Understanding the pH dependence of protein solubility is a key factor in the formulation of stable protein solutions. Protein solubility depends on pH, salt concentration, ion type, temperature and the solvent. Protein molecules such as lysozymes, insulin or albumin have polyelectrolyte characteristics with ionic side groups. At the isoelectric point (IEP) the protein solubility is zero. Hence, aggregation can occur.
The Stabino® Experiment
With the COLLOID METRIX Stabino®, titrations with pH, salt or polyelectrolyte solutions on a 10 ml, 0.001 to 10% v/v concentration dispersion can be performed within a few minutes. The monitor signal is the streaming potential SP, which correlates to the strength of the ionic charge at the interface of the particles or macromolecules. The potential also correlates to the solubility of the protein solution. The oscillating piston induces the signal SP, whilst mixing the titrand into the sample and preventing the suspension from sedimenting. Two titration units are integrated in the Stabino® to allow efficient particle and polyelectrolyte charge titrations.
The left diagram shows pH titrations on 10 ml of 1% w/v hen (OVE) and bovine (BOV) albumins. The IEP of the 1 nm OVE albumin is at pH = 4.7, the IEP of the 5 nm bovine albumin is at pH = 5.2. At the end of the titration the samples were coagulated.
The size measurement was performed with the 180° DLS heterodyne backscattering NANO-flex® measurement probe.
With Stabino®, an efficient tool is available to do titrations without need of sample parameters. Screening work is favored.