Joe,
As David said many different types of salt crystals can grow in the
presence of phosphate, even when the concentrations of PO4 or many
divalent cations are VERY low (10s of micromolar or less). Struvite
is common (NH4MgPO4) when ammonium sulfate is the precipitant, mM
magnesium is present, and the pH is >7; struvite is a common mineral
in kidney stones. If we have removed phosphate just before
crystallization, as with a spin desalting column (best way) or by
repeated spin concentration (poorer way), we always redouble check
"hits" in divalent cation containing conditions with a variety of
controls (e.g., keep the filtrate fraction from the last spin
concentration step).
Michael
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R. Michael Garavito, Ph.D.
Professor of Biochemistry & Molecular Biology
513 Biochemistry Bldg.
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On Jan 23, 2008, at 4:06 AM, David Briggs wrote:
Hi Joe,
I've known most salt crystals in Phosphate - and I think most people
are weary of phosphate.
Also, Calcium Sulphate is a fairly common one, esp if your buffers are
titrated with sulphuric acid. Fluoride Ions are also prone to form
salt crystals with transition metal ions.
HTH,
Dave
On 22/01/2008, Joe Krahn <[EMAIL PROTECTED]> wrote:
Salt crystals are common in macromolecular crystallography. Has
anyone
tried to tabulate salt crystal forms that commonly occur?
I just identified a salt crystal as Mirabilite, made of Na2SO4·10H2O.
The high water content makes them rather soft, and may not be
recognized
as salt right away. In this case, it probably happened because the
buffer was made with Na·Citrate + HCl instead of citric acid, while
trying to optimize conditions. So, characterization of salt
crystals can
help to avoid the conditions that cause them.
There is probably a reasonably small number of salt crystal forms
that
are very common in crystallization trials. Maybe it would be
useful to
tabulate common salt crystals to help guide optimization experiments.
Has anyone else tried to use salt crystal information beyond ensuring
that it is not protein?
Joe Krahn
--
============================
David C. Briggs PhD
Father & Crystallographer
http://www.dbriggs.talktalk.net
AIM ID: dbassophile
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