Strong hydrogen bonding in molecules and enzymatic complexes

The physicochemical properties of low barrier hydrogen bonds (LBHBs) and single well hydrogen bonds (SWHBs) are contrasted with the properties of conventional hydrogen bonds. The LBHBs are much more common than the rare, symmetrical SWHBs. LBHBs are characterized by shortened hydrogen bond lengths and correspondingly lengthened covalent bonding, shortened distances between participating heteroatoms, far downfield proton NMR signals, low deuterium fractionation factors and high values of the enthalpy of activation for exchange of the proton with solvent protons. For many years, it was thought that LBHBs could exist only in aprotic media or crystalline or vapor states. Recently, however, the downfield protons associated with hydrogen maleate, hydrogen cis-cyclohexane-1,2-dicarboxylate and hydrogen 2,2-dimethylmalonate have been observed in 0.32 mol fraction water in acetone-d6 at -50°C. The activation enthalpy for exchange of the downfield proton in hydrogen maleate with solvent water has been found to be 7.2 kcal mol-1, consistent with strong hydrogen bonding. Two examples of LBHBs in enzymes are the downfield proton bridging His 57 and Asp 102 in transition state analog complexes of chymotrypsin, and the proton bridging Tyr 14 and the equilenin-OH group in its complex with 3-ketosteroid isomerase. These protons display the structural, spectroscopic and chemical properties of LBHBs. Their presence in transition state analogue complexes suggests their importance in the transition states of enzymatic reactions. Copyright © 2001 John Wiley & Sons, Ltd.