Chemical Relaxation Study of the Electron Transfer Between Laccase and External Redox Couples

The temperature jump chemical relaxation method was employed to study the electron transfer reactions between fungal laccase and two different, single electron redox couples: Mo(CN) 4−/3− 8 and Ru(CN) 4−/3− 6 . The reactions were followed by measurement of the light absorption bands at 610 and 330 run. In the perturbed equilibrium of laccase with the Mo(CN) 4−/3− 8 only a single relaxation time was observed at 610 nm, and from its concentration dependence, the second‐order rate constants of electron transfer to and from the type 1 Cu(II) were evaluated. Similarly, at 610 nm a single concentration‐dependent relaxation time was observed in the laccase −Ru(CN) 4−/3− 6 equilibrium. At the 330 nni band a concentration‐independent chemical relaxation was resolved. Fluoride ions were found to have no effect on the observed rates of electron exchange with the type 1 copper ions. These results are in line with the kinetic data obtained by stopped flow and pulse radiolysis showing the type 1 Cu(II) to be the first electron accepting site. In addition, it has been found that this site may be oxidized by single electron acceptors at rather fast rates. In the reaction with the Ru(CN) 4−/3− 6 couple an intramolecular electron transfer to and from the 330 nm chromophore is proposed.