Ion-exchange chromatography of proteins near the isoelectric points.

The retention and the resolution of beta-lactoglobulin A and B (LgA, LgB) were investigated with various ion-exchange chromatography media. The number of sites involved in the retention (adsorption) decreased as the mobile phase pH approached the isoelectric points pI (=5.1-5.2). However, even at pH 5.2 both LgA and LgB were retained on anion- and cation-exchange chromatography columns. The separation (resolution) of LgA and LgB became better when the pH approached the pI in anion-exchange chromatography columns where the number of adsorption site values are small (ca. 2-3). The two proteins were not separated on cation-exchange chromatography columns. Factors affecting the resolution and the retention near the pI were discussed.