VPB 112: General Veterinary Biochemistry (1+1)

ISOELECTRIC pH AND ZWITTER ION

• Charge of amino acid is dependent on pH.
• At acidic pH amino acids exists as cation (positively charged). The carboxyl group is unionized and the amino group is protonated.
• At alkaline pH as anion (negatively charged). The carboxyl group is ionized and the amino group is unionized.
• At neutral pH as dipolar ions (Zwitter ions) (dipolar means a molecule containing negatively charged and positively group in equal number). In the dipolar form of amino acid, the amino group is protonated and the carboxyl group is dissociated (- NH₃⁺, -COO^-).
• The pH at which the amino acids possess equal number of negative and positive charges is the isoelectric pH (denoted as pI). At this pH the net charge of amino acid is ‘0’, it is electrically neutral.