The Role of Surface Diffusion in the Growth Mechanism of Triosephosphate Isomerase Crystals

In the protein crystallization process, a growth unit has two possible molecular pathways it can follow from solution to the crystal bulk, namely, the process of direct incorporation from solution or the process of surface diffusion preceded by surface adsorption. We use real time in situ atomic force microscopy to monitor the molecular processes that govern the crystallization of the protein triosephosphate isomerase. With this technique, we study the step edge dynamics on a molecular scale. We conclude that step reorganization as a result of line diffusion has a negligible effect on step dynamics. Therefore, step displacements are attributed to the exchange of growth units with the surrounding phases, i.e., the terrace and the solution. Triosephosphate isomerase (TIM) tetramers are identified to be the dominating growth units. From the statistics of molecular attachment and detachment from the step, we conclude that the incorporation of growth units occurs through surface diffusion. Additionally, in the tested supersaturation range, normal growth is dominated by the two-dimensional nucleation of triangular islands. The step edges of these islands have equal step formation energy.