Active-site selective reagents properties

Goeldner, Hirth and colleagues have presented evidence that the efficiency of labeling by certain photoaffinity reagents is improved in the environment provided by the receptor. They define a photosuicide inhibitor as a ligand analog of an enzyme or a receptor, the photodecomposition of which is selectively induced by the intrinsic physico-chemical properties of an active site (Goeldner et al., 1982). 

More promising from an industrial perspective, however, is the separation of the oxidation zone from the aqueous one effected by the catalytic material itself, through the selective adsorption of the reagents. The introduction of Titanium Silicalite-1 (TS-1), in which the hydrophobic properties of the pores protect the active sites from the inhibition of the external aqueous medium, was a demonstration of the concept. The catalyst, the substrate and the aqueous soluhon of hydrogen peroxide can, in this case, be mixed together, with a great simplification of the process and also a reduction of the hazards. Three commercial processes. 

TS-1 is an efficient and selective catalyst for the oxidation of various organic molecules with H O sulphur and nitrogen compounds, alcohols, olefins, aromatic and aliphatic C-H bonds [11, 92-93]. Selectivity is the result of the electrophilic properties of active oxidant species and of the shape selectivity. The latter arises from diffusion of reagents and products and from steric constraints in the transition state (restricted transition state shape selectivity). Molecules having a cross section larger than about 0.6 nm cannot difhise to TS-1 active sites and are not oxidised. This restricts TS-1 catalysis to almost linear molecules and mononuclear aromatic compounds, bearing small or no substituents. On the other hand, small molecules can be selectively oxidised in the presence of bulkier ones. 

The interactions of a reagent with a zeolite are governed by the shape-selective properties and the confinement effects previously mentioned. The cavities and pores of a zeolite can be considered as microreactors, containing stabilized active sites, which can be designed to have particular properties. This is comparable to the active site of an enzyme, where the catalytic activity depends on the three-dimensional arrangement of the functional groups on the side-chains of the amino acids of the protein. Several of these zeolite mimics of enzyme action have been developed, and three are worthy of mention here. 

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