Intermediate Topic 827 of 1102 Organic Chemistry
Zwitterions
Dipolar ions bearing both positive and negative charges, common for amino acids at neutral pH.
In-depth explanation of Zwitterions
A zwitterion is a molecule that carries both a positive and a negative charge while being neutral overall. At neutral pH, amino acids exist mostly as zwitterions, with the amino group protonated to NH3+ and the carboxyl group deprotonated to COO-. This internal salt character gives amino acids high melting points, good water solubility and low solubility in nonpolar solvents. The isoelectric point, pI, is the pH at which the net charge of the zwitterion is zero.
Going deeper: Zwitterions
Key ideas: At the isoelectric point (pI) an amino acid is net neutral with NH3+ and COO-. For neutral amino acids, pI equals the mean of pKa1 and pKa2; for acidic or basic side chains average the two pKa values that bracket the neutral species. Solubility is minimum at pI, and in electrophoresis the molecule does not move. Below the pI it is cationic, above anionic.
Common mistakes and connections: Students use the wrong two pKa values for lysine or glutamic acid. Another error is saying a zwitterion has no charges; it has both but zero net charge. Amino acids have high melting points (over 200 C) because of ionic character. Links to buffers, titration curves and protein separation.
Visualization of Zwitterions
Glycine exists as the zwitterion between pH 2.3 and 9.6; the distribution shows species fraction as pH changes (pKa 2.34 shown). The interactive lesson shows this as a live animation or chart.
Worked example and use case
Glycine has an isoelectric point of about 6.0, where it carries no net charge.
Frequently asked questions about Zwitterions
What is Zwitterions?
Dipolar ions bearing both positive and negative charges, common for amino acids at neutral pH.
Is Zwitterions a beginner, intermediate or advanced topic?
Zwitterions is a intermediate topic in Organic Chemistry. It builds on the vocabulary of beginner topics in this area.
What should I learn before Zwitterions?
Study these earlier topics in Organic Chemistry first: Glucose Ring Forms, Reducing Sugars, Amino Acids.
What comes after Zwitterions?
The next topics in Organic Chemistry are Peptide Bond and Lipids.
Can I practise Zwitterions with quizzes?
Yes. The interactive lesson includes a self-check question on Zwitterions, and the Organic Chemistry quiz draws on every topic in this area.
Where Zwitterions fits in Organic Chemistry
Zwitterions is topic 99 of 148 in Organic Chemistry.