Spectroscopy and kinetics of tyrosinase catalyzed trans-resveratrol oxidation

The spectroscopy and kinetics of the tyrosinase catalyzed trans-resveratrol oxidation were investigated by measuring both UV–vis absorption spectra over the 200–500 nm range and Raman spectra over the 600–1800 cm–1 region. Room temperature UV–vis absorption spectra, as a function of time, showed the presence of two isosbestic points located at λ1 = 270 nm and λ2 = 345.5 nm delimiting two different regions: the reactant region around 300 nm, where the absorption decreased with time, and the product region over the low wavelength (λ 390 nm) wavelength zone in which the absorption increased with time until, in both cases, constant values were achieved. A first-order kinetics was deduced with a rate coefficient of k1 = (0.10 ± 0.001) min–1, which turned out to be independent of substrate concentration over the 50–5 μM range; a feature that was rationalized by invoking the limiting case of the Michaelis–Menten scheme appropriate for substrate concentration much lower than the respective Michaelis constant. The observation of the distinct resonance enhanced Raman lines, specifically those peaking at 830 cm–1, 753 cm–1, and 642 cm–1 together with their time evolution, permitted us to gain insight into some crucial features and steps of the catalytic reaction. Namely, that the formation of the so-called trans-resveratrol and tyrosinase SP complex with its O–O bridge plays a crucial role in the first steps of this enzymatic reaction and that the hydroxylation of the ortho C–H bond of the trans-resveratrol OH group occurs after O–O bond cleavage in the tyrosinase active site. The present study makes clear that a class of potential inhibitors of tyrosinase can be found in compounds able to bind the two Cu (II) ions of the enzyme bidentate form.