## **Core Concept**
The isoelectric pH, also known as the isoelectric point (pI), is the pH at which a particular molecule or surface carries no net electrical charge. This concept is crucial in understanding the behavior of proteins in different pH environments, particularly in the context of electrophoresis and protein separation techniques.

## **Why the Correct Answer is Right**
At the isoelectric pH, proteins have an equal number of positive and negative charges, resulting in a **net charge of 0**. This occurs because the pI is the pH at which the protein has an equal number of acidic and basic residues. As a result, the protein does not migrate in an electric field when the pH of the solution equals the pI of the protein. This property is fundamental for various laboratory techniques, including isoelectric focusing, which separates proteins based on their isoelectric points.

## **Why Each Wrong Option is Incorrect**
* **Option B:** Are positively charged - This is incorrect because at the isoelectric pH, the positive and negative charges on the protein are balanced, resulting in no net positive charge.
* **Option C:** Are negatively charged - Similarly, this is incorrect because at the isoelectric pH, there is no net negative charge on the protein.
* **Option B and C are essentially describing conditions that occur at pH values below or above the pI, respectively.**

## **Why Option D is Incorrect**
* **Option D:** Don't migrate - While it's true that proteins do not migrate in an electric field at their isoelectric pH due to having no net charge, option A directly addresses the question's focus on the net charge.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the isoelectric point is critical for understanding protein behavior in various biochemical assays. For example, in **isoelectric focusing**, proteins are separated based on their pI values, which can be useful in identifying specific proteins in a mixture.

## **Correct Answer: A. Have Net charge '0'**