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Tri-tert-butoksysilanotiolanowe kompleksy cynku(II), kadmu(II) i kobaltu(II) jako modele koordynacji jonu cynku w miejscu aktywnym dehydrogenazy alkoholowej i niektórych innych enzymów

Dołęga A.

Wiadomości Chemiczne

2010

[Z] 64, 5-6

389-411

Zinc is ubiquitous in living organisms and second only to iron among the transition metals. Catalytic zinc sites have been found in all six classes of enzymes. Liver alcohol dehydrogenase (EC 1.1.1.1, LADH, Fig. 1) is a zinc enzyme, which catalyzes reversible oxidation of alcohols to aldehydes utilizing as a co-substrate NAD+. The mechanism of an action of the enzyme is summarized by an equation: R1R2CHOH + NAD+ = R1R2CO + NADH + H+ The coordination of zinc in LADH is usually described as pseudotetrahedral but there were several findings supporting penta-coordination of zinc during the catalytic cycle (Fig. 1 and 2) [1–7]. Studies on synthetic analogues of functional sites of the enzymes are used to elucidate the substrate – metal interactions [8]. Tri-tert-butoxysilanethiol which was used as a ligand to model cysteinate – metal ion centers has several advantageous features. It secures formation of mononuclear transition-metal complexes, exhibits resistance towards hydrolysis, and additionally its pKa value is close to that of cysteine mercapto group [9–11]. Over 20 tri-tertbutoxysilanethiolate complexes with MS2NO2 (M = transition metal) cores were synthesized to model the enzymatic centre of alcohol dehydrogenase, including complexes with water, methanol and ethanol (Fig. 5). Crystal structures of zinc(II), cobalt(II), cadmium(II), and manganese(II) complexes were determined and compared to the structures of LADH [12–17]. Structural features of zinc(II) and cadmium(II) complexes were analyzed (Fig. 9, Tab. 2) [15]. 113Cd CP/MAS NMR spectra of tri-tert-butoxysilanethiolate cadmium(II) complexes were determined and compared with the 113Cd NMR spectra of the cadmium-substituted alcohol dehydrogenase. Solid state 113Cd NMR shifts of five-coordinate cadmium(II) complexes with the CdNO2S2 coordination core were found to be identical to that of cadmium-substituted alcohol dehydrogenase in complex with NAD+ [15,18]. The equilibrium character of the reactions between bis(bis(tri-tert-butoxysilanethiolate) cadmium(II) and nitrogen ligands, with a low equilibrium constant of heteroleptic complex formation, were proved by 113Cd NMR studies in solution [18, 19]. The electronic absorption spectra of heteroleptic cobalt(II) complexes with tri-tert-butoxysilanethiolate and imidazole ligands were measured in methanol and solid state. The equilibrium reactions in the methanolic solutions of cobalt complexes were described on the basis of the analysis of charge transfer bands S→dCo LMCT and the molecular structures of the obtained complexes [17]. EPR spectra revealed the high-spin configuration of the studied cobalt(II) compounds [17]. Quantum mechanical calculations performed on the model zinc complex indicate ca10–20% decrease in enthalpy of ethanol deprotonation due to complexation with Zn2+ [15].