Direct Visualization of Orientation Competition in Protein Epitaxial Crystallization.

The formation of two-dimensional protein crystallites on solid surfaces is crucial in both natural biological processes and bottom-up nanofabrication. However, direct molecular-level insight into the dynamic evolution of epitaxial interfaces has remained challenging. Here, using in situ atomic force microscopy, we reveal the two-dimensional crystallization dynamics of streptavidin on muscovite mica with single-molecule resolution. We directly imaged the initial formation of multiple orientation domains during the crystallization process, a direct consequence of epitaxial matching to the three-fold symmetry of the mica lattice. This metastable poly-domain structure then evolves into a single-oriented crystallite through two coexisting classical ripening pathways, i. e. Ostwald ripening and Smoluchowski-type coalescence. The dominance of a single domain orientation arises from a competitive selection process during ripening, in which the initially largest domain ultimately consumes the neighboring domains of other orientations. Our work establishes a paradigm of domain coarsening in biomolecular epitaxy, providing a solid foundation for the rational design of functional bio-nano interfaces.