MEASUREMENTS OF CALCITE NUCLEATION KINETICS AND BINDING ENERGIES RECONCILE DISPARATE VIEWS OF TEMPLATE-DIRECTED NUCLEATION

The physical basis for how organisms use macromolecules to regulate the onset of mineral formation in calcifying tissues is not well established. The biomineralization community has widely assumed that the organic matrix promotes nucleation through stereochemical matching to guide the organization of solute ions, while materials synthesis groups use simple binding assays to correlate high binding strength with good promoters of nucleation. This study reconciles the two views and provides a mechanistic explanation for template-directed nucleation by correlating heterogeneous nucleation barriers with crystal-substrate binding free energies.

Using surface assembled monolayers (SAM), we first measure the kinetics of calcite nucleation onto model substrates that present different functional group chemistries (carboxyl, thiol, phosphate, hydroxyl) and conformations (C11, C16 chain lengths). We find rates are substrate-specific and obey predictions of classical nucleation theory at supersaturations that extend above the solubility of amorphous calcium carbonate (ACC). Analysis of the kinetic data shows the thermodynamic barrier to nucleation is reduced by minimizing the interfacial free energy of the system, g. We then use dynamic force spectroscopy to independently measure calcite-substrate binding free energies, DG_b. Moreover, we show that within the classical theory of nucleation, g and DG_b should be linearly related. The results bear out this prediction and demonstrate that low energy barriers to nucleation correlate with strong crystal-substrate binding. This relationship is general to all functional group chemistries and conformations.

These findings reconcile the long-standing concept of templated nucleation through stereochemical matching with the conventional wisdom that ‘good binders are good nucleators’. The alternative perspectives become internally consistent when viewed through the lens of crystal-substrate binding and provide a physical basis for the compound-specific ability of organics to promote carbonate nucleation.