pKa and pI of Amino Acids

Dissociation of amino acids

Amino acids are a class of amphoteric organic compounds with amphoteric dissociation properties, and the dissociation mode depends on the pH value of the solution.

The pKa value of α-COOH is about 2.0. When pH>3.5, α-COOH exists as α-COO-.

The pKa value of α-NH2 is about 9.4. When pH<8.0, α-NH2 exists as α-NH3+.

When the pH of the solution is in the range of 3.5-8.0, amino acids exist as zwitterions. When the pH of the solution is at the pKa value, the buffering capacity of amino acids is strongest.

Amino acid pKa and pI values

Amino acid isoelectronic point (pI)

Isoelectronic point (pI) is the pH value of a molecule when it does not carry a net charge or is electrically neutral in the statistical mean. Amphoteric molecules called zwitterions contain both positively and negatively charged functional groups. The total charge of the entire molecule is determined by the pH value of its surrounding environment. Depending on the pH value, the entire molecule may have a positive or negative charge. The reason for this is because such molecules may absorb or lose protons (H+) in different pH environments. At pH equal to the isoelectric point, the positive and negative charges carried by such molecules cancel each other out, making the entire molecule uncharged. Amino acids are typical amphoteric molecules, that is, they contain acidic carboxyl groups and basic amino groups. The isoelectric point of amino acids is calculated in three cases.

Neutral side chain

If an amino acid contains a neutral side chain, it contains two pKa, which are pKa1 for the carboxyl group and pKa2 for the amino group. At this time, the isoelectric point is the average value of the two pKa, that is, pI = 1/2 (pKa1 + pKa2). For example, the simplest amino acid glycine, has pKa1= 2.34, pKa2 = 9.6, and pI = 5.97.

Acidic side chains

If the amino acid contains an acidic side chain, pI will be at a lower pH because of the introduction of an additional negative charge. For example, aspartic acid, the neutral molecule is between pH 1.88 and 3.65. pI = 1/2 (pKa1 + pKa3)，pI = 2.77.

Basic side chains

If the amino acid contains a basic side chain, pI will be at a higher pH because of the introduction of an additional positive charge. Histidine, a neutral molecule, exists between pH 6.00 and 9.17. pI = 1/2 (pKa2 + pKa3)，pI = 7.59.