Mushroom mediated selective bioreduction of S-(+)-carvone to cis-(-)-dihydrocarvone: approach towards a safer biocatalysis

Dihydrocarvone, possessing four stereoisomers is an important flavour and chiral building block in chemical synthesis. Ascomycetes are well known for the selective bioreduction of carvone to dihydrocarvone. Often, these fungi produce mycotoxins which may contaminate the biocatalytic product. Herein, Ganoderma sessile, a polypore mushroom, selectively reduced S-(+)-carvone to cis-(-)-dihydrocarvone (DHC) in its submerged culture. In an optimised condition (0.75 g/L, 18 h, pH 3-5, 30 °C and 150 rpm), 82.7% cis-(-)-DHC was obtained in gas chromatography-mass spectrometry profile of the fermented product. The absolute titre of cis-(-)-DHC in fermentation medium was 0.35 ± 0.01 g/L. However, substrate toxicity (IC₅₀ 0.15 g/L) drastically reduced the transformation at higher carvone concentration (≥1.0 g/L). On the other hand, R-(-)-carvone was less selective and efficient in producing the desired isomer i.e. trans-(+)-DHC. G. sessile is the member of a group of non-toxic medicinal mushrooms and may be a safer yet efficient option for producing cis-(-)-DHC biocatalytically.