You can typically assign the histidine orientation based on analysis of the hydrogen bond network. In ambiguous cases you might have to look a few residues deep. WHAT_CHECK does this for you by a global optimization of the hydrogen bond network.
Cheers, Robbie Sent with my Windows Phone ________________________________ Van: Smith Liu<mailto:smith_liu...@163.com> Verzonden: 20-5-2015 14:12 Aan: CCP4BB@JISCMAIL.AC.UK<mailto:CCP4BB@JISCMAIL.AC.UK> Onderwerp: [ccp4bb] HIS related crystallography issue Dear All, Suppose the protein crystal resolution is about 2-3A, then in the map it should be rather difficult to distinguish the C and N in the sidechain of HIS. In this way we may regard the sidechain of HIS is flippable. But suppose in one flipped conformation of the HIS, the free N in the sidechain of HIS can form H-bond with the neighbour H of the OH of the Thr, in the other flipped conformation of the HIS, the free N in the sidechain of HIS cannot form H-bond with the neighbour H of the OH of the Thr (caused by distance issue). Suppose whether the H-bond forms between the free N in the sidechain of HIS and the neighbour H of the OH of the Thr was very important biologically, in this situation how can we distinguish whether the H-bond forms? Smith
