Binuclear complexes, metal-binding

The main problem with the reaction of metal centres with 02 is its tendency to react irreversibly with oxidation of the metal centre, to form either cis dioxo species or pi oxo bridged binuclear complexes (Scheme 2.4a). However, in cases where there is a strong driving force towards lower oxidation states, the reaction is somewhat reversible, e.g. reaction with Vaska s compound (Scheme 2.4b). The role of the haem centre in haemoglobin is to ensure not only that is 02 binding reversible, but that both its complexation and release occur rapidly and at the correct concentrations. These concentrations, or partial pressures, must correspond to those found in the lung and intracellular medium, respectively. 

Binuclear Zn complexes compared to mononuclear species were also shown to be more efficient by a factor of about 3 for hydrolyzing phosphate mono- and diesters (308). These complexes are based on dimers derived from 1,4,7-triazacyclododecane and 1,5,9-triazacyclo-tetradecane ligands (14 and 15, Fig. 16). A 10 rate enhancement over the noncatalyzed reaction was obtained. The distance between the metal binding sites influenced the reagent reactivity. Short spacers... 

This binuclear Zn(II) coordination complex in ZntR, coupled with the bound anion, was totally unexpected since Hg-MerR requires Just one trigonally coordinated Hg(II) ion that bridges the MerR dimer to effect metalloregulation of the /Her operon. The extent to which occupancy of one or both metal sites in each of two symmetry-related metal-binding loops in ZntR regulates transcriptional activation has yet to... 

Sternlicht, Jones, and Kustin (5) have found evidence that the metal ion binds, in such complexes, to the base of one molecule and the phosphate of another molecule. Models demonstrate that our NMR data can be explained if there is a binuclear complex in which each metal ion is bound to the base of one molecule and to the phosphate of another (Figure 9). 

Structure and Mechanism. Cytochrome-c oxidase catalyzes the four-electron reduction of molecular oxygen to water and couples these redox processes to proton transfer across the mitochondrial membrane.As depicted in Fig. 24, the enzyme is structurally complex and contains four metal centers which are redox active, two copper ions and two heme a groups. One copper ion, Cug, and one of the heme a groups, cytochrome <13, (cyt a ), form a binuclear center which binds dioxygen. Electrons are ... 

---