What characteristic of proteins affects their movement in isoelectric focusing?

The correct answer is that the isoelectric point (pI) of proteins significantly affects their movement in isoelectric focusing. Isoelectric focusing is a technique used to separate proteins based on their charge. Each protein has a specific pI, which is the pH at which it carries no net electrical charge.

During isoelectric focusing, a pH gradient is established within a gel or an electric field. Proteins will migrate within this gradient until they reach the point where the pH equals their pI. At this point, the protein does not migrate any further because it has no net charge, meaning that the electrochemical forces acting on it are balanced. Therefore, the specific pI of a protein determines where it will stop moving within the gradient.

While size, thermal stability, and charge density may influence protein behavior in other contexts, they do not fundamentally dictate the specific pH at which proteins will cease to migrate in isoelectric focusing. This makes the isoelectric point the primary characteristic influencing the movement of proteins in this particular method of separation.