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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: [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]> >>> ******************************************* >> -- *********************************** 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: [log in to unmask] Tel. Office: +49-(0)7531 88 3205 Tel. Lab : +49-(0)7531 88 3687 Fax: +49-(0)7531 88 2966