Describe amino-acid zwitterions and the isoelectric point
Amino acids can exist as zwitterions with –NH₃⁺ and –COO⁻; at the isoelectric point their net charge is zero.
Section 34 · Nitrogen compounds
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 34. Use the stated reagents, conditions and product scope. Written practice is self-checked This online lesson cannot demonstrate supervised laboratory skills.
Before you begin
- Recall nucleophilic substitution, reduction, acyl-chloride addition–elimination, acid–base equilibria and covalent bonding.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the net charge of an amino-acid zwitterion at its isoelectric point?
What you will learn
- Draw the zwitterionic form of an amino acid
- Explain amphoteric acid–base behavior
- Define the isoelectric point by net charge
Amino acids can carry both charges
An amino acid with –NH₂ and –COOH can transfer a proton internally to form a zwitterion: ⁺H₃N–CHR–COO⁻.
The zwitterion has a positive ammonium group and a negative carboxylate group in the same molecule. Its total charge is zero, although the molecule still contains charged regions.
The isoelectric point is a pH condition
At its isoelectric point, an amino acid has zero overall charge on average. It can still exist mainly as zwitterions; ‘zero net charge’ does not mean ‘no charged groups’.
Amino acids are amphoteric: they can accept H⁺ at the amino group or donate H⁺ from the carboxyl group. Below the isoelectric point they tend to be more positively charged; above it they tend to be more negatively charged.
Pause and explain
Which groups appear in the zwitterion form?
Worked example
Write the zwitterion of a generic amino acid H₂N–CHR–COOH and state its net charge.
Show the worked solution
- Transfer H⁺ from –COOH to –NH₂.
- Write –NH₃⁺ and –COO⁻.
- Add the formal charges.
Answer ⁺H₃N–CHR–COO⁻; the overall charge is zero.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Convert H₂N–CH(CH₃)–COOH into its zwitterion and calculate its net charge.
Give me a hint
Transfer a proton from the carboxyl group to the amino group.
Compare my reasoning
- Change –NH₂ to –NH₃⁺.
- Change –COOH to –COO⁻.
- Sum +1 and −1.
⁺H₃N–CH(CH₃)–COO⁻; net charge 0.
Look for these in your work
- Ammonium form
- Carboxylate form
- Net charge
Explain why an amino acid is amphoteric.
Give me a hint
Identify how each functional group can respond to H⁺.
Compare my reasoning
- The amino group can accept H⁺.
- The carboxyl group can donate H⁺.
- Therefore the molecule can act as both base and acid.
Amino acids can accept a proton at –NH₂ and donate one from –COOH, so they are amphoteric.
Look for these in your work
- Base behavior
- Acid behavior
- Term explained
A response says ‘at the isoelectric point, an amino acid has no charges’. Refine this statement.
Give me a hint
Distinguish local formal charges from total charge.
Compare my reasoning
- The amino acid can be a zwitterion.
- It carries –NH₃⁺ and –COO⁻ simultaneously.
- The charges cancel to give zero net charge.
At pI the overall charge is zero, but the zwitterion still contains positive and negative charged groups.
Look for these in your work
- Zwitterion named
- Both groups
- Net charge clarified
Common mistakes
- Drawing –NH₂ and –COOH as the only ionic form at neutral pH
- Calling a zwitterion uncharged at every site
- Defining pI as the pH where the molecule has no polar bonds
- Reversing charge trends above and below pI
What can you explain now?
What is the net charge of an amino-acid zwitterion at its isoelectric point?
Compare with the explanation
Zero
The positive and negative formal charges balance overall.
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Make it stick
What makes a zwitterion different from a molecule with no formal charges?
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