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It depends what else is in your crystallization mixture. I’d bet $100 that it’s not sulfate (or sulphate.) What about K+? JPK From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of Chris Fage Sent: Sunday, March 20, 2016 4:05 PM To: [log in to unmask] Subject: [ccp4bb] Sulfate ion density? Dear All, A sulfate ion appears to fit well into a blob of my electron density (please see image at https://drive.google.com/file/d/0B6ny9TrPnOP8YWRXcUZZa2VqaUU/view?usp=sharing). Water is too small to account for the density and the coordination number is too low for a monatomic ion (I have labeled distances to the closest atoms; all others are >3.9 angstroms away). I wonder if sulfate could originate from the M9 minimal media in which the protein was expressed. Has anyone else observed this? There is no sulfate in the crystallization condition or protein solution. A physiological role for sulfate is also unlikely for this enzyme. Thank you. Kind regards, Chris