PARTITION OF DIPEPTIDES IN AQUEOUS POLYMER 2-PHASE SYSTEMS AS A FUNCTION OF PH IN THE PRESENCE OF SALTS

Dipeptides are partitioned in aqueous two-phase systems containing Pol y(ethylene glycol) and dextran. NaCl and Na2SO4 are added to the syste m with a concentration of 40 mmol/kg. The pH of the system is adjusted with HCl or NaOH directly to keep the concentrations of the electroly tes to a minimum. By partition experiments with a charged and an uncha rged test substance it is shown that neither the basic two-phase syste m nor the electrostatic potential difference between the coexisting ph ases is influenced by the electrolytes in the pH-range studied. The re sults show that the model proposed by Albertsson (1986) can successful ly be applied to the partition behaviour of proteins over the entire p H-range investigated, if their acid-base equilibria are taken into acc ount appropriately. This is done by assuming that the differently char ged subspecies of each dipeptide partition independently of each other and that the equilibria between these subspecies can be described bas ed on their pK(i)-values. Simultaneously the difference between the el ectrostatic potential differences, DELTAphi(Na2So4) - DELTAphi(NaCl, h as been unambiguously determined to be 4.8 +/- 0.5 mV for the system c hosen.