EDTA-Magnesium relationship
Michael Allen
I was wondering if any of you could tell me how many divalent cations
(Mg2+ specifically) a single EDTA (ethylenediaminetetraacetic acid)
molecule can bind at one time.
I have searched high and low for this, but could not find a good answer.
Thanks in advance
Mike Allen
University of Waterloo
Ontario, Canada
J. Martinez-Irujo
divalent cations.(Sorry, no catalog at hand).
Michael Allen wrote:
--
Juan J. Martinez Irujo
Departamento de Bioquimica
Universidad de Navarra
Pamplona, Spain
---
Robert Hartley
Martinez-Irujo") wrote:
OK I may be talking complete nonsense here BUT :-)
think about it:- the ED-tetra-acetic-acid has 4x 0minus on the end and no
direct ionic bond. Therefore WRT divalent cations; it cant be 2 and it
cant be more, therefore it must be one or none, but it cant be none so it
must be one. :-)))))
I must get out more.
> Only one. I remember that Merck catalog had a picture showing how EDTA binds
> divalent cations.(Sorry, no catalog at hand).
>
> Michael Allen wrote:
>
> > Hello Bionetters
> >
> > I was wondering if any of you could tell me how many divalent cations
> > (Mg2+ specifically) a single EDTA (ethylenediaminetetraacetic acid)
> > molecule can bind at one time.
now this is the tricky bit. you have to compare the preferential
relationship between Mg2+ and Ca2+ and decide which percentage of EDTA
will have Mg bound and Ca bound. Then you must consider the concentrations
of Mg and Ca in the system and then do a calculation.
> >
> > I have searched high and low for this, but could not find a good answer.
> >
> > Thanks in advance
cheers
bob
--
Robert Hartley,
Centre for Cell Engineering,University of Glasgow,UK.
mail: r...@mblab.gla.ac.uk, Tel: ++44 (0)141 330 4756
Web : http://www.gla.ac.uk/Inter/CellEngineering
tfitz...@gilead.com
>From: Michael Allen (m3a...@sciborg.uwaterloo.ca)
>Date: Thu 30 Mar 2000 - 20:33:02 BST
>I was wondering if any of you could tell me how many divalent cations
>(Mg2+ specifically) a single EDTA (ethylenediaminetetraacetic acid)
>molecule can bind at one time.
>I have searched high and low for this, but could not find a good answer.
>Thanks in advance
>Mike Allen
>University of Waterloo
>Ontario, Canada
Chelation of magnesium by EDTA occurs at a 1:1 molar ratio. The stability
constants of EDTA for Ca++ and Mg++ are 10.61 and 8.83. The log10 of the
difference (1.78) is approximately 60, therefore the ratio of complexed
Ca++ to Mg++ in an equimolar mix is about 60:1. EGTA has a lower stability
constant for Mg++, so the ratio is about 4 e 5.
The Ki at pH 7 or 8 is the "apparent" value at a particular pH which
considers all forms of complexing species, rather than only the most
anionic. EDTA has 4 pKa values (1.99, 2.67, 6.16 and 10.26) and, at pH
7.0, a = 2.08 e 3, so that log a = 3.3. For Mg++:EDTA at pH 7.0: log Ki
(apparent) = 8.7 - 3.3 = 5.4. See R. M. C. Dawson, D. C. Elliot, W. H.
Elliot and K. M. Jones 1969, Data for Biochemical Research. Oxford Univ.
Press, New York.
www.biotechniques.com/ has a free chelator calculator program available to
subscribers:
PACKCHEL.ZIP  Size: 54521 byte(s).
Chelator Calculation of total and free divalent cations obtained using
metal chelators. The program automatically recalculates metal-chelator
stability constants for effects of ionic strength, temperature, and pH. By
editing the
datafile, one may add/delete/change chelators, metals, or pH buffers
present in the original CHELDFLT.DAT datafile or any other datafiles
created with CHELATOR.
Tim Fitzwater
Gilead Sciences
---
Wolfgang Schechinger
It depends on how much EDTA is ionized.
