Contrasting the effects of aspartic acid and glycine in free amino acid and peptide form on the growth rate, morphology, composition and structure of synthetic aragonites

Corals and mollusks produce aragonite skeletons and shells containing highly acidic proteins, rich in aspartic acid (Asp) and glycine (Gly). These biomolecules are pivotal in controlling biomineral formation. We explore the effects of l-Asp, Gly, and two peptides: glycyl-l-aspartic acid (Gly-Asp) and tetra-aspartic acid (Asp₄) on the precipitation rate, crystal morphology, and CO₃ group rotational disorder (inferred from Raman spectroscopy) in aragonite precipitated in vitro at the approximate pH, [Ca²⁺], and Ω_ar occurring in coral calcification media. All of the biomolecules, except Gly, inhibit aragonite precipitation. Biomolecules are incorporated into the aragonite and create CO₃ group rotational disorder in the following order: Asp₄ > Asp = Gly-Asp > Gly. Asp₄ inhibits aragonite precipitation more than Asp at comparable solution concentrations, but Asp reduces aragonite precipitation more effectively than Asp₄ for each Asp residue incorporated into the aragonite. At the highest solution concentration, the molar ratio of Asp₄:CaCO₃ in the aragonite is 1:690. We observe a significant inverse relationship between the aragonite precipitation rate and aragonite Raman spectrum ν₁ peak fwhm across the entire data set. Tetra-aspartic acid inhibits aragonite precipitation at all concentrations, suggesting that the aspartic acid-rich domains of coral skeletal proteins influence biomineralization by suppressing mineral formation, thereby shaping skeletal morphology and preventing uncontrolled precipitation.