Core ConceptsThis article will discuss zwitterion’s structure, pH, net charge, and examples.IntroductionA zwitterion is a molecular ion with a net charge of zero, but with constituents that carry positive and negative formal charges. The most common example of a zwitterion is an amino acid, where the amine nitrogen can carry a +1 charge, and the carboxylic acid group can carry a -1 charge. While amino acids are the most common, they are not the only zwitterions possible. The general structure of a zwitterion is shown in the figure below.Some more specific examples of zwitterions include sulfamic acid (left) and psilocybin (right), as shown below. In both examples, the molecule contains a positive and negative charge that balance to a net charge of zero. The two charges may be quite close together, in the case of sulfamic acid, or very far apart, as in psilocybin. Both examples still have a neutral charge overall.Isoelectric PointZwitterions are unique because at a certain pH, they have no net charge. This pH is called the isoelectric point.Recall when the pH is lower than the pka of a functional group, it will be protonated. If the pH is higher than the pKa of a functional group, it will be deprotonated. This is a key concept for understanding isoelectric point.The isoelectric point occurs because the pka of a carboxylic acid is around 2 and the pka of an amine is around 9. At a pH between the pka values (around 7) a net charge of zero occurs.This amine (NH3) has a pKa of around 9. Since the pH is less than the pKa, the amine carries a positive charge on the nitrogen. The carboxylic acid (COOH) has a pka of about 2. Since the pH is higher than the pKa, the carboxylic acid loses the proton and carries a negative charge on the oxygen. The positive charge on nitrogen and the negative charge on oxygen create an overall net charge of zero.The isoelectric point (pI) can be calculated from the following formula:Net Charge and pHIn acidic or low pH, there is an abundance of hydrogen. The acidic environment leads to the carboxylic acid being protonated and being neutral. However, the amine will have three hydrogens giving the nitrogen a positive charge. With an overall positive net charge of +1, a cation is formed as shown below.In basic or high pH, there is an abundance of –OH. This leads to the amine having two hydrogens and being neutral. However, the carboxylic acid is deprotonated, and the oxygen becomes negative. With an overall negative net charge of -1, an anion is formed as shown below.Putting it all together, a pH scale is shown with its respective cation, zwitterion, or anion.Amino Acids as ZwitterionsAmino acids are the most well-known example of zwitterions. They contain carboxylic acid and an amine. This means they contain positively and negatively charged functional groups at a pH of around 7. An example of Alanine is shown below. The R group becomes a CH3 for this specific amino acid.ConclusionZwitterions are biological molecules that vary in charge based on the respective pH of their environment. By specifying the R group, specific amino acids exemplify zwitterions structure and behavior.
