There
may be PDB files which shows different protonation state of
HIS, at least there is PDB file shows double protonated HIS.
At which resolution of the crystal
can allow us to distinguish different protonation states of
HIS?
an example:
M. Blakeley, F. Ruiz, R. Cachau, I. Hazemann, F. Meilleur, A.
Mitschler, S. Ginell, P. Afonine, O. Ventura, A. Cousido, M.
Haertlein, A. Joachimiak, D. Myles & A. Podjarny. Quantum model
of catalysis based on a mobile proton revealed by subatomic x-ray
and neutron diffraction studies of h-aldose reductase. (2008). PNAS.
105, 1844-1848.