Williopsis californica, Williopsis saturnus, and Pachysolen tannophilus: novel microorganisms for stereoselective oxidation of secondary alcohols.

A screening of 416 microorganisms from different taxonomical groups (bacteria, actinomycetes, yeasts, and filamentous fungi) has been performed looking for active strains in the stereoselective oxidation of secondary alcohols. The working collection was composed of 71 bacterial strains, 45 actinomycetes, 59 yeasts, 60 basidiomycetes, 33 marine fungi, and 148 filamentous fungi. All microorganisms selected were mesophilic. Yeasts were the most active microbial group in the whole-cell-catalyzed oxidation. Williopsis californica, Williopsis saturnus, and Pachysolen tannophilus were the strains of greatest interest, both as growing cells and as resting cells. The oxidation of the alcohols takes place when cells are in the stationary growth phase (after 48 h of culture). These three strains are S-stereoselective for the oxidation of racemic secondary alkanols and show stereospecificity in the oxidation of menthol or neo-menthol, whereas iso-menthol is not oxidized. In the case of the 1-tetrahydronaphtol enantiomers, only the S-enantiomer is oxidized. The three strains were immobilized by entrapment using agarose and agar from algae of the Gracilaria genus. The agarose derivatives displayed significant improvement in the stereospecificity of the reactions.