Hydroxide bridging

Coordination networks can be crystallized from the zinc ion and 4,4 -bipyridyl.283,284 Some of these networks, including that of Hoskins et al. show interpenetration of an independent grid such as square sheets.2 These do not form spontaneously with bridging hydroxides as is observed with flexible bipyridyls. 

Zinc hydroxide and alkoxide species are particularly relevant to catalytic processes, often forming the active species. The cooperative effects of more than one zinc ion and bridged hydroxides are exploited in some enzymatic systems. Zinc alkyl phosphate and carboxylate materials have been important in the formation of framework compounds, often containing large amounts of free space for the inclusion of guest molecules. Aldehyde and ketone compounds are of low stability due to the poor donor capabilities of the ligands however, a number of examples have recently been characterized. 

However as well as electrostatics the making and breaking of bonds must be considered. The Be-0 bond has considerable covalent character so that it is not obvious, a priori, that replacement of water molecules by bridging hydroxide should be favored by a negative enthalpy... 

There are three mechanistic possibilities for catalysis by two-metal ion sites (Fig. 10). The first of these is the classic two-metal ion catalysis in which one metal plays the dominant role in activating the substrate toward nucleophilic attack, while the other metal ion furnishes the bound hydroxide as the nucleophile (Fig. 10 a). Upon substrate binding, the previously bridged hydroxide shifts to coordinate predominately with one metal ion. Enzymes believed to function through such a mechanism include a purple acid phosphatase [79], DNA polymerase I [80], inositol monophosphatase [81],fructose-1,6-bisphosphatase [82], Bam HI [83], and ribozymes [63]. 

Fig. 10. Participation of a bi-nuclear metal site in enzymatic catalysis (a) the substrate coordinates to one metal ion while the other activates the nucleophile (b) the substrate is stabilized by amino acid side chains, and the metal ions provide the bridging hydroxide as the nucleophile (c) the bridging hydroxide is displaced by the substrate, and the nucleophile is another water, which may be activated by an amino acid side chain in the active site...

Finally we should briefly mention the purple acid phosphatases, which, unlike the alkaline phosphatases, are able to hydrolyse phosphate esters at acid pH values. Their purple colour is associated with a Tyr to Fe(III) charge transfer band. The mammalian purple acid phosphatase is a dinuclear Fe(II)-Fe(III) enzyme, whereas the dinuclear site in kidney bean purple acid phosphatase (Figure 12.13) has a Zn(II), Fe(III) centre with bridging hydroxide and Asp ligands. It is postulated that the iron centre has a terminal hydroxide ligand, whereas the zinc has an aqua ligand. We do not discuss the mechanism here, but it must be different from the alkaline phosphatase because the reaction proceeds with inversion of configuration at phosphorus. 

The half-life of Fe2(OH)2" at room temperature is a few seconds. An improved model for the kinetics of dissociation of this dinuclear cation recognizes significan articipation by Fe2(OH)3 + at higher pHs, thus clearing up earlier slight anomalies in this area. Phosphate ester hydrolysis at the di-iron center of uteroferrin has now been shown to involve nucleophilic attack by bridging hydroxide (as proposed but not conclusively demonstrated for several M—OH—M-containing catalytic species) rather than by hydroxide bonded to just one Fe. ... 

The suggestion that the electrophile is activated by coordination to the zinc ion is prevalent in the experimental literature but usually does not find support in theoretical investigations. The binuclear zinc enzymes can be an exception because one zinc center can coordinate the electrophile while the other is used to generate the nucleophile. A question often raised for binuclear zinc enzymes is whether the nucleophile (usually OH ) is bridged or coordinates to only one of the zinc atoms. This question is often made more difficult by the fact that bridging hydroxides are often found in X-ray structures... 

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