Theoretically, 1 molecule may bind 2 ions Mg (four negative charges
will complex 2 * 2 positive charges), but since EDTA is a weak acid,
the molecule will not be 100% ionized in average.
In the end, one would have to do some calculations using the 4
different pK values for EDTA assuming that Mg2+ comes from 100%
dissociated MgCl2. Of course all depends from the solution's pH.
Happy calculating. Maybe a nice problem for next weeks freshmen's
anorganic course? Is there any?
It's probably easier to make some Mg2+/EDTA mixtures and add some
other chelator that will form a coloured complex with free Mg, and
put the stuff into the photometer.
Have a nice weekend!
Wolfgang
> think about it:- the ED-tetra-acetic-acid has 4x 0minus on the end
> and no direct ionic bond. Therefore WRT divalent cations; it cant be
> 2 and it cant be more, therefore it must be one or none, but it cant
> be none so it must be one. :-)))))
>
> I must get out more.
> > Only one. I remember that Merck catalog had a picture showing how
> > EDTA binds divalent cations.(Sorry, no catalog at hand).
> >
> > Michael Allen wrote:
> >
> > > Hello Bionetters
> > >
> > > I was wondering if any of you could tell me how many divalent
> > > cations (Mg2+ specifically) a single EDTA
> > > (ethylenediaminetetraacetic acid) molecule can bind at one time.
> now this is the tricky bit. you have to compare the preferential
> relationship between Mg2+ and Ca2+ and decide which percentage of
> EDTA will have Mg bound and Ca bound. Then you must consider the
> concentrations of Mg and Ca in the system and then do a calculation.
>
> > >
> > > I have searched high and low for this, but could not find a good
> > > answer.
> > >
> > > Thanks in advance
>
> cheers
> bob
>
> --
> Robert Hartley,
> Centre for Cell Engineering,University of Glasgow,UK.
> mail: r...@mblab.gla.ac.uk, Tel: ++44 (0)141 330 4756
> Web : http://www.gla.ac.uk/Inter/CellEngineering
-----
This message is encrypted. Use your brain to decode it.
-----
Dr. Wolfgang Schechinger, Dept. of Pathobiochemistry
University of Tuebingen, Germany
email: wolfgang.s...@med.uni-tuebingen.de
wwWait: http://www.medizin.uni-tuebingen.de/~wgschech/start.htm
-----
usual disclaimers apply
-----
---
J. Martinez-Irujo
molecule. The matter is not how many charges EDTA has, but what makes EDTA
so efficient in chelating cations. EDTA behaves like an octopus: each
nitrogen forms a dative bond with the cation and oxygens from the carboxylic
groups forms additional coordination bonds. As a result, the cation remains
tightly bound at the center of the structure (and only a cation can be bound
in this way).
It is of interest that immobilized iminodiacetic acid (IDA, half and EDTA
molecule), commonly used in the purification of his-tagged proteins, is
also able to chelate cations. In this case, the remaining coordination bonds
are formed by water, or by histidines if the protein is bound. As reported
in this newsgroup, a method to elute these proteins is by adding EDTA, that
is able to easily remove cations from IDA and histidines.
Incidentally , EDTA was sometimes used to titrate calcium solutions, in the
absence of other divalent cations, in the way proposed by Wolfgang.
Eriochrome black T is used as indicator in these titrations (blue in the
free form, dark red when calcium is bound), and, of course, a 1:1 proportion
is used to calculate calcium concentration.
ChenHA
" J. Martinez-Irujo" wrote:
>
> I agree with Tim Fitzwater in that a single cation is bound per EDTA
> molecule. The matter is not how many charges EDTA has, but what makes EDTA
> so efficient in chelating cations. EDTA behaves like an octopus: each
> nitrogen forms a dative bond with the cation and oxygens from the carboxylic
> groups forms additional coordination bonds. As a result, the cation remains
> tightly bound at the center of the structure (and only a cation can be bound
> in this way).
>
If anyone is interested, here's a picture of EDTA chelating
a metal ion using its six binding sites:
http://www.chem.neu.edu/Courses/1221Tom/99Lecture15/sld003.htm