Isoelectric point

soelectric point. The isoelectric point is defined as the pH at which the number of positive charges equals the number of negative charges provided by the ionizable groups of a molecule. At the isoelectric point the net charge of the molecule is zero (0). In the amino acid ionizable groups include groups carboxyl , amino, and phenolic thiol.


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  • 1 Introduction
    • 1 Isoelectric point of amino acids and proteins
    • 2 IEP
      • 2.1 Features
    • 2 Source
    • 3 See also


As a protein, gelatin exhibits amphoteric behavior due to the presence of functional amino acid groups and terminal amino and carboxyl groups. In acidic media, that is, in the presence of high concentrations of H + ions, gelatin has a positive charge. In alkaline media, that is, in the presence of OH- ions, gelatin is negatively charged. At the isoelectric point (IEP) the positive charges of the NH3 + radicals equal the negative charges of the COO- radicals.

Isoelectric point of amino acids and proteins

Isoelectric points provide useful information for reasoning about the behavior of amino acids and proteins in solution. Thus, the presence of ionizable groups in these molecules has important consequences on solubility.

Amino acids and proteins are less soluble at their isoelectric point if the other conditions remain the same. This is because the dipole ions have no net charge and crystallize as insoluble salts at that pH.


The IEP is an intrinsic property of gelatine, determined by the treatments of the raw materials and the type of process: Type A (acidic) gelatins have an IEP that ranges between 6 – 9.5. However, pretreatment of certain raw materials (such as the leather or skin hair removal process) can lower the IEP to less than 6. Type B (alkaline) gelatins have an IEP that ranges from 4.5 – 5.6. Gelatin with a low IEP is due to the amino acid deamidation phenomenon. Type A and B gelatins from raw materials treated or pretreated with alkaline substances that remove amide groups, have a low IEP.


Protein functionalities are affected when they approach the IEP, due to the electrostatic attraction of groups with opposite charge. In this way, in the IEP the properties of the gelatin coincide with the maximum or minimum values. Minimum: hydration , viscosity and gelation  ; maximums: turbidity, gel strength, foaming power and syneresis-

The IEP is useful in explaining the possible interactions of gelatin with other ingredients, in particular anionic polymers. For example, a Type B gelatin interacts best with an anionic polymer (eg carrageenans in chocolate mousse) to prevent precipitation by incompatibility, which depends on pH conditions.


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