The change in hydrogen bond strength accompanying charge rearrangement: implications for enzymatic catalysis

The equilibrium for formation of the intramolecular hydrogen bond (KHB) in a series of substituted salicylate monoanions was investigated as a function of delta pKa, the difference between the pKa values of the hydrogen bond donor and acceptor, in both water and dimethyl sulfoxide. The dependence of log KHB upon delta pKa is linear in both solvents, but is steeper in dimethyl sulfoxide (slope = 0.73) than in water (slope = 0.05). Thus, hydrogen bond strength can undergo substantially larger increases in nonaqueous media than aqueous solutions as the charge density on the donor or acceptor atom increases. These results support a general mechanism for enzymatic catalysis, in which hydrogen bonding to a substrate is strengthened as charge rearranges in going from the ground state to the transition state; the strengthening of the hydrogen bond would be greater in a nonaqueous enzymatic active site than in water, thus providing a rate enhancement for an enzymatic reaction relative to the solution reaction. We suggest that binding energy of an enzyme is used to fix the substrate in the low-dielectric active site, where the strengthening of the hydrogen bond in the course of a reaction is increased.