Isoelektrisk

Isoelektrisk, or isoelectric point, is the pH at which a molecule carries no net electric charge. For amino acids and proteins, this point corresponds to the pH where the molecule exists mainly as a zwitterion, with balanced positive and negative charges.

Amino acids have at least two ionizable groups: a carboxyl group and an amino group. The isoelectric point is commonly estimated as the average of the pKa values surrounding the neutral form of the molecule. For neutral amino acids with no charged side chain, pI is roughly (pKa1 + pKa2)/2. Amino acids with acidic side chains (such as aspartic acid and glutamic acid) have lower pIs, while those with basic side chains (such as lysine, arginine, and histidine) have higher pIs. Typical values are around pI ~6 for many neutral amino acids, ~3 for acidic residues, and ~9–10 for basic residues.

In proteins, the overall isoelectric point depends on the combined contributions of all ionizable groups, including side chains and terminal groups. The pI influences solubility, aggregation, and interaction with surfaces, and it is a key parameter in purification and analysis.

Practical applications include isoelectric focusing, a technique that separates molecules by their pI in a pH gradient, and various chromatography methods that exploit charge differences. Environmental factors such as temperature, ionic strength, and the presence of other ions can slightly shift the apparent pI of a molecule.