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	9

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 you select? 

Concept:-

• Based on their affinity for ion exchangers, ions and polar compounds can be separated using ion exchange chromatography (also known as ion chromatography).
• Thus, the reversible exchange of ions between the target ions in the sample solution and the ions on the ion exchangers constitutes the separation principle.
• Positively charged ions will be drawn to cationic exchangers with negatively charged groups. Because their negative charges originate from the ionization of acidic groups, these exchangers are also known as "Acidic ion exchange" materials.
• Positively charged ions will be drawn to negatively charged anions by anionic exchangers. Also known as "Basic ion exchange" materials, these.
• The isoelectric point (pI) is the pH value at which the molecule carries no electrical charge.
• The concept is particularly important for zwitterionic molecules such as amino acids, peptides, and proteins.

Explanation:-

Option:- pH 8, cation exchanger resin

• If pH>pI then it exhibits a net negative charge and binds to an anion exchanger.
• If pH=pI then no net charge and no binding to the ion exchanger.
• And if pH<pI then it s a positive charge and will bind to the cation exchanger
• Here, pH is less than pI, and if pH<pI then it s a positive charge and will bind to the cation exchanger.

Conclusion:-

Hence, the pH 8, cation exchanger resin is correct.