Rotate chi2 bond (CB-CG) and have two alternative conformations for the His. Add two water molecules (50% occupancy) each making a H-bond with the NE2 atoms (alternative  1 and 2)  of the His.

George

From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of SD Y
Sent: Saturday, March 08, 2014 7:29 AM
To: [log in to unmask]
Subject: [ccp4bb] density near Histidine

Hi,

During refinement, I see a strong density next to His and I have tried metal ions  and water but it seems to be different.  Known components of crystallization are Tris, NaCl, BME, MgOAc, PEG3350. Its only seen in one chain of AU. Are there any suggestions?

Thanks in advance

SDY