A chromophore-supported structural and functional model of dinuclear copper enzymes, for facilitating mechanism of action studies

Abstract

Type III dicopper centres are the heart of the reactive sites of enzymes that catalyze the oxidation of catechols. Numerous synthetic model complexes have been prepared to uncover the fundamental chemistry involved in these processes, but progress is still lagging much behind that for heme enzymes. One reason is that the latter gain very much from the informative spectroscopic features of their porphyrin-based metal-chelating ligand. We now introduce sapphyrin-chelated dicopper complexes and show that they may be isolated in different oxidation states and coordination geometries, with distinctive colors and electronic spectra due to the heme-like ligands. The dicopper(i) complex 1-Cu2 was characterized by ¹H and ¹⁹F NMR spectroscopy of the metal-chelating sapphyrin, the oxygenated dicopper(ii) complex 1-Cu2O2 by EPR, and crystallographic data was obtained for the tetracopper(ii)-bis-sapphyrin complex [1-Cu2O2]2. This uncovered a non-heme [Cu₄(OH)₄]^4- cluster, held together with the aid of two sapphyrin ligands, with structural features reminiscent of those of catechol oxidase. Biomimetic activity was demonstrated by the 1-Cu2O2 catalyzed aerobic oxidation of catechol to quinone; the sapphyrin ligand aided very much in gaining information about reactive intermediates and the rate-limiting step of the reaction.

Fig. 1. Structures of heme b (a), type III copper (b) and bimetallic sapphyrin ((c) substituents omitted).

Fig. 2. Insertion of copper into sapphyrin 1-H2 ((a) Ar = pentafluorophenyl, [Cu^I] = [CuCl(BTMSA)]2), the three kinds of copper complexes obtained under various conditions, and (b) the electronic spectra of 1-Cu2O2 (orange trace), 1-Cu2 (pink trace) and [1-Cu2O2]2 (blue trace) in CHCl₃.

Fig. 3. X-ray crystal structure of [1-Cu2O2]2. (a) Front view with emphasis on Cu–O–Cu angles and distance between two Cu atoms coordinated by the same sapphyrin. (b) Side view and coordination environment of the Cu atoms (inset).

Fig. 4. (a) Aerobic oxidation of DTBC to DTBQvia1-Cu2O2 catalysis, monitored using changes in the absorption spectra of the latter for one hour with time intervals of 5 min (inset: reaction conditions and results obtained via¹H NMR investigations). (b) UV-Vis titration of 1-Cu2O2 with increasing amounts of TCCAT and (inset) intensity changes at 518 nm as a function of the TCCAT/1-Cu2O2 molar ratio. (c) Proposed reaction mechanism with symmetrical binding of catechol to the dicopper centre as the key intermediate and reoxidation of the reduced catalyst as the rate-limiting step.