Influence of model globular proteins with different isoelectric points on the precipitation of calcium carbonate

Several mechanisms have been proposed to explain the interactions between proteins and mineral surfaces formed during biomineralization. To investigate the effect of the surface charge of proteins on calcium carbonate precipitation, a group of globular proteins with similar sizes and molecular weights but with different isoelectric points (iep) has been selected to be added to a CaCl2 solution in free-drift calcium carbonate precipitation experiments. These proteins are lysozyme (Lyz; theoretical iep 9.32), ribonuclease-A (Rib-A; theoretical iep 8.64), myoglobin (Myo; theoretical iep 7.36), and alpha-lactalbumin alpha-La; theoretical iep 4.83). Depending on their isoelectric point and their concentration in the solution, these proteins affected the nucleation, growth, polymorphism and growth morphology of calcium carbonate in different manners, evidencing different types of protein-surface interactions. For the protein with an acidic isoelectric point (alpha-La), electrostatic interactions were predominant. For proteins with isoelectric points around neutrality or slightly higher (Myo and Rib-A), the foremost interactions were hydrophobic with a certain electrostatic contribution. For the protein with a basic isoelectric point (Lyz), there was not any notable effect on most of the analyzed precipitation properties, evidencing a weaker protein-surface interaction.