Chapter 3: Proton Tunnelling and Proton-coupled Electron Transfer in Biological Systems: Theory and Experimental Analysis

This chapter discusses various aspects of proton tunnelling in biomolecules. Two experimental model systems are explored in detail: a “proton wire” in green fluorescent protein and proton-coupled electron transfer in the enzyme soybean lipoxygenase. These two systems demonstrate, respectively, the application of electronically adiabatic and electronically non-adiabatic analytic models of proton tunnelling that provide useful insight into methods of experimental data analysis. Surprisingly rapid (sub-nanosecond) time constants are found for the inherent ground-state tunnelling rates at 298 K and it is demonstrated how these absolute rates help to constrain other parameters, particularly for the electronically adiabatic reaction where the agreement between experiment and theory is found to be excellent. Approximations that are often made in analytical models, such as the Born–Oppenheimer separation of heavy and light atom motion and the “linear approximation” to the exponential coupling of donor–acceptor displacement, are tested by comparison to exact results. The important effect of anharmonic donor–acceptor interactions is illustrated using a simple model that, when applied to the tunnelling kinetics of the lipoxygenase enzyme, helps to determine the magnitude of the electric field-induced force that compresses the donor and acceptor atoms at the active site. Tunnelling-based kinetic trapping is also suggested as a potential mechanism by which protons can be moved along a protein-based “wire” in a preferred direction.