The energetics of hydrogen bonds in model systems: Implications for enzymatic catalysis

Low-barrier or short, strong hydrogen bonds have been proposed to contribute 10 to 20 kilocalories per mole to transition-state stabilization in enzymatic catalysis. The proposal invokes a large increase in hydrogen bond energy when the pK(a) values of the donor and acceptor (where K-a is the acid constant) become matched in the transition state (Delta pK(a) = 0). This hypothesis was tested by investigating the energetics of hydrogen bonds as a function of Delta pK(a) for homologous series of compounds under nonaqueous conditions that are conducive to the formation of low-barrier hydrogen bonds. In all cases, there was a linear correlation between the increase in hydrogen-bond energy and the decrease in Delta pK(a), as expected from simple electrostatic effects. However, no additional energetic contribution to the hydrogen bond was observed at Delta pK(a) = 0. These results and those of other model studies suggest alternative mechanisms by which hydrogen bonds can contribute to enzymatic catalysis, in accord with conventional electrcstatic considerations.