Tabled stability constants are e.g. in 'NIST Critically Selected
Stability Constants of Metal Complexes' an electronic database. I am
sure you find also tables in the library.
Another source is
http://old.iupac.org/publications/pac/1997/pdf/6907x1549.pdf; here
enthalpies are given for several at different ionic strength etc. but
not for imidazol with Mg2+ or Ca2+.
Jacob Keller wrote:
Could those who responded with numbers for affinities of imidazole for
metal ions please divulge their sources? It is not that I doubt their
veracities, but it would be a nice reference to have on hand.
For those wondering about why I was asking about imidazole's affinity
for metal ions, I was wondering whether the presence of imidazole
would affect a metal-ion-dependent reaction. With, for example, 200 mM
imidazole and 10 mM Ca++ or Mg++, what would be the amount of free
metal? This can of course be calculated from imidazole's binding
constant for these ions, which is another reason I ask for the sources
of the numbers quoted in a couple of the responses.
Thanks for all of the helpful responses so far,
Jacob Keller
*******************************************
Jacob Pearson Keller
Northwestern University
Medical Scientist Training Program
Dallos Laboratory
F. Searle 1-240
2240 Campus Drive
Evanston IL 60208
lab: 847.491.2438
cel: 773.608.9185
email: [EMAIL PROTECTED]
*******************************************
----- Original Message ----- From: "Nadir T. Mrabet"
<[EMAIL PROTECTED]>
To: "Jacob Keller" <[EMAIL PROTECTED]>
Cc: <CCP4BB@JISCMAIL.AC.UK>
Sent: Friday, July 18, 2008 8:55 AM
Subject: Re: [ccp4bb] Imidazole's ability to chelate metal ions
Imidazole can indeed complex (monodentate) metal ions but not chelate
them (bidendate, at least).
However, the stability constant, K, of such complexes is rather low,
eg log K = 0.1 for Mg, 3.3 for Fe and 4.2 for Cu.
In comparison, metal chelates are formed with EDTA, for which log K =
10.6 for Mg, 14.2 for Fe and 18.8 for Cu.
So the difference amounts to several orders of magnitude.
It should also be pointed out that the competitive effect of
imidazole in IMAC does not involve binding to free metal ions,
but instead coordination to immobilized metal chelates, eg
Ni(II)-nitrilotriacetate (Ni-NTA, where NTA is the chelator).
In any situation where one assays a protein whose activity and/or
stability and/or else is/are metal dependent, one should
rather use buffers (see below) that do not interfere (eg Good's
buffers).
I suspect the imidazole in your case is either a buffer (pKa 7.0) or
else results from competitive elution from an IMAC column.
What should be done depends on your exact conditions.
hth,
Nadir
--
Pr. Nadir T. Mrabet
Cellular & Molecular Biochemistry
INSERM U-724
Nancy University, School of Medicine
9, Avenue de la Foret de Haye, BP 184
54505 Vandoeuvre-les-Nancy Cedex
France
Phone: +33 (0)3.83.68.32.73
Fax: +33 (0)3.83.68.32.79
E-mail: [EMAIL PROTECTED]
Jacob Keller wrote:
Dear Crystallographers,
Does anybody happen to know whether imidazole is able to chelate
metal ions in solution? It seems reasonable that since it can
compete for binding to IMAC resins, it should have some affinity for
at least Ni++ and Co++, but what about metal ions like Ca++ and
Mg++? I assume that the affinity is weak, but at the concentrations
at which we are wont to use it in our elutions (~100-500 mM), does
it not seem likely that other metal ions are being competed away
from our proteins as well?
Jacob Keller
*******************************************
Jacob Pearson Keller
Northwestern University
Medical Scientist Training Program
Dallos Laboratory
F. Searle 1-240
2240 Campus Drive
Evanston IL 60208
lab: 847.491.2438
cel: 773.608.9185
email: [EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>
*******************************************
--
***********************************
Priv.Doz.Dr. Guenter Fritz
Fachbereich Biologie
Sektion Naturwissenschaften
Universitaet Konstanz
http://www.biologie.uni-konstanz.de/fritz
Universitaetsstrasse 10
Postfach M665
D-78457 Konstanz
e-mail: [EMAIL PROTECTED]
Tel. Office: +49-(0)7531 88 3205
Tel. Lab : +49-(0)7531 88 3687
Fax: +49-(0)7531 88 2966
