Calculation of Mg-ions in solution

Frank Nygaard frank at
Thu Dec 3 17:22:02 EST 1998


During a kinetic study of an enzyme requiring magnesium both as free ions as
well as MgATP, I need to calculate the concentration of each component in my
assay condition. A typical assay condition could be: 50 mM Tris-HCl, 50 mM
Pi, 2 mM EGTA, 2 mM ATP and 5 mM MgCl2 at pH 8,2 - 37degree.
My main concern binding of magnesium to Pi and Tris. I have tried two
programs: "cabuffer" by Jochen Kleinschmit and a program by  Karl-Josef
Foehr and Wojciech Warchol which I think is called "calcium". The programs
give similar results in the absence of Pi and Tris. The binding of Mg++ to
Pi however are estimated differently by the two programs where cabuffer
gives a relatively high concentration of MgPi compared to MgATP whereas
"calcium" have very low binding of Mg++ to Pi.
I've found a stability constant for MgHPO4 of 500 M-1 corresponding to a Kd
of 2 mM which would obvious contribute to binding of Mg++.  None of the
programs calculate the binding of Mg++ to Tris. I've not been able to find
any stability/dissociation constant for this complex but the dissociation
constant for Mn-Tris complex have been reported to be 250 mM.

Does anybody have experience using these programs, or know of similar
programs for calculating the destribution af Mg-ions in solution?

Thanks in advance

Frank Nygaard

Centre for Crystallographic Studies
University of Copenhagen
frank at

More information about the Proteins mailing list