In simple terms
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Life's Versatile Building Blocks
Amino acids are molecules with both acidic and basic parts, allowing them to act as buffers and link together to form proteins. Their unique side chains (R groups) give proteins their diverse functions.
Imagine a person with two hands, one that can shake hands with an 'acid' group and another that can shake hands with a 'base' group. This person can also link hands with others like them to form a long chain, just like amino acids form proteins.
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The general structure is H₂N–CH(R)–COOH. All are chiral and optically active except glycine (where R = H), which has no chiral centre. In solution, they exist as zwitterions: H₃N⁺–CH(R)–COO⁻.
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The isoelectric point (pI) is the pH where the amino acid exists as a zwitterion with a net charge of zero. This property is used to separate amino acids using electrophoresis.
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Amino acids join via condensation reactions to form a dipeptide, eliminating water. The resulting amide link, –CO–NH–, is called a peptide bond and forms the primary structure of proteins.
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There are 20 common amino acids found in proteins, each with a different R group. This R group determines if the amino acid is acidic, basic, polar, or non-polar, dictating the protein's final structure and function.
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Full topic notes
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General Structure and Chirality
All 20 common amino acids that make up proteins are α-amino acids. This means the amine group is attached to the α-carbon, which is the carbon atom directly bonded to the carboxyl group. This α-carbon is also bonded to a hydrogen atom and a variable side chain, known as the R group. The identity of the R group distinguishes one amino acid from another.
General structure of an α-amino acid:
H₂N—CH(R)—COOH
The α-carbon is a chiral centre for 19 of the 20 common amino acids, as it is bonded to four different groups (–H, –NH₂, –COOH, and –R).
This chirality means that amino acids (except glycine) can exist as two non-superimposable mirror images, or enantiomers (L- and D-forms).
The exception is glycine, where the R group is a hydrogen atom. Since the α-carbon is bonded to two hydrogen atoms, it is achiral and does not exhibit optical isomerism.
Zwitterions and Acid-Base Properties
Due to the presence of a basic amine group and an acidic carboxyl group, amino acids are amphoteric. In the solid state, and in aqueous solution at a certain pH, an internal acid-base reaction occurs. The –COOH group donates a proton to the –NH₂ group, forming a dipolar ion called a zwitterion. A zwitterion has both a positive charge (–NH₃⁺) and a negative charge (–COO⁻) but is electrically neutral overall.
H₂N-CH(R)-COOH \rightleftharpoons H₃N⁺-