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My 2-cents worth.
All pKa are given at 20 °C.
pKa (malate ; pK2) = 5.13
pKa (imidazole) = 6.95
Hence, usefull buffer range goes from 4.13 to 7.95 with better buffer
capacity from 4.4-7.7.
I wouldn't dare using the mixture as a buffer beyond pH 7.7 and
certainly not at pH 8.5.
If you need to cover the pH 5.5-8.5 range, I would suggest using an
equimolar mixture of MES (pKa = 6.15), MOPS (pKa = 7.2) and HEPPS (pKa =
8.0), all in acidic forms with titration with KOH (the amount of which
can be exactly calculated using the Henderson-Hasselbalck equation to
yield the desired pH value, in a highly reproductible fashion).
Furthermore, using switterionic buffers, you no longer have to worry
about activity coefficients as is the case with malate.
Good luck,
Nadir Mrabet
Pr. Nadir T. Mrabet
Cellular & Molecular Biochemistry
INSERM U-724
UHP - Nancy 1, School of Medicine
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]
Cell.: +33 (0)6.11.35.69.09
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Oganesyan, Vaheh wrote:
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Thanks to Marc Graille, Robyn Stanfield, Artem Evdokimov and Pietro
Roversi for sharing their experience on how to make Imidazole-Malate
buffer.
Essentially it is same as making phosphate buffer. In the range of pH's
from 5.5 to 8.5 mix the un-adjusted d-Malic acid and Imidazole solutions
in
specific proportions to get desired pH.
Regards,
Vaheh Oganesyan, PhD
Scientist II
MedImmune, Inc.
One MedImmune Way
Gaithersburg, MD 20878
USA
Phone: (301)398-5851
Facsimile: (301)398-9851
www.medimmune.com
-----Original Message-----
From: Pietro Roversi [mailto:[EMAIL PROTECTED]
Sent: Tuesday, January 09, 2007 4:33 AM
To: Oganesyan, Vaheh
Cc: [email protected]
Subject: Re: [ccp4bb]: Imidazole-Malate Buffer
Dear Vaheh,
I usually prepare the 2M solutions and then:
1. If a final basic pH is needed, I add malic acid to the imidazole
solution until the desired pH is achieved
2. If a final acidic pH is needed, I add the imidazole to the malic acid
solution until the desired pH is achieved
With best wishes
Pietro
--
Pietro Roversi
Sir William Dunn School of Pathology, Oxford University
South Parks Road, Oxford OX1 3ER, England UK
Tel. 0044-1865-275385
