for model proteins. Surfaces with different densities of amino or sulfonated groups have been prepared. Crystallization trials were carried out using the hanging-drop vapour-diffusion method. The results show that using these surfaces the starting protein concentration necessary to form crystals is reduced. The effect of these surfaces on the crystallization process may be the consequence of electrostatic interactions between charged residues of proteins and ionisable groups on surfaces. These interactions can be attractive or repulsive, depending on the relative charge of the protein and the surface at the crystallization pH. Both phenomena can induce an increase of the local protein concentration on the surface or in its proximity, favouring nucleation. Moreover, a reduction of the waiting time (an estimation of the nucleation time) was also observed for some proteins, suggesting a surface-stabilization effect on crystal nuclei.