Surface Dependence of Protein Nanocrystal Formation

The self-assembly kinetics and nanocrystal formation of the bacterial surface-layer-protein SbpA are studied with a combination of quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM). Silane coupling agents, aminopropyltriethoxysilane (APTS) and octadecyltrichlorosilane (OTS), are used to vary the protein-surface interaction in order to induce new recrystallization pathways. The results show that the final S-layer crystal lattice parameters (a = b = 14 nm, = 90°), the layer thickness (15 nm), and the adsorbed mass density (1700 ng cm-2) are independent of the surface chemistry. Nevertheless, the adsorption rate is five times faster on APTS and OTS than on SiO2, strongly affecting protein nucleation and growth. As a consequence, protein crystalline domains of 0.02 µm2 for APTS and 0.05 µm2 for OTS are formed, while for silicon dioxide the protein domains have a typical size of about 32 µm2. In addition, more-rigid crystalline protein layers are formed on hydrophobic substrates. In situ AFM experiments reveal three different kinetic steps: adsorption, self-assembly, and crystalline-domain reorganization. These steps are corroborated by frequency-dissipation curves. Finally, it is shown that protein adsorption is a diffusion-driven process. Experiments at different protein concentrations demonstrate that protein adsorption saturates at 0.05 mg mL-1 on silane-coated substrates and at 0.07 mg mL-1 on hydrophilic silicon dioxide.