Sorry for the late posting on this subject, but I wanted to add that you might want to also use knowledge of the preferred tautomer of neutral histidine when assigning the ionization state and orientation of the imidazole. Hong's group has done some very nice work on this: Li, S., and Hong, M. (2011) Protonation, tautomerization, and rotameric structure of histidine: a comprehensive study by magic-angle-spinning solid-state NMR, J Am Chem Soc 133, 1534-1544. On Apr 6, 2012, at 10:05 AM, Pavel Afonine wrote: Hi, 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. Pavel _______________________________________________ phenixbb mailing list [email protected]mailto:[email protected] http://phenix-online.org/mailman/listinfo/phenixbb