Wa. Alves et al., Equilibria and tyrosinase activity of a dinuclear and its analogous tetranuclear imidazolate-bridged copper(II) complexes, INORG CHIM, 321(1-2), 2001, pp. 11-21
A copper(II) complex, [Cu(apip)(imH)](2+), containing an imidazole ligand,
in addition to a discrete tridentate imine (apip = 2-[2-(2-pyridyl)ethylimi
no-1-ethyl]pyridine), was prepared. Both mono- and dinuclear species coexis
ted in aqueous solution exhibiting a pH-dependent monomer-dimer interconver
sion and were monitored by different techniques. Similar data were obtained
with the complex [SECu(imH)](+), where SE = 2-salicylidene-aminoethane. Mo
reover, these species were isolated as perchlorate salts and characterized
by cyclic voltammetry and spectroscopic techniques (U-V-Vis, IR, Raman, and
EPR). Experiments by capillary electrophoresis, in addition to parallel EP
R spectra at different pH values, permitted to estimate the equilibrium con
stant, corresponding to the equation: 2[LCu(imH)](2+) + OH- reversible arro
w [LCu(im)CuL](3+) + H2O + imH, K= (8 +/- 3) x 10(7) mol(-1)dm(3), in reaso
nable agreement to the value determined by spectrophotometric measurements,
K= (0.12 +/- 0.01) x 10(7) mol(-1)dm(3). A further tetranuclear species, [
Cu-4(apip)(4)im(4)](4+), was obtained by a pH-dependent self-assembly proce
ss, separated from very alkaline solutions of the corresponding mononuclear
complex. The tetranuclear and dinuclear species were observed to behave as
good functional models of the tyrosinase enzyme, catalyzing the aerobic ox
idation of 2,6-di-tert-butylphenol, in methanolic solution, and that of 3,4
-dihydroxyphenylalanine (L-dopa) in aqueous solution, with a biphasic behav
ior in the range of pH 7-11. Applying the Michaelis- Menten approach, the k
inetic parameters determined indicated that the tetranuclear species is a b
etter catalyst than the dinuclear ones, exhibiting a higher rate constant,
k(3), as well as a higher Km value. The hydrophobic cavity in the polynucle
ar complex seems to facilitate its interaction with the phenol substrate, s
ince the structural parameters determined for both the complexes, di- and t
etranuclear species were very similar. (C) 2001 Elsevier Science B.V. All r
ights reserved.