Three FMN-dependent oxidoreductases, YcnD and YhdA from Bacillus subtilis and Lot6p from Saccharomyces cerevisiae, oxidised α,β-unsaturated carbonyl compounds and a thioether, respectively, to furnish the corresponding racemic epoxides or sulfoxide, respectively. The mechanism of this enzyme-mediated (rather than enzyme-catalysed) oxidation was shown to proceed via the NADH-dependent reduction of O₂, forming H₂O₂, which acted as oxidant in a spontaneous (non-enzymatic) fashion.