CONTROLS OF FUNCTIONAL GROUP CHEMISTRY ON CALCIUM CARBONATE NUCLEATION AND INSIGHTS FOR POLYMORPH SELECTION

Living organisms produce skeletal structures within a complex matrix of organic macromolecules that guide mineral nucleation and growth into the organic-inorganic composites we know as biominerals. The fossil record shows this type of template-directed mineralization dates to the Ediacaran (ca. 549 Ma). The mechanistic basis for how macromolecules control the nucleation kinetics and thermodynamics of carbonate polymorphs in template-directed nucleation is not well established.

We test the hypothesis that the kinetics and thermodynamics of calcium carbonate (CaCO₃) formation is regulated by a systematic relationship to the functional group chemistry of macromolecules. A long-term goal is to establish the energetic basis for biochemical motifs that are seen (and not seen) at sites of calcification across the phylogenetic tree. We first measured nucleation rates on model biomolecular substrates with termini found in the proteins and polysaccharides of calcifying tissues (-COOH, -PO₄, and -SH) and two alkanethiol chain lengths (16-C and 11-C) at variable chemical driving forces. Functional group chemistry and molecule conformation regulate rates by a predictable relation to interfacial free energy.

A second study tested the claim that Mg:Ca ratio determines the CaCO3 polymorph that forms by conducting a series of experiments using C16-COOH substrates, pH = 8.5-10.5, and variable solution Mg:Ca. Calcite is the only phase to nucleate until Mg:Ca exceeds approximately 2.5, above which vaterite precipitates as a rough film. There is no evidence for aragonite formation in any experiment.

The findings demonstrate organic surfaces regulate the thermodynamic barrier to nucleation through interfacial free energy and suggest the need to revisit the assumption that polymorph selection is determined by solution Mg: Ca. This is consistent with evidence (Wang et al., 2012, Faraday Disc.) that kinetic factors regulate the pathway to calcite versus aragonite formation.