Isoelectric point


  • pI

Defined as the pH in which a peptide has a net charge of zero. The presence of different charges in proteins is generated by potential charges of amino acid side chains and peptide termini which is dependent on the pKa values of the groups and the solution pH. 

Once an isoelectric point (pI) is determined, it is established that at a lower pH from the pI the peptide net charge will be positive and at a higher pH from the pI the net charge will be negative. The presence of predominant negative or positive charges in a protein in solution different from the pI can produce repulsion forces. Therefore, at the pI the negative and positive charges are balanced and the repulsive forces are reduced producing aggregation and precipitation due to predominant attraction forces.1


  1. Novak P. and Havlicek V. (2016). Proteomic profiling and analytical chemistry (2nd ed.). Elsevier.