Laccase-catalyzed mediated oxidation of benzyl alcohol: the role of TEMPO and formation of products including benzonitrile studied by nanoelectrospray ionization Fourier transform ion cyclotron resonance mass spectrometry.

Substituted benzyl alcohol was oxidized enzymatically with a laccase-mediator system and the products were investigated as a function of time by nanoelectrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (nanoESI-FTICRMS). With Trametes versicolor laccase (TVL), the mediator, 2,2',6,6'-tetramethylpiperidine-N-oxyl radical (TEMPO), undergoes oxidation and forms oxoammonium ion. Oxidized TEMPO oxidizes the alcohol and is simultaneously reduced to the N-OH form. The laccase then restores TEMPO back to the normal radical form and the oxidation cycle starts again. The role of TEMPO and the structures of its oxidized and reduced forms in the enzymatic oxidation process were clarified in collision-induced dissociation experiments and gas-phase hydrogen/deuterium (H/D) exchange reactions. The amounts of enzyme and mediator were significant for product formation: with greater amounts overoxidation products, the corresponding benzoic acid and benzonitrile were formed. Smaller amounts of laccase and mediator generated benzaldehyde in high yield. The reaction pathway for benzonitrile formation is discussed and it is suggested to start from benzaldehyde and the ammonia in the ammonium acetate buffer.