How Does Pressure Affect Barrier Compression and Isotope Effects in an Enzymatic Hydrogen Tunneling Reaction?

摘要

The shape of the reaction barrier is crucial for enzymatic hydrogen tunneling reactions. Thus, the concept of barrier compression is of fundamental importance. Such compression is believed to arise from "promoting vibrations"-rapid, sub-picosecond donor-acceptor (DA) vibrational modes that transiently compress the reaction barrier, thus enhancing the probability of tunneling and of over-the-barrier transfers and hence the rate of H-transfer. For reactions that are dominated by H-tunneling, this has experimentally observable impact on the kinetic isotope effect (KIE) and its temperature-dependence, △△H~+: by shortening the tunneling distance, DA compression decreases the KIE, while the temperature-dependence of the vibration gives rise to an elevated △△H~+. However, the role of promoting vibrations in H-tunneling reactions is highly contentious. We present a computational study on the effect of pressure on DA compression in an enzymatic H-tunneling reaction, demonstrating that experimental observations are directly coupled to the promoting vibration.