Enantioseparation of dl-tryptophan by spiral tube assembly counter-current chromatography and evaluation of mass transfer rate for enantiomers

Abstract Spiral tube assembly counter-current chromatography was successfully applied in enantioseparation of dl -tryptophan using bovine serum albumin as chiral selector. An improved biphasic aqueous–aqueous solvent system 12.0% (w/w) polyethyleneglycol 8000-9.0% (w/w) dibasic potassium phosphate-0.1% ammonia-78.9% water was used as the solvent system for counter-current chromatography, in which bovine serum albumin was predominantly distributed in the lower phase of the two-phase aqueous system. The aqueous–aqueous solvent system gave a very high enantioselectivity for d - and l -tryptophan at α  = 2.605 along with distribution ratio D D  = 1.200 and D L  = 0.461. High peak resolution was obtained for enantioseparation of 2.0 mg of dl -tryptophan by spiral tube assembly counter-current chromatography under room temperature. It was found that 0.1% ammonia added in the aqueous–aqueous solvent system greatly improved the enantioseparations. An unusual extremely broad peak for l -tryptophan was observed during enantioseparations. In order to give an explanation, mass transfer rates of d - and l -enantiomers through the interface between the two phases were measured. It was found that l -tryptophan showed lower mass transfer rate than d -tryptophan. Further discussions were proposed for possible reasons for mass transfer rate difference between the enantiomers.