Enolate anions, kinetic reduction

Whatever the explanation, the effect of acids is less marked than the selectivffy in alkaline solutions, where a attack is largely suppressed. The effect of alkali may depend upon the formation and selective reduction of enolate anions. The A2 4-dienolate anion, which is the major product of kinetically-controlled enolisation by bases (see p. 156) is seen from a molecular model to have a somewhat "folded conformation of the A/B ring system (ii). The convex / -face of the A/B ring system and the absence of an axial 2jS-proton should favour approach to the catalyst from this direction, whereas the a-face of the A -bond is severely hindered by the axial hydrogens at C(7) and C<9>. 

X 10 if the enolate anion intermediate were not stabilized in the active site this value is 10 -fold less than the observed value for the kcat, 500 s. Recall that an enolate anion is necessarily on the reaction coordinate, so the value of AG° must be reduced for the enolate anion to be kinetically competent irrespective of whether AG int can be reduced. Thus, the active site of mandelate racemase must decrease AG° from the value predicted from the values of the substrate carbon acid and the active site base in solution. The obvious strategy to accomplish this reduction is preferential stabilization of the enolate anion intermediate relative to the carbon acid substrate, the increased negative charge on (or proton affinity of) the carbonyl/carboxylate oxygen of the enolate anion intermediate provides a convenient handle for enhanced electrostatic or hydrogen bonding interactions with the active site. 

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