CARBOXYLATED BIOMOLECULES CONTROL MAGNESIUM CONTENT OF AMORPHOUS CALCIUM CARBONATE: INSIGHTS FOR CALCIFICATION

A new paradigm for carbonate formation is emerging with the realization that many calcified skeletons form by non-classical mineralization processes that involve a transient amorphous calcium carbonate phase (ACC). In contrast to classical crystal growth by step growth via the terrace-ledge-kink (TLK) model, this type of mineralization begins by stabilizing ACC as a reactive intermediate that later transforms to one or more crystalline carbonate polymorphs such as calcite or aragonite. There is evidence that the Mg content in biogenic ACC is modulated by carboxylated (acidic) proteins and other macromolecules, but the physical basis for such a regulatory process is unknown.

We test the hypothesis that ACC compositions express a systematic relationship with the chemistry of carboxyl-rich biomolecules. A series of inorganic control experiments are conducted to establish the dependence of Mg/Ca signatures in ACC on solution composition. We then determine the influence of a suite of simple carboxylated organic acids on Mg content. Molecules with a strong affinity for binding Ca compared to Mg promote the formation of Mg-enriched ACC that is compositionally equivalent to high Mg-calcites and dolomite.

Measurements show Mg/Ca signatures are controlled by a predictable dependence upon the binding properties of the organic molecules. The trend appears rooted in the conformation and electrostatic potential topology of each molecule but dynamic factors may also be involved. The dependence suggests a physical basis for reports that specific sequences of calcifying proteins are critical to modulating mineralization. Many of sedimentary environments are organic graveyards that are rich with macromolecules and microbial activity. Insights from this study suggest a physical basis for anecdotal evidence that these environments influence mineralization by showing that carboxyl-rich organic environments can significantly increase Mg levels in ACC into the range of dolomites and other Mg-rich carbonate cements. This enhancement is possible without requiring the extreme levels of Mg enrichment in solution that have been suggested.