Binding Site in Close Proximity to a Catalytic or Reactive Center

Binding Site in Close Proximity to a Catalytic or Reactive Center 

The key element of the strategy described in this section is the provision of a binding site for the substrate close to a catalytic center. Of the mechanistic effects described by Jencks (I), the following contribute proximity and orientation of reactants and covalent catalysis. 

Page and Jencks (16) estimated that the entropy changes in solution may correspond to an effective concentration of about 6 x 10 M, and high EM values [10 -10 M] have been measured for simple cyclization reactions (17). However, with rare exceptions, the simple approximation of reactants with the help of synthetic supramolecular assemblies results in rate increases that are tiny by comparison with those achieved by enzymes, as evidenced by typical EM values of less than 10 M. 

FIGURE 1 Molecular structures of I, II, and a-cyclodextrin (a-CD) alkoxide. Schematic representation of the inclusion complexes and reaction intermediates involved in the hydrolysis of I affording acetic acid and m-ferf-butyl phenol. The inset shows the CAChe-minimized structure of the ternary catalytic complex I C a-CD alkoxide-ll proposed by Bender et al. (7S). The putative hydrogen bond between the alkoxide of a-CD and II is indicated by a solid line. a-CD alkoxide is shown in stick representation, and only polar hydrogen atoms are specified. I is shown in CPK representation and II in ball and stick. (See Color Insert in the back of this book.) 

Komiyama s work (18) on m-ferf-butylphenyl acetate was aimed at shedding light on the role of the charge relay catalytic system. However, the manner of combination of the three groups in the model system is apparently different from that occurring in the enzymes. 

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