Stabilization of Calcium Oxalate Metastable Phases by Oligo(l-glutamic acid): Effect of Peptide Chain Length

The understanding of the crystal growth of calcium oxalate and its inhibition is of great importance not only in medicine and biomineralization, but also in industry, as undesired deposits of this mineral are often formed. In this work, oligo( l -glutamic acid)s of different and well-defined chain lengths were synthesized by solid-phase synthesis to serve as a model system for acid-rich biogenic macromolecules during the precipitation of calcium oxalate. The kinetic parameters of the precipitation process in the presence of the model peptides and the characteristics of the products were investigated. The peptide additives inhibit the nucleation and crystal growth of calcium oxalate and control the morphology and the phase, being calcium oxalate dihydrate the promoted phase with long-chain peptides. The results showed that the chain length, rather than simply the amount of carboxylic groups, plays a deciding role in the morphology, phase, and stability of the resulting crystals. We showed how the presence of oligo( l -glutamic acid)s is able to stabilize metastable calcium oxalate dihydrate (COD) and calcium oxalate trihydrate (COT). Although COD and COT convert commonly under thermodynamic conditions to the thermodynamically stable monohydrate phase (COM), in the presence of the homopeptidic additives, the formed metastable phases remained stabilized over very long periods of time. To prove the incorporation of the additives in the crystals, carboxyl-functionalized latex nanoparticles—which are easy to observe by electron microscopy—were used as models of carboxylic macromolecular additives. The formation of calcium oxalate in the presence of the latex particles showed that the additives become incorporated into the crystal, which can explain the stabilization of the metastable phases.