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Enormous Hydrogen Bond Strength Enhancement through π-Conjugation Gain: Implications for Enzyme Catalysis.

Surprisingly large resonance-assistance effects may explain how some enzymes form extremely short, strong hydrogen bonds to stabilize reactive oxyanion intermediates and facilitate catalysis. Computational models for several enzymic residue-substrate interactions reveal that when a π-conjugated, hydrogen bond donor (XH) forms a hydrogen bond to a charged substrate (Y-), XH can become significantly more π-electron delocalized, and this "extra" stabilization may boost the [XH···Y-] hydrogen bond strength by ≥15 kcal/mol. This reciprocal relationship departs from the widespread pKa concept (i.e., the idea that short, strong hydrogen bonds form when the interacting moieties have matching pKa values), which has been the rationale for enzymic acid-base reactions. The findings presented here provide new insight into how short, strong hydrogen bonds could form in enzymes.