Hydrogen transfer in large systems

Studying hydrogen transfer reactions experimentally is difficult because the reactions occur on ultrafast femtosecond time scales and the systems of enzymes are too large to obtain fine spectra. Describing them theoretically in large systems such as enzymes is challenging due to a proton's quantum dynamics.

Accurate computational algorithms require a balance between accounting for strong quantum effects among small degrees of freedom and weak quantum effects in the large system as a whole. Scientists have now developed such a method with EU support of the project 'VASPT2: A method for targeted quantum dynamics of hydrogen transfer reactions' (VASPT2).

Researchers divided the system into active regions (small and local) and bath regions (large and global). The team then applied a computationally heavy approach to the active regions and treated the rest of the system and coupling between two regions with a mean-field approach. The latter focuses on one particle or entity and replaces all interactions with the other entities with an average (mean) interaction. The new method was applied to formic acid, a prototype system with weak and strong correlations. Theoretical predictions of vibrational wave functions (fundamental spectral bands) were shown to match experimental values quite well.

The team also developed a method to describe semi-global potential energy surfaces related to hydrogen transfer reactions. Again, there is a trade-off between computational load and the need to describe quantum dynamics. VASPT2 members used a novel linear regression approach to fit the semi-global potential energy surface that minimises 'over-fitting' but does not create unphysical holes.

Implemented in a new programme suite for quantum dynamics called DYNAMOL, the novel frameworks provide computationally efficient and accurate descriptions of hydrogen transfer reactions. They are expected to help answer one of the most important open questions in biochemistry, namely whether or not quantum effects are important for enzymatic reactions. VASPT2 has thus made an invaluable contribution to the design of improved catalysis that is so important to many industrially relevant reactions.