Toward a Mechanism-Based Understanding of Skeletal Formation: Toolbox for Biomineralization

Since the onset of the Cambrian radiation (~540 Ma.), organisms have developed the ability to control the nucleation and growth of amorphous and crystalline earth materials to form skeletal structures. Observations that similar skeletal materials are utilized across multiple branches of the phylogenetic tree are cited as evidence that biomineralization strategies evolved independently by similar biochemical pathways that developed early in evolutionary history. Understanding these relations is critical to deciphering earth history, yet until recently, insights into biomineralization processes were inferred largely from macroscopic experiments and structural characterizations.

Our research group has pursued the long-term goal of establishing a mechanistic understanding of biologically controlled mineralization. Using simple model systems to probe underlying controls on mineralization, we have conducted molecular-scale studies of calcium carbonate growth using in situ atomic force microscopy and computational methods. We have learned that from amino acids and peptides to full proteins, acidic biomolecules enhance calcite mineralization rate to 25X by a systematic relationship. This suggests functional roles for aspartate- and glutamate-enriched macromolecules long-known to be associated with calcification. Remarkably, acidic biomolecules promote magnesium incorporation up to 3 mol%. Comparing a modest 2% increase to laboratory-based proxy models that correlate Mg content with temperature show this corresponds to a temperature offset of 7-14°C.

With the realization that some biogenic minerals form by nonclassical processes from amorphous precursors, new studies are focused on establishing factors that drive the transformation of intermediate phases to final biomineral products. New model systems to probe how biological substrates can modulate the onset of mineral formation, will establish: 1) Principles of mineral formation by nonclassical processes and influences on resultant signatures; 2) Thermodynamic and kinetic role(s) of the organic matrix as a trigger/inhibitor to the timing and location of mineral formation; and 3) Interplay of solvation and organic matrix in polymorph selection.