Understanding Isoelectric Point (pI) in Protein Purification and Analysis

Isoelectric point (pI) is the pH at which a protein or other molecule has no net charge, meaning that the number of positively charged amino acid residues is equal to the number of negatively charged residues. At this point, the molecule is electrically neutral and does not have an overall charge.

For example, if a protein has a pI of 5.0, it means that the protein has an equal number of positively charged (acidic) and negatively charged (basic) amino acid residues at a pH of 5.0. If the pH is higher or lower than the pI, the protein will have a net charge due to the unequal distribution of charges.

The isoelectric point is important in protein purification and analysis because it can be used to separate proteins based on their charge. For example, proteins with a high pI are typically more basic and tend to be positively charged at lower pH values, while proteins with a low pI are typically more acidic and tend to be negatively charged at higher pH values. This can be useful for separating proteins from different sources or for identifying specific protein bands on a Western blot.