MODEL SYSTEMS FOR FLAVOENZYME ACTIVITY - ONE-ELECTRON AND 2-ELECTRON REDUCTION OF FLAVINS IN APROTIC HYDROPHOBIC ENVIRONMENTS

The processes occurring during the electrochemically irreversible redu ction of flavins in aprotic organic medium have been the cause of cons iderable controversy. To provide insight into these events we have inv estigated the reduction of flavins alkylated at N(3). We have demonstr ated that the reversible reduction of N(3)-methylated flavin can be co nverted to the irreversible reduction observed with unalkylated flavin s through addition of the weak external proton donor phthalimide. Prot on transfer to the flavin radical anion generated at the electrode lea ds to partial formation of the neutral flavin radical; which is instan taneously further reduced to the fully reduced flavin anion. This mirr ors the reduction of unalkylated flavins, where the proton source is t he imide proton at N(3) of fully oxidized flavin in bulk solution. Sim ultaneous electrochemistry and EPR experiments confirm the disappearan ce of electrogenerated flavin radical anion both methylated and non-me thylated at N(3), upon addition of phthalimide. UV/vis spectroelectroc hemistry likewise confirmed the generation of the radical anion in the absence of proton donors and fully reduced flavin in the presence of proton donors.