Catalytic Coupling of CH4 with CO2 and CO by a Modified Human Carbonic Anhydrase Combined with Oriented External Electric Fields: Mechanistic Insights from DFT Calculations

The leading edge of biocatalysis is human intervention to expand the specificity of the reactions, so that enzymes can catalyze an impressive range of challenging chemical reactions. Here density functional theory (DFT) calculations were used to explore the catalytic coupling of CH4 with CO2 and CO into value-added chemicals by a rhodium­(I)-substituted human carbonic anhydrase [Rh­(hCAII)] under the oriented external electric fields (OEEFs), and possible mechanisms and OEEF effects have been discussed. The DFT calculations show that the rate-determining step for the catalytic coupling of CH4 and CO2 into acetic acid is CO2 insertion and the formation of a C–C bond, and the application of OEEF (Fx = +0.0075 au) can remarkably reduce the free energy span from 37.4 to 19.3 kcal/mol. The coupling of CH4 with CO into acetaldehyde has a barrier requirement of below 28.0 kcal/mol. On the basis of the present results, the artificial carbonic anhydrase is very promising for the conversion of carbon-based small molecules with the judicious use of OEEFs.