Enantioselective extraction of d,l-tryptophan by a new chiral selector: Complex formation with di(2-ethylhexyl)phosphoric acid and O,O′-dibenzoyl-(2R,3R)-tartaric acid

The paper presents a new complex chiral selector of di(2-ethylhexyl)phosphoric acid (D2EHPA) and O,O′-dibenzoyl-(2R,3R)-tartaric acid ((−)-DBTA) and its enantioselectivity on d,l-tryptophan (Trp). The influence of the initial concentration of d,l-Trp, (−)-DBTA, D2EHPA were studied. Both the distribution ratio and enantioselectivity of d,l-Trp are greatly improved with the complex extractant instead of (−)-DBTA individually. It was also found that the enantioselectivity of the complex extractant was changed by the D2EHPA content, although pure D2EHPA has no chiral separation ability. By adjusting the equilibrium pH in the aqueous phase, a high chiral separation efficiency with a maximum enantioselectivity of 5.3 and an enantiomeric excess (e.e.) of up to 57% in aqueous phase were reached at PI point. Finally, the most reasonable extraction mechanisms of the complex to Trp enantiomers have been suggested.