You are trying to purify a protein with a pI of 8.2 by cation exchange chromatography. Which pH would be best for the buffer when initially binding the protein to the column?

Question

You are trying to purify a protein with a pI of 8.2 by cation exchange chromatography. Which pH would be best for the buffer when initially binding the protein to the column?

BlackTom AI · Accepted Answer

To decide the buffer pH for initial binding in cation exchange chromatography, we need the protein to carry a net positive charge so it will bind to the negatively charged (cation exchanger) resin. A protein’s net charge depends on the pH relative to its isoelectric point (pI). When pH < pI, amino groups tend to be protonated and the protein is positively charged; when pH > pI, it tends to be negatively charged.

Option 7.2: This pH is below the pI of 8.2, so the protein will be positively charged and will interact with the cation exchanger, promoting binding. This makes 7.2 a favorable choice for initial binding.

Option 9.2: This pH is above the pI, meaning the protein would be negatively charged at this pH. It would not bind well to a cation exchanger and might instead flow through or bind weakly, which is undesirable for the initial capture step.

Option 8.2: This pH is equal to the pI. At the pI, the protein has no net charge on average, leading to minimal binding to a charged resin and poor interaction with the column, making this suboptimal for binding.

In summary, the pH must be kept below the pI to ensure a positive net charge for effective binding to a cation exchanger; among the options, 7.2 best satisfies this requirement while 9.2 and 8.2 do not.

You are trying to purify a protein with a pI of 8.2 by cation exchange chromatography. Which pH would be best for the buffer when initially binding the protein to the column?Single choice

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