The presence of acid (-COOH) and basic (-NH 2 ) groups gives amino acids characteristic acid-base properties.
In strong acid media, both the amino group and the acid group are protonated and the amino acid has the following form:
As the pH rises, the more acidic group, H with a lower pKa, is deprotonated, forming a neutral species called Zwitterion.
![isoelectric ph 02](/images/stories/organica-avanzada/productos-naturales/aminoacidos-peptidos/punto-isoelectrico/ph-isoelectrico-02.png)
When the amino acid is in basic media, it loses the proton of the amino group, giving rise to the deprotonated species.
![isoelectric-point punto-isoelectrico](/images/stories/organica-avanzada/productos-naturales/aminoacidos-peptidos/punto-isoelectrico/punto-isoelectrico-03.png)
The pH at which the Zwitterion concentration is maximum (the amino acid has no net charge) is called isoelectric pH or isoelectric point.
Other definition of isoelectric point is: pH at which the concentration of protonated and deprotonated species equalize.
![isoelectric-point punto-isoelectrico](/images/stories/organica-avanzada/productos-naturales/aminoacidos-peptidos/punto-isoelectrico/punto-isoelectrico-04.png)
The isoelectric pH is calculated as the average of pK a,1 and pK a,2 , that is, the average of the pKas of the stages that form and decompose the Zwitterion.