ORGANIZATION AND REORGANIZATION DURING MATRIX ASSEMBLY AND MINERALIZATION: THE NON-CLASSICAL ROUTE TO THE CRYSTALLINE STATE

Assembly of organic matrices and subsequent directed nucleation of mineral constituents is a widespread paradigm in biomineralization; the architecture of the underlying matrix imposes order on the nucleating mineral species. In recent years, the importance of amorphous or disordered precursor phases and multi-stage mineralization pathways during matrix-directed mineral growth has become increasingly apparent. However, the influence of the matrix on these complex pathways or the process by which the matrices themselves become organized remains unclear. Here we present results from in situ optical, force and electron microscopy as well as polarization-dependent NEXAFS investigations on three systems in which we examine the dynamics of organization and reorganization during matrix assembly and directed mineralization. These systems include surface-layer proteins from microbial systems, which can serve as matrices for calcium carbonate formation, calcium carbonate on self-assembled monolayers, and calcium phosphate on collagen. In each case, the system passes through an initial amorphous state before reaching the final ordered state. Moreover, where quantitative analyses of nucleation rates have been possible, they indicate that the thermodynamic barrier is not a significant factor in determining the pathway. For example, even when the free energy barrier for nucleation of amorphous calcium phosphate (ACP) is two orders of magnitude larger than that for hydroxyapatite (HAP), ACP is the first phase to form. The system then transforms to octacalcium phosphate before finally forming HAP. Consequently, kinetic barriers that appear in the pre-exponential factor of the nucleation rate equations are likely to be rate limiting. In the case of the surface-layer proteins, the important barrier appears to be that associated with the folding of the monomers into their tetrameric configuration. In the case of the mineralizing systems, the nature of these barriers is unknown, but we hypothesize that they are associated with factors that direct the structure of the solvated species.