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Isoelectric points of proteins and exchange columns

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You have a protein that is well behaved at pH=7.0 (The ph of the buffer is 7.) and the pI of your protein is equal to 4.3 (pI=4.3). You have decided to use ion exchange chromatography to help isolate your protein.
1.Using this information, would you use a cationic column (anion exchange) or an anionic column (cation exchange)? Why?
2.What residues in your protein do you think would be in a majority?
3.Same question as in question 1 with the exception that your protein has a pI=9.2.

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1) Since the pI (the pH at which the protein has 0 charge) is 4.3, when we are at pH=7, we will be generating more DEPROTONATION and thus more negative charge. So we will be generating anions, ...

Solution Summary

In this solution the use of the isoelectric point is explained. Also we explain how changes in the pH of the buffer containing a protein will alter its charge if we know its isoelectric point. A brief discussion of cation and anion exchange columns is also included, focusing on how to pick the correct column for the given conditions.

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Galactose represser protein

The galactose represser protein from E. coli has a pI of about 5.9. While purification protocols were being designed, it was found to bind to a Mono-S column at pH values of 7 and below. (Mono-S columns have S-type sulfonic acid groups attached to the resin and are strong cation exchangers.)

What is unusual about this observation, and explain these results?

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