Kinetic Modifications in Calcium Phosphate Precipitation with Oligo(l-glutamic acids)

Objectives: Biomineralization of bone and dentin is modulated by acidic proteins that contain contiguous negatively charged amino acid sequences. Well-defined short peptides are of particular interest to understand calcium phosphate biomineralization because these peptides serve as a suitable model to mimic the acidic sequences present in the natural proteins. Previous results from our group suggested that peptides with longer chain length stabilize the amorphous calcium phosphate (ACP) precursor phase and delay ACP transformation to crystalline hydroxyapatite (HAP). The occlusion of peptides within the particles was proposed to be responsible for the delay in the phase transformation. The current work aims to investigate the precipitation of calcium phosphate in the presence of different lengths of oligo(l-glutamic acids) under different kinetic variable conditions in vitro. The modified variables include the rate of phosphate addition, Ca/P supersaturation ratio, and temperature.
Methods: Solution kinetics were analyzed using inductively coupled plasma-optical emission spectroscopy, while the calcium phosphate precipitates were characterized using a combination of electron microscopy, dynamic light scattering, and x-ray diffraction.
Results: The results suggest that the peptide with the longest chain delays the phase transformation the most compared to shorter chains, similar to our previous results. The longest peptide is able to reduce the supersaturation of the aqueous media by binding more calcium ions. Faster rate of phosphate introduction to the solution and higher temperature increase the rate of ion uptake from the solution to the growing precipitate. The increased rate of ion uptake eventually accelerates the time required for ACP phase transformation to HAP.
Conclusions: In summary, the varying kinetic variables do not alter the chain length effect of oligo(l-glutamic acids) in affecting the precipitation of calcium phosphate in solution. These variables can be explored to tune mineral properties in the design of collagen-based composite materials.