Light at the end of the tunnel? The rate and kinetic isotope effect of an enzymatic H-tunneling reaction are not dependent on solution viscosity (see picture), despite clear evidence that this reaction is coupled to promoting motions within the protein. This finding is in contrast to the viscosity dependence of conformationally gated electron-transfer reactions and suggests that the mechanisms of environmental gating of these reactions are different.
Hay, S., Pudney, C., Sutcliffe, M. J., & Scrutton, N. S. (2008). Are environmentally coupled enzymatic hydrogen tunneling reactions influenced by changes in solution viscosity? Angewandte Chemie-International Edition, 47(3), 537-540. https://doi.org/10.1002/anie.200704484