Acid-base activity, changed coordination

Changes in acid-base activity. Water bound to a metal ion frequently is more acidic than free water, and coordination to proteins enhances this effect still more. This results in M — OH species that can then react with other substrates. Mg and Zn are common metal ions that serve this function. 

It is known that a vast variety of enzymes use metal ions in acid/base catalysts. In some cases the role of the metal is to activate water directly, e.g. Zn(OH)2 becomes Zn(OH ) in carbonic anhydrase, but in others it may be that the metal just forms a particularly constructive (useful) H-bond network, e.g. calcium in phospholipase A2 and in staph, nuclease. Substitution of one metal by other metals is now a critical test of the precision of the catalytic site and we know that nickel does not substitute for zinc in carbonic anhydrase, although it binds, and that Sr(II) has a different activity in lipases and nucleases from Ca(II). It is the water in the coordination sphere which is partly responsible for these changes. 

The postulated catalytic cycle of the asymmetric epoxidation reaction is shown in Figure 13.10. A lanthanide metal alkoxide moiety changes to a rare earth metal-peroxide through proton exchange (I). In this step, lanthanide metal alkoxide moiety functions as a Bronsted base. The rare earth metal-BINOL complex also functions as a Lewis acid to activate electron-deficient olefins through monoden-tate coordination (II). Enantioselective 1,4-addition of rare earth metal-peroxide gives intermediate enolate (III), followed by epoxide formation to regenerate the catalyst (IV). 

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