The water hydration shell around proteins strongly influences the protein dynamical properties, and quantum tunnelling effects have likely implications for biological activity. An example of the interplay between hydration water and protein behaviour is the globular enzyme lysozyme. This protein-water system is of particular interest due to the presence, at room temperature, of a transition in the proton conductivity at a critical hydration level coinciding with the onset of enzymatic activity. Using Vesuvio to probe the local proton environment at ultra-short time scales, we determined the momentum distribution of hydrated lysozyme. The momentum distribution in the hydration shell shows a higher population at low- and high-momentum states with respect to bulk water, indicating a likely occurrence of quantum effects. Numerical simulation of protons performed using the experimentally-determined momentum distribution confirm this idea. These results support the presence of proton tunnelling associated with biological function.
F Bruni, SE Pagnotta, (Università degli Studi Roma TRE, Italy), A Pietropaolo, R Senesi (Università degli Studi di Roma 'Tor Vergata', Italy)
Contact: Prof F Bruni, email@example.com
Research date: December 2008
R Senesi et al., Phys. Rev. Lett. 98 (2007) 138102