In Situ Atomic Force Microscopy Imaging of Octacalcium Phosphate Crystallization and Its Modulation by Amelogenin’s C-Terminus

Abstract

© 2017 American Chemical Society. Amelogenin proteins play a critical role in controlling crystal growth and orientation into highly organized calcium phosphate (Ca-P) minerals during tooth enamel formation. However, real-time observations for understanding the kinetics and mechanisms of Ca-P surface crystallization and its modulation by amelogenin have been lacking. We monitor the kinetics of the (100) surface growth of octacalcium phosphate (OCP) with precisely defined thermodynamic driving forces in the presence of amelogenin’s C-terminus peptides inside a fluid cell of an atomic force microscope with a controlled near-physiological environment. During in situ growth via a nonclassical particle attachment pathway, an obviously elongated aggregation of Ca-P nanoparticles induced by the assembly of amelogenin’s C-termini was observed. The nanostructured fibrous assemblies, reminiscent of extracellular matrix, are able to bind Ca-P nanoparticles and direct OCP mineralization. This was analyzed and rationalized through single-molecule determination of the binding free energy of the C-terminal fragment adsorbed to the (100) face of OCP. Combining in situ growth kinetics with force spectroscopy reveals the shape evolution from spherical particles to elongated nanorods resembling the nanostructure of enamel crystallites. The findings improve the fundamental understanding of natural biomineralization through nonclassical crystallization routes and amelogenin self-assembly.