Excess electron capture by hydrated histidine side-chain group

An excess electron in condensed phase models of hydrated protonated histidine residue (HisH+) in proteins has been investigated using ab initio calculations and molecular dynamics simulations, which focuses on electron-binding mode and evolution mechanism of the captured electron in the HisH+ side chain hydrated clusters. Results indicate that distribution of an excess electron is highly associated with the number of water molecules and the geometric configurations of the hydrated clusters. An electron can stably localize in three cases depending on the size of the hydrated clusters. First, an excess electron always occupies the π* orbital of the imidazole ring not only after vertical binding but also in relaxation process. Second, an excess electron firstly vertically localizes in a Rydberg-like orbital, and then relaxes to reside in the π* LUMO localizing in the imidazole ring. Third, an electron always occupies the Rydberg-like orbital both after vertical binding and after relaxation. Furthermore, molecular dynamics simulations reveal that the captured excess electron prefers to localize around HisH+ no mater how it distributes at the initially bound states (localized versus delocalized), and the evolution time for the solvated electron from the initially bound state to the finally localized state at HisH+ is about 220 fs, as demonstrated from an excess electron-bound HisH+(H2O)40 model cluster.