Abstract
Abstract Background levo-Menthol is an important flavoring chemical, which can be prepared by enantioselective enzymatic hydrolysis of dl-menthyl esters. A recombinant esterase (BsE) cloned from Bacillus subtilis 0554 shows excellent enantioselectivity to dl-menthyl acetate and has been immobilized using cross-linked enzyme aggregates. Though BsE has relatively high substrate tolerance, the conversion of dl-menthyl acetate decreased sharply with the increase of substrate loading from 1 to 3 M in mono-aqueous system, which might be due to the severe inhibition of enzyme activity at extremely high load of substrate or product. In this work, enzymatic hydrolysis of dl-menthyl acetate with an extremely high load using the immobilized CLEA-BsE was investigated in an organic-aqueous biphasic system containing surfactant to establish a promising bioprocess for large-scale production of l-menthol. Results An efficient biphasic reaction system of pentanol-water containing sodium dodecyl sulfate (SDS) was developed for improving enantioselective hydrolysis of dl-menthyl acetate to produce l-menthol by immobilized BsE. Under the optimized reaction conditions, l-menthol was produced in >97% enantiomeric excess (ee) at a substrate load of up to 3.0 M with >40% conversion. Conclusions All the positive features demonstrate the potential applicability of the bioprocess for the large-scale production of l-menthol.
Highlights
Menthol is an important cyclic monoterpene alcohol having eight optical isomers because of three chiral centers
Enzymatic hydrolysis of dl-menthyl acetate in mono-aqueous system The activity of the free BsE was determined as 1,304 s−1, and the activity recovery of the immobilized BsE was as high as 70%
Immobilized BsE could efficiently catalyze the enantioselective hydrolysis of dl-menthyl acetate to produce l-menthol in mono-aqueous phase containing 10% ethanol
Summary
Menthol is an important cyclic monoterpene alcohol having eight optical isomers because of three chiral centers. L-menthol is one of the most important flavoring chemicals used extensively in oral products, pharmaceuticals, tobacco products, confectionaries, and shaving products [1,2], and a useful chiral resolving reagent [3,4]. Symrise Incorporation (Germany) first developed a chemical synthesis process for the industrial production of l-menthol using thymol as a raw material [6,7]. Takasago International Corporation (Japan) developed an elegant route from myrcene based on the catalytic asymmetric isomerization of geranyldiethylamine by using a chiral catalyst (S)-BINAP-Rh, invented by Nobel laureate Ryoji Noyori. The production of l-menthol has reached 1,000 t annually [8,9,10,11]
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