​The pI of four proteins (A, B, C, D) are shown in the table below:​

​Protein​​	pI
A	4
B	5.5
C	7
D	4

​To purify 'D' from a mixture of these four proteins in a single step, using ion-exchange chromatography, what combination of buffer pH and ion-exchange resin would be used?​

Correct option is D

Explanation-

To purify protein D (pI = 9) from a mixture of proteins A (pI = 4), B (pI = 5.5), and C (pI = 7), we can use the principle of ion-exchange chromatography, where:
                                             At a pH below its pI, a protein is positively charged → binds to a cation exchanger.
                                             At a pH above its pI, a protein is negatively charged → binds to an anion exchanger.
Goal:
We want protein D to bind the resin while the others do not (they should flow through).
Protein D has a pI of 9. So ,  at pH 8, D will be positively charged.
Proteins A, B, and C (pI 4, 5.5, and 7 respectively) will all be negatively charged at pH 8.
Therefore:
Use cation exchange resin (which binds positively charged proteins like D at pH 8). Proteins A, B, and C (negatively charged at pH 8) will not bind and elute out. Protein D will bind, and you can elute it later by increasing salt or changing pH.

Correct Option:
Option d : pH 8, cation exchanger resin