Final answer:
Proteins stop moving and separate in preparative isoelectric focusing because of their isoelectric pH (pI). At the pI, proteins have an equal number of positive and negative charges and become least soluble, resulting in their precipitation.
Step-by-step explanation:
Proteins stop moving and separate in preparative isoelectric focusing because of their isoelectric pH (pI). Isoelectric pH is the pH at which a protein has an equal number of positive and negative charges.
When proteins are subjected to an electric field, they do not move towards the anode or cathode at their pI. Instead, they become least soluble and precipitate. This property is used to isolate proteins in preparative isoelectric focusing.