Last updated: 24 August 2026
An amino acid looks at first like a molecule containing two familiar functional groups: an amine and a carboxylic acid. Put these two groups into the same molecule, however, and something interesting happens. One end can accept H⁺ while the other can donate H⁺, so the molecule develops acid–base behaviour that neither functional group shows on its own.
We will use aminoethanoic acid, commonly called glycine, as the main example. Its simple structure makes it ideal for seeing why amino acids form zwitterions, why they have unusually high melting points, how their charge changes with pH and how they join together to form peptide chains.
Try these five questions before reading the chapter. Pay particular attention to where H⁺ is gained or lost and to the functional groups involved in peptide formation.
- 5 diagnostic questions
- Structures and reaction schemes where useful
- Instant explanation for every answer option
What is an amino acid?
An amino acid is an organic compound containing both an amino group, –NH₂, and a carboxyl group, –COOH. The amino group can behave as a base because the nitrogen atom has a lone pair that can accept H⁺, while the carboxyl group can behave as an acid because it can donate H⁺.
In an α-amino acid, the amino group and carboxyl group are attached to the same carbon atom, known as the α-carbon. A convenient general representation is:
The group represented by R is the side chain. Changing R gives different amino acids while leaving the amino and carboxyl functional groups in place.
The characteristic chemistry of an amino acid comes from having both a basic –NH₂ group and an acidic –COOH group in the same molecule.
Aminoethanoic acid
The simplest α-amino acid is aminoethanoic acid, commonly called glycine. Here, R = H, so its structural formula is NH₂CH₂COOH.
Why aminoethanoic acid is not optically active
Most α-amino acids can contain a chiral α-carbon because that carbon may be bonded to four different groups: –NH₂, –COOH, –H and R. Aminoethanoic acid is the important exception.
In aminoethanoic acid, R is also H. Its central carbon is therefore attached to two identical hydrogen atoms, so it is not a chiral carbon and aminoethanoic acid does not show optical isomerism.
General α-amino acid: The α-carbon may be bonded to four different groups and may therefore be chiral.
Aminoethanoic acid: The α-carbon has two H atoms attached, so it is achiral.
Why amino acids form zwitterions
Writing aminoethanoic acid as NH₂CH₂COOH is useful because it makes the two functional groups easy to recognise. However, this is not the best representation of the compound in the solid state.
The –COOH group can donate H⁺, while the –NH₂ group can accept H⁺. Because both groups occur in the same molecule, a proton can be transferred internally from the carboxyl group to the amino group.
The product contains both a positive and a negative charge, but its overall charge is zero. A species containing oppositely charged groups within the same molecule is called a zwitterion.
In the solid state, amino acids exist predominantly as zwitterions rather than as completely uncharged molecules.
Why amino acids have relatively high melting points
Many small organic molecules are molecular substances held together mainly by intermolecular forces. Amino acids behave differently because their zwitterions contain full positive and negative charges.
In the solid, neighbouring zwitterions are held together by strong electrostatic attractions between oppositely charged ions. Considerable energy is therefore needed to overcome these attractions, giving amino acids relatively high melting points compared with many molecular organic compounds of similar size.
If an examination asks why an amino acid has a high melting point, connect the explanation explicitly to its zwitterionic structure and the strong electrostatic attractions between oppositely charged groups. Simply stating that it has “strong intermolecular forces” is too vague.
Why amino acids can dissolve in water
The charged –NH₃⁺ and –COO⁻ groups interact strongly with polar water molecules through ion–dipole attractions. The amino and carboxyl groups can also participate in hydrogen bonding with water.
Actual solubility varies from one amino acid to another because the side chain R also affects the overall interaction with water, but the ionic character of the zwitterion explains why amino acids behave very differently from non-polar organic molecules.
Amino acids are amphoteric
An amphoteric substance can react with both acids and bases. Amino acids are amphoteric because they contain sites that can either accept or donate H⁺.
