Heavy-Atom Skeleton Quantization and Proton Tunneling in “Intermediate-Barrier” Hydrogen Bonds

Quantum effects on proton transfer through barriers of several kcal/mol in hydrogen bonds are investigated theoretically in malonaldehyde. Such "intermediate-barrier" proton transfer processes play a key role in the catalytic activity of some enzymes. Tunneling is shown to be significant in this reaction even at room temperature. More importantly, the quantum nature of the heavy molecular frame atoms is found to substantially enhance proton tunneling. These findings have far-reaching implications for common modeling strategies of proton transfer in complex systems such as biomolecules.