A Simple Method for Determining the Relative Strengths of Normal and Low-Barrier Hydrogen Bonds in Solution: Implications to Enzyme Catalysis

A simple method has been developed to study the relative strengths of normal and low-barrier hydrogen bonds in solution using the maleic/fumaric and mesaconic/citraconic acid equilibria. Isomerization of maleic and fumaric acids was catalyzed by thiourea and isomerization of mesaconic and citraconic acids was catalyzed by the thiophenyl radical, with equilibrium isomeric ratios determined by HPLC. The monoanion of mesaconic and citraconic acid was found to greatly favor the cis isomer in aprotic solvents under conditions in which the cis isomer forms an intramolecular low-barrier hydrogen bond, but to slightly favor the trans isomer in protic solvent. The trans isomer is also favored with the diacid and with the corresponding monoamide, for which normal pK_a-mismatched intramolecular hydrogen bonds are formed by the cis isomers. The cis–trans equilibria were used to estimate the relative strength of the intramolecular hydrogen bonds formed by the cis isomers. The low-barrier hydrogen bond of citraconic monoanion in DMSO is estimated to be about 4.4 kcal/mol stronger than the pK_a-mismatched hydrogen bonds of the diacid and monoamide. These data provide a basis for prediction of the potential differences in strengths of hydrogen bonds between ground state and reactive or transition state complexes in enzyme catalysis.