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

• dl-tryptophan was enantioseparated by spiral countercurrent chromatography.
• An improved biphasic aqueous solvent system was found for enantioseparations.
• Mass transfer rate of enantiomers in the solvent system were evaluated by a rotary device.
• The chromatographic retention mechanism for enantioseparation was presented.
• Possible reasons for mass transfer rate difference between enantiomers were proposed.

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 DD = 1.200 and DL = 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.