Zwitterion confusion: what is the charge of an amino acid at different pH, and how are amino acids classified?
Ananya Reddy11th Sample
2 Oct 2026 · 1 min read
I'm doing the amino acids part of Biomolecules with four colours of pens (😅) and my notes on zwitterion are a mess. This is how I wrote it.
Structure: amino acids are "substituted methanes" with four groups on the α-carbon: a hydrogen, an amino group (NH2), a carboxyl group (COOH), and a variable R group. Glycine has R = H, alanine has R = CH3, serine has R = CH2OH.
Classification: depends on the R group. Acidic: glutamic acid. Basic: lysine. Neutral: valine. Aromatic: tyrosine, phenylalanine, tryptophan.
Zwitterion: In water, the NH2 takes a proton and COOH gives a proton, so we get NH3+ and COO−. NCERT says the structure changes with pH. But I don't know what happens in acid vs base, or what charge the zwitterion has. I wrote "it has no charge" in the margin and then a teacher scribbled "?" next to it.
What is going on, and why does it matter for NEET?
8 answers & replies
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[+][–]Tanvi Deshmukh12thSample· 17d ago
Your structure and classification notes are perfect. Quick extension, an amino acid joins to another through a peptide bond (–CO–NH–), formed between the COOH of one and the NH2 of the next with loss of water. That's how proteins are built, and 20 types of amino acids are in proteins. The "α" just means the amino group is on the carbon next to the COOH.
[+][–]Pooja Iyer12thSample· 17d ago
Zwitterion = has no charge at all, all the charges cancel and it's neutral, because NH3+ and COO− neutralise each other and they're gone. So the zwitterion is just a neutral molecule, like the unionised form H2N–CHR–COOH.
[+][–]Siddharth RaoDropperSample· 17d ago
Pooja, not quite. The zwitterion has two charges, positive on the NH3+ and negative on the COO−, so it's a dipolar ion. The net charge is zero, but the charges are still there. The un-ionised form (H2N–CHR–COOH) and the zwitterion (H3N+–CHR–COO−) are different structures. Why does it matter: it explains high melting points, solubility in water, and behaviour in electric field.
[+][–]Ananya Reddy11thSample· 17d agoAuthor
net zero but not "no charge", got it!! I'll redraw with + and − marked
[+][–]Pooja Iyer12thSample· 17d ago
yes siddharth, I mixed up "net" with "no". Makes sense now, sorry ananya
[+][–]Zoya Khan12thSample· 17d ago
What happens with pH (for a neutral amino acid like glycine):
- Acidic solution (low pH): extra H+ protonates COO−, so we get H3N+–CHR–COOH, net positive. Moves towards cathode.
- At a certain pH (the isoelectric point), it's the zwitterion, net zero. Doesn't move in an electric field.
- Basic solution (high pH): NH3+ loses a proton to give H2N–CHR–COO−, net negative. Moves towards the anode. So amino acids can act as both acids and bases (they are amphoteric), and that's why proteins act as buffers.
[+][–]Kavya NaikDropperSample· 17d ago
Two small additions. NCERT's table classifies them by R group as acidic, basic or neutral (acidic like glutamic acid has an extra COOH in R, basic like lysine has an extra NH2). Aromatic ones have a ring in R. Also glycine is the simplest and the only one in the list without a chiral α-carbon, because its R is H, so it has two identical H groups, and thus no optical isomers.
[+][–]Zoya Khan12thSample· 17d ago
good point kavya. And in NEET it's often the question "which amino acid is optically inactive" and the answer is glycine.
