The isoelectric point (pI) is the acidity of the medium (pH) at which a certain molecule or surface does not carry an electric charge. Amphoteric molecules (zwitterions) contain both positive and negative charges, the presence of which is determined by the pH of the solution. The charge of various functional groups of such molecules can change as a result of binding or, on the contrary, loss of protons H +.Many biological molecules, such as amino acids and proteins, are inherently amphoteric, since they contain both acidic and basic functional groups. So for example, glycine has pI = 7.13, at pH above this value, it has a positive charge, below - negative. This property underlies the buffer systems to maintain pH. This property underlies the buffer systems to maintain pH. With an increase in pH, the amino acid takes a positive charge, thereby reducing the value of this parameter in solution.