Isoelectric point

Isoelectric point (pI) is a term in biochemistry that refers to the pH at which a particular molecule or surface carries no net electrical charge. The concept of isoelectric point is particularly important in the fields of protein chemistry and electrophoresis.

Overview

The isoelectric point is the pH at which a particular molecule carries no net electrical charge. To understand the isoelectric point, it is important to understand that most biological molecules are polymeric and can carry both positive and negative charges. The exact charge on a molecule is determined by the pK values of the various ionizable groups in the molecule, which in turn are determined by the pH of the surrounding medium.

Calculation

The isoelectric point can be calculated using the Henderson-Hasselbalch equation, which relates the pH, pKa, and the ratio of the concentrations of the ionized and unionized forms of a weak acid or base. In the case of proteins, the isoelectric point is typically calculated using the pK values of the amino and carboxyl groups at the ends of the protein, as well as the pK values of any ionizable side chains.

Applications

The isoelectric point has several practical applications in the field of biochemistry. For example, it is used in the design of buffer solutions that maintain the pH of a system at a constant value. It is also used in the separation of proteins by two-dimensional gel electrophoresis, a technique that separates proteins based on their isoelectric point and molecular weight.

See also

• pH
• Protein
• Electrophoresis
• Henderson-Hasselbalch equation
• Buffer solution