High Amino Acid Lattice Loading at Nonambient Conditions Causes Changes in Structure and Expansion Coefficient of Calcite

Biogenic crystals produced by organisms have been known for several decades to exhibit intracrystalline organic macromolecules. Here, using a reductionist approach, we tackle the question of whether the incorporation of single amino acids is driven by kinetics or by thermodynamics. We show that when calcite is grown in the presence of amino acids under nonambient conditions, extremely high loading levels of up to 6.12 mol % of aspartic acid (Asp) are achieved. This incorporation leads to marked changes in the host calcite crystal's structure and expansion coefficient. The latter is as much as twice as high as that of pure calcite. This is the first example showing that an organic molecule incorporated into an inorganic host can strongly affect the expansion coefficient. Most importantly, we show that the incorporation of amino acids in calcite is controlled by their thermodynamic solubility in calcite rather than kinetically and that hybrid amino acid-calcite crystals can indeed be considered a solid solution.