The easiest way to understand their behaviour is to begin with the zwitterion and ask what happens when the concentration of H⁺ is changed.
In acidic solution
In acidic solution there is a high concentration of H⁺ ions. The negatively charged –COO⁻ group of the zwitterion acts as a base and accepts H⁺.
The product now has an overall charge of +1. Notice that the –NH₃⁺ group remains protonated while the –COO⁻ group becomes –COOH.
In alkaline solution
In alkaline solution, the –NH₃⁺ group can donate H⁺. The OH⁻ ions remove this proton to form water, leaving the amino group as –NH₂.
The resulting amino acid species has an overall charge of −1.
| Conditions | Predominant form of aminoethanoic acid | Overall charge |
|---|---|---|
| Strongly acidic | ⁺H₃NCH₂COOH | +1 |
| Near its isoelectric region | ⁺H₃NCH₂COO⁻ | 0 |
| Strongly alkaline | NH₂CH₂COO⁻ | −1 |
Acidic conditions add H⁺; alkaline conditions remove H⁺. Always track the charge on both –NH₃⁺/–NH₂ and –COOH/–COO⁻ before deciding the overall charge.
The isoelectric point
For a particular amino acid, there is a pH at which its average overall charge is zero. This is called its isoelectric point, pI.
At this pH the zwitterionic form predominates, so the amino acid has no net migration towards either electrode in an electric field. At lower pH it becomes more positively charged, while at higher pH it becomes more negatively charged.
How amino acids form peptide bonds
Amino acids can react with one another because each molecule contains both a carboxyl group and an amino group. The carboxyl group of one amino acid reacts with the amino group of another.
An –OH from the carboxyl group and an H from the amino group are removed as H₂O. The carbonyl carbon then becomes bonded to nitrogen. This is a condensation reaction.
The peptide bond is an amide linkage
The newly formed linkage is:
This is called a peptide bond. Chemically, it is an amide linkage.
Before condensation: One amino acid contributes –COOH and the other contributes –NH₂.
After condensation: H₂O has been eliminated and the amino acids are connected by –CONH–.
From dipeptides to polypeptides
A molecule made from two amino acid residues is a dipeptide. Because the dipeptide still has functional groups capable of reacting with further amino acids, repeated condensation reactions can produce long chains.
These chains are called polypeptides. Proteins contain one or more polypeptide chains constructed from amino acid residues linked through peptide bonds.
When locating a peptide bond in a displayed structure, look for a carbonyl group whose carbon atom is bonded directly to nitrogen: –C(=O)–NH–. Do not count the C–N bond of an ordinary amino group as a peptide bond.
Hydrolysis breaks peptide bonds
Condensation joins amino acids by eliminating water. Hydrolysis is the reverse process: water is used to break the peptide bond and regenerate the functional groups from which it was formed.
Peptide bonds do not hydrolyse rapidly enough under ordinary laboratory conditions, so heating with aqueous acid or aqueous alkali is normally required.
Acid hydrolysis
A peptide can be hydrolysed by heating it with a dilute aqueous acid such as HCl, commonly under reflux. Water breaks the peptide bond.
Because the mixture is acidic, the amino groups in the products become protonated. The products are therefore obtained as ammonium salts of the amino acids rather than simply as NH₂-containing molecules.
Alkaline hydrolysis
Peptide bonds can also be hydrolysed by heating with aqueous NaOH, commonly under reflux.
In alkaline solution, carboxylic acid groups are deprotonated. The products therefore contain carboxylate ions, –COO⁻, present as their sodium salts when NaOH is used.
| Method | Typical conditions | Form of amino-acid-derived products |
|---|---|---|
| Acid hydrolysis | Dilute aqueous acid, heat under reflux | –COOH and protonated –NH₃⁺ groups |
| Alkaline hydrolysis | Aqueous NaOH, heat under reflux | –COO⁻Na⁺ and –NH₂ groups |
Do not stop at “amino acids are formed”. The protonation state of the products depends on the hydrolysis conditions: acid gives protonated amino groups, while alkali gives carboxylate salts.
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Drawing NH₂CH₂COOH as the only form of aminoethanoic acid in the solid state | Amino acids undergo internal proton transfer and exist predominantly as zwitterions in the solid. | Draw ⁺H₃NCH₂COO⁻ when discussing the solid-state structure and ionic properties. |
| Leaving the amino group as –NH₂ in strongly acidic solution | The amino group is protonated under acidic conditions. | Track protonation explicitly: the acidic form contains –NH₃⁺. |
| Calling any C–N bond in a peptide a peptide bond | A peptide bond is specifically the amide linkage involving a carbonyl carbon bonded to nitrogen. | Look for –C(=O)–NH– or –CONH–. |
| Drawing neutral amino acids after acid or alkaline hydrolysis | The reaction medium controls the protonation state of the hydrolysis products. | Under acid use –NH₃⁺; under NaOH use –COO⁻Na⁺. |
What you should be able to do
Recognise amino acids
Identify the amino and carboxyl functional groups and recognise the general structure of an α-amino acid.
Draw aminoethanoic acid
Draw NH₂CH₂COOH and explain why its α-carbon is not chiral.
Explain zwitterion formation
Show the internal transfer of H⁺ and draw the zwitterion ⁺H₃NCH₂COO⁻.
Predict the effect of pH
Draw the predominant cationic, zwitterionic or anionic form under acidic, intermediate and alkaline conditions.
Form peptide bonds
Show how amino acids undergo condensation to produce the –CONH– linkage and H₂O.
Hydrolyse peptide bonds
State suitable acidic or alkaline conditions and draw the products in the correct ionic forms.
These ten questions test whether you can apply the chapter to zwitterions, acid–base behaviour, peptide formation and hydrolysis.
- 10 mastery questions
- Structures and reaction schemes where useful
- Instant explanation for every answer option
Frequently asked questions
Common questions about amino acids
Once zwitterion formation is understood, most amino-acid questions become a matter of following H⁺ and recognising the –CONH– peptide linkage. These are some of the points students most often need to clarify.
Why do we write aminoethanoic acid as NH₂CH₂COOH if it exists as a zwitterion?
NH₂CH₂COOH is a useful structural representation because it makes the amino and carboxyl functional groups immediately visible. However, when discussing the solid state and the physical properties of amino acids, the zwitterionic structure ⁺H₃NCH₂COO⁻ is more appropriate.
Does a zwitterion have zero charge?
Its overall charge is zero, but this does not mean that there are no charges in the molecule. A zwitterion contains both a positively charged group and a negatively charged group. For aminoethanoic acid these are –NH₃⁺ and –COO⁻.
Why does an amino acid become positively charged in acid but negatively charged in alkali?
Acidic solution supplies H⁺, so the –COO⁻ group of the zwitterion becomes –COOH and the molecule has an overall +1 charge. In alkaline solution, OH⁻ removes H⁺ from –NH₃⁺, producing –NH₂ while –COO⁻ remains, so the overall charge becomes −1.
Is a peptide bond the same as an amide bond?
A peptide bond is an amide linkage formed specifically between amino acid residues. Its characteristic structure is –C(=O)–NH–, usually written more simply as –CONH–.
Why are the products of acid and alkaline peptide hydrolysis drawn differently?
Hydrolysis breaks the same peptide bond in both cases, but the surrounding solution changes the protonation state of the products. Acidic hydrolysis gives protonated –NH₃⁺ groups, whereas alkaline hydrolysis gives carboxylate groups such as –COO⁻Na⁺ when NaOH is used.
Amino-acid chemistry becomes much easier once you see it as one connected story. The –COOH and –NH₂ groups first explain zwitterion formation and amphoteric behaviour; the same two groups then provide the sites that join amino acids through peptide bonds. From there, hydrolysis is simply the reverse process — breaking those amide linkages and restoring the amino-acid-derived functional groups.