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: [log in to unmask]
*******************************************
----- Original Message -----
From: "Nadir T. Mrabet" <[log in to unmask]>
To: "Jacob Keller" <[log in to unmask]>
Cc: <[log in to unmask]>
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: [log in to unmask]
>
>
>
> 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: [log in to unmask] <mailto:[log in to unmask]>
>> *******************************************
>
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