The effect of aggregation into fractals or microgels on the charge density and the isoionic point of globular proteins

• The isoionic point of β-lactoglobulin is at pH 5.0 and IIP = 4.9 for WPI.
• The pH shifts towards the IIP at higher salt or protein concentration.
• After aggregation the pH depends more strongly on the charge density.
• The dependence is stronger for microgels than for fractals.
• The dependence is independent of the aggregate size.

Potentiometric titration curves were obtained for aqueous solutions of native whey protein isolate (WPI) and β-lactoglobulin (β-lg) at different NaCl concentrations. The curves crossed at approximately the same pH, which is considered to be the isoionic point (IIP). Titration with NaCl confirmed that the pH was independent of the salt concentration up to 0.1 M at the IIP, which was 4.9 for WPI and 5.0 for β-lg. Fractal aggregates and microgels of different sizes were formed by heating protein solutions at different pH and different concentrations. The titration curves of the aggregates depended on the type of aggregates, but not on their size. The IIP increased by at most 0.2 pH units after aggregation. For a given pH larger than IIP, the charge density of the proteins (α) was reduced after denaturation and aggregation. The reduction was stronger for microgels than for fractal aggregates. Addition of NaCl or increasing the protein concentration mitigated the effect. Comparison between WPI and β-lg showed that the pH dependence of α was almost the same for pH > 5.0 both for native and aggregated proteins.

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