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Dear Sudhir, A blob surrounded by three Asp residues looks like a positively charged ion. What else (buffer, additives) is in your crystallization drop? I would also try to fit cysteine/serine. At 2 Å you will quickly see whether or not this is correct. Good luck! Herman ________________________________ From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of Sudhir Kumar Sent: Friday, February 10, 2012 7:06 AM To: [log in to unmask] Subject: [ccp4bb] Unidentified density Dear all, I have a 2 A structure of an enzyme which show 12 molecules per asymmetric unit. While placing waters i found out some blobs where i could not model any ligand. It is surrounded by 3 Asp molecules. I have attached screenshot of the blob at 1.5 sigma cutoff 2fo-fc density. Crystallization condition had following precipitants: PEG 1000, Peg 3350, MPD, Ethylene glycol,and protein contained Glycerol. My protein is known to bind Cysteine/Serine, though i haven't added any in crystallization buffer. any suggestion what it might be are welcome. thanks in advance -- best regards Sudhir Kumar Research Scholar C/O Dr. S. Gourinath Structural Biology Laboratory SLS, JNU, New Delhi-110067