Contrasting the Effects of Aspartic Acid and Glycine\nin Free Amino Acid and Peptide Forms on the Growth Rate, Morphology,\nComposition, and Structure of Synthetic Aragonites

Corals and mollusks\nproduce aragonite skeletons and shells containing\nhighly acidic proteins, rich in aspartic acid (Asp) and glycine (Gly).\nThese biomolecules are pivotal in controlling biomineral formation.\nWe explore the effects of l-Asp, Gly, and two peptides: glycyl-l-aspartic acid (Gly-Asp) and tetra-aspartic acid (Asp<sub>4</sub>) on the precipitation rate, crystal morphology, and CO<sub>3</sub> group rotational disorder (inferred from Raman spectroscopy) in\naragonite precipitated in vitro at the approximate pH, [Ca<sup>2+</sup>], and Ω<sub>ar</sub> occurring in coral calcification media.\nAll of the biomolecules, except Gly, inhibit aragonite precipitation.\nBiomolecules are incorporated into the aragonite and create CO<sub>3</sub> group rotational disorder in the following order: Asp<sub>4</sub> > Asp = Gly-Asp > Gly. Asp<sub>4</sub> inhibits aragonite\nprecipitation more than Asp at comparable solution concentrations,\nbut Asp reduces aragonite precipitation more effectively than Asp<sub>4</sub> for each Asp residue incorporated into the aragonite. At\nthe highest solution concentration, the molar ratio of Asp<sub>4</sub>:CaCO<sub>3</sub> in the aragonite is 1:690. We observe a significant\ninverse relationship between the aragonite precipitation rate and\naragonite Raman spectrum ν<sub>1</sub> peak fwhm across the\nentire data set. Tetra-aspartic acid inhibits aragonite precipitation\nat all concentrations, suggesting that the aspartic acid-rich domains\nof coral skeletal proteins influence biomineralization by suppressing\nmineral formation, thereby shaping skeletal morphology and preventing\nuncontrolled precipitation.