In electrophoresis, what does the isoelectric point (pI) represent?

The isoelectric point (pI) is defined as the pH at which a molecule, such as a protein, carries no net electrical charge. At this specific pH, the number of positive charges on the molecule equals the number of negative charges, resulting in a neutral overall charge. This property is crucial in techniques like electrophoresis, where the movement of molecules in an electric field is influenced by their net charge.

When a protein is at its pI, it does not migrate in an electric field because there is no driving force exerted on it due to charge. Instead, the protein tends to precipitate out of solution because of reduced solubility at this point. Understanding the pI is essential when separating proteins by electrophoresis, as manipulating the pH of the buffer can enhance resolution in protein separation.

The other choices do not accurately describe the isoelectric point. While option A refers to a point where a molecule becomes positively charged, this occurs at a pH below the pI. Options C and D relate to protein stability and denaturation, which are separate concepts that do not directly pertain to the definition of the isoelectric point.