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And how will you know if they are the calcium citrate xtals Sam asked about, and not
sodium citrate xtals?  Sodium citrate in the Hampton Crystal Screen condition will
crystallize out at 4 degrees.  It's solubiility is pH and temperature dependent.

You'll need to go to your friendly neighborhood synchrotron and either look at the
x-ray fluorescence emission spectrum to see if calcium is there, or collect a dataset 
and solve the structure. (Setting myself up here for a reply from Bruker about how
easy the latter would be).

Pete 

-----Original Message-----
From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of Jim Pflugrath
Sent: Tuesday, April 29, 2008 4:24 PM
To: [log in to unmask]
Subject: Re: [ccp4bb] Are Calcium Citrate Crystals A Common False Positive?

That's an easy hypothesis to test.  Simply set up your drops with the same conditions except without protein and see if you get crystals.  Please let us know the results.  Thanks!

Jim

On Tue, 29 Apr 2008, Sam Stephenson wrote:

> Are calcium citrate crystals a common false positive in trays with up 
> to 200mM of each?  There is absolutely no phosphate in the trays so 
> I'm almost positive they're not calcium phosphate. Cheers, Sam