Catalytically active sites chemical shift

Adsorption of toluene on zeolites Li-X, Na-X, K-X, Rb-X, and Cs-X has been investigated with quantum chemical methods. Calculations of geometries, Mulliken partial charges, and C chemical shift parameters of clusters representing the catalytically active site are presented. The polarisation of the toluene carbons is the first step in alkylation reactions catalysed by zeolites and, at an early stage, will influence the outcome of the reaction. We show the simultaneous influence of the Lewis acidic cation and the basicity of the zeolite is responsible for altering the electron distribution within the toluene and thus affecting the outcome of an alkylation reaction. 

He attributed the positively enhanced lines In Figure 6b to the active site His 119 on the basis of Its known chemical shifts (Markley, 1975a). This Is nicely confirmed by a photo-CIDNP experiment In the presence of the competitive Inhibitor cytldlne 2 -monophosphate, which Is known to bind with Its phosphate group to His 119. In figure 7 the normal photo-CIDNP difference spectrum of 1.5 mH RNase A Is compared with that In the presence of 8 mM 2 -CMP. It can be seen In Figure 7b that the positive lines have disappeared from the spectrum. Thus, the Inhibitor blocks accessto His 119 or otherwise Inactivates this residue towards the photoreaction. Since histidines play an In rtant role In the catalytic mechanism of many enzymes, the enhancement of histidine resonances Toy CIDNP Is of particular Interest. 

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