Question

You are using ammonium sulphate to purify protein Q (pI = 5.0) by salting out from a solution at pH 7.0. How should you adjust the pH of the mixture to maximize the amount of protein Q that precipitates?

Short answer

By lowering the pH of the mixture it will maximize the amount of protein Q that gets precipitated.

Step-by-step solution

Definition of protein

Huge molecules known as proteins are composed of basic building pieces known as amino acids.

How to adjust the pH to maximize the protein

The pI and pH will be equal to 5.0 if the pH is reduced from 7.0 to 5.0. If they both equalise the protein, then it will be the least soluble since the net charge will be zero (and it will promote precipitation).