Vancomycin partitioning in aqueous two-phase systems: effects of pH, salts, and an affinity ligand

The partitioning of vancomycin in polyethylene glycol (PEG)-dextran and PEG-phosphate aqueous two-phase systems was studied at different pHs, at varying concentrations of neutral salts, and with an affinity ligand attached to methoxy polyethylene glycol (MPEG). Vancomycin is found to partition preferentially into the PEG-rich top phase, and its partition coefficient increases nearly exponentially with the addition of water structure-making salts, such as sodium sulfate and sodium chloride, but is independent of sodium phosphate concentration. In the PEG-dextran system the vancomycin partition coefficient increases 3-fold in acidic and neutral solutions, while in the PEG-phosphate system it increases about 30-fold on the addition of the same amount of sodium chloride (1. 5 mol/kg). In basic solution, above its isoelectric point, the vancomycin partition coefficient increases slightly with NaCI concentration in the PEG-dextran system. We also examined the use of the dipeptide D-ala-D-ala as an affinity ligand on MPEG to extract vancomycin into the PEG-rich phase. The vancomycin partition coefficient increased almost 7-fold upon adding the MPEG-ligand in an amount equal to approximately 3% of the total PEG in the system. Finally, fractionation of the polydisperse phase-forming polymers in the two-phase PEG-dextran system was observed. The effect of this polymer fractionation on the partition coefficient of vancomycin is discussed.