Finally, we note that a recent neutron structure suggested that also the HIE form of histidine could function as proton donor, leading to formation of OOH and a doubly deprotonated histidine. Setting out from 2 HIE and 2 red HIE , we investigated this latter possibility with linear transit calculations using TPSS/def2-SV(P). Both transfers are strictly uphill, and releasing the restraint on the O d ...H ε2 distance leads to H ε2 transferring back to HIE.