Purification of (S)-3-cyano-5-methylhexanoic acid from bioconversion broth using an acetone/ammonium sulfate aqueous two-phase system

( S )-3-Cyano-5-methylhexanoic acid (( S )-CMHA) is the key chiral intermediate of pregabalin . In this paper, an aqueous two-phase system (ATPS) was developed to extract ( S )-CMHA from nitrilase-catalyzed bioconversion broth. Inorganic salts and hydrophilic solvents were screened to form ATPS, among which an acetone/ammonium sulfate ATPS was investigated in detail, including phase diagram, effect of phase composition and stability of ( S )-CMHA. The maximum product recovery of 99.15% was obtained by an optimized ATPS system composed of 15% (w/w) ammonium sulfate and 35% (w/w) acetone with the removal of 99% cells and 86.27% proteins. The total ( S )-CMHA yield reached 92.11% after back-extraction. The recycling use of ammonium sulfate was investigated, and 93.10% of salt in the salt-rich phase was recovered with the addition of methanol. The results demonstrated the efficiency of the two-step extraction process for separation of ( S )-CMHA.