All subjects · Chemistry · WCH15 (Unit 5)

Amino acids, peptides and proteins

Amino acids contain an amine group and a carboxylic acid group. They exist as zwitterions, most are chiral, and they join through peptide links to make proteins.

Structure

2-amino acids have the general formula H2NCH(R)COOH. Glycine has R = H; alanine has R = CH3. Every amino acid except glycine has a chiral centre, so solutions of a single natural amino acid rotate plane-polarised monochromatic light (spec 19.11 ii).

Acid-base properties and zwitterions (spec 19.11 i)

The -COOH group can donate H+ and the -NH2 group can accept it, so an amino acid reacts with acids and bases. In the solid and in neutral solution it exists mainly as a zwitterion, +H3NCH(R)COO-, an ion with both a positive and a negative charge. Strong attractions between zwitterions give amino acids high melting points and solubility in water.

Peptides and proteins (spec 19.11 iii)

The -COOH of one amino acid and the -NH2 of another join by condensation, losing water and forming a peptide (amide) link, -CONH-. Two amino acids give a dipeptide; glycine and alanine can give two different dipeptides (Gly-Ala and Ala-Gly). Proteins are long polypeptides. Heating a protein under reflux with concentrated (about 6 mol dm-3) hydrochloric acid hydrolyses the peptide links back to amino acids, which can be separated by chromatography and located with a reagent such as ninhydrin, then identified by Rf values.

Worked example

Write the formulae of alanine in strongly acidic and in strongly alkaline solution.

  1. In acid, -NH2 accepts H+ and -COO- is protonated.
  2. In alkali, -COOH loses H+.

Answer: Acid: +H3NCH(CH3)COOH; alkali: H2NCH(CH3)COO-

Key idea: Amino acids H2NCH(R)COOH: zwitterions +H3NCH(R)COO-, high melting points. Chiral except glycine. Peptide link -CONH- forms by condensation; acid hydrolysis breaks it.

Check yourself

  1. What is a zwitterion?
    • An ion with a 2+ charge
    • An ion with a positive and a negative charge, with no overall charge
    • A molecule with no charges
    • A negative ion of an amino acid
    Show answer

    An ion with a positive and a negative charge, with no overall charge. For glycine: +H3NCH2COO-.

  2. Why do amino acids have high melting points?
    • They have very large molecules
    • They form covalent networks
    • They contain metal ions
    • They exist as zwitterions with strong ionic attractions between them
    Show answer

    They exist as zwitterions with strong ionic attractions between them. Ionic attractions between zwitterions need much energy to overcome.

  3. What is the structure of glycine in strongly acidic solution?
    • H2NCH2COO-
    • +H3NCH2COOH
    • +H3NCH2COO-
    • H2NCH2COOH
    Show answer

    +H3NCH2COOH. The amine group is protonated and the carboxylate gains H+.

  4. What is the structure of alanine in strongly alkaline solution?
    • +H3NCH(CH3)COOH
    • +H3NCH(CH3)COO-
    • H2NCH(CH3)COO-
    • H2NCH(CH3)COOH
    Show answer

    H2NCH(CH3)COO-. The -COOH group loses H+; -NH2 stays as it is.

  5. Which amino acid does not rotate plane-polarised light?
    • Glycine
    • Alanine
    • Serine
    • Valine
    Show answer

    Glycine. Glycine's central carbon has two H atoms, so it is not chiral.

  6. How many different dipeptides can be made from one molecule of glycine and one molecule of alanine?
    Show answer

    2. Gly-Ala and Ala-Gly differ in which amino acid keeps the free -NH2 group.

  7. Which group is the peptide link?
    • -CONH-
    • -COO-
    • -NH2
    • -N=N-
    Show answer

    -CONH-. An amide group joins the amino acids.

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Last updated 7 November 2026. Spotted a mistake? Tell us.