Characterization of Low-Barrier Hydrogen Bonds. 6. Cavity Polarity Effects on the Formic Acid−Formate Anion Model System. An ab Initio and DFT Investigation

High-level ab initio molecular orbital and density functional theory calculations incorporating cavity polarity effects via the use of self-consistent reaction field (SCIPCM) simulations reveal that the short, strong hydrogen bond formed between a formic acid molecule and a formate anion is significantly, but nowhere near completely, weakened by the presence of an extremely polar cavity. These results suggest that even if an enzyme active site were to present an environment as polar as aqueous water, the formation of a low-barrier, or short-strong, hydrogen bond would still be some 8 kcal/mol more favorable than the corresponding neutral, traditional, weak hydrogen bondlike the one formed between two formic acid molecules. The short, strong hydrogen bond formed between a formic acid and a formate anion is clearly much more sensitive to the effects of its environment than is a typical weak traditional hydrogen bond. However, even in the most polar of cavities, the calculated hydrogen bond energy for formic acid−formate anion is greater than 12 kcal/mol, whereas the calculated hydrogen bond energy for formic acid−formic acid is less than 4 kcal/mol. These results suggest that cavity polarity effects alone are insufficient grounds to rule out the low-barrier hydrogen bond facilitated mechanism, as proposed by Gerlt, Gassman, Cleland, and Kreevoy several years ago.