A Model for the Active Sites

The active site for type I hydrogen adsorption appears to consist of isolated, noninteracting Zn—O pair sites which are not affected by the oxida- 

The essential features of the above picture are shown schematically in Fig. 6. Hydrogen or deuterium adsorbs and desorbs rapidly on these sites (half-time less than 1 min) hence, the slower H2-D2 equilibration (half-time about 8 min) appears to be determined by the site-to-site migration required for exchange. Poisoning experiments show that water also prefers these sites in agreement with the limited IR data, Fig. 6 shows the adsorbed water yields surface hydroxyls (10). 

Adsorption of ethylene as an olefinic species would not be likely to occur on the zinc half of the active site. A rigid ethylene molecule could not approach the sequestered zinc ions because of steric restrictions hence, ethylene would be confined primarily to the oxide part of this layer. In 

Let us now look at the chemistry of the reaction of water and hydrogen with the active sites. When water reacts with the active site, it seems quite clear that this should be viewed as heterolytic fission of an OH bond with the proton adding to the oxide ion and the hydroxide ion adding to the zinc ion. This is shown schematically below  

in this reaction the active site functions as an acid-base pair and the adsorption of water is an acid-base reaction. The driving force for this reaction is the resulting reduction of the charge separation. In a similar fashion we can view hydrogen adsorption as heterolytic fission at the highly polar active site, viz  

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Q. To proceed further at this point one has to specify a pore model for the catalyst, and a model for the active site distribution. Froment and co-workers have examined a variety of cases such as single pore models (single-ended pores and pores open on both sides) with both deterministic and stochastic active site distributions, the bundle of parallel pores model and various tree-like models of the porous structure, which were earlier used by Pismen (40) to describe transport and reaction in porous systems. Such treelike models contain interconnected pores but lack any closed loops and are usually called Bethe networks or lattices. They are completely characterized by their coordination number Z, which is the number of pores connected to the same site of the network. 

Nakamura, N., Kohzuma, T., Kuma, H., and Suzuki, S., 1992, The first topa containing copper (II) complex, [Cu(DL-topa)(bpy)(H20)]BF4 as a model for the active site in copper-cotaining amine oxidases, J. Am. Chem. Soc. 114 6550n6552. 

Berg, J. M., and Holm, R. H., 1985, A model for the active sites of oxo-transfer molybdo-enzymes reactivity, kinetics and catalysis, J. Am. Chem. Soc. 107 925n932. 

Compound 28.30 reacts with Zn(C104)2 6H20 to give a complex [Zn(28.30)(OH)] that is a model for the active site of carbonic anhydrase. Suggest a structure for this complex. What properties does 28.30 possess that (a) mimic the coordination site in carbonic anhydrase and (b) control the coordination geometry around the Zn ion in the model complex. 

H. Flohr, N, Paunhorst, and J. Retey (1976), Synthesis, structure determination, and rearrangement of a model for the active site of methylmalonyl-CoA mutase with incorporated substrate. Angew. Chem. Int. Ed. Engl. 15, 561 -562. 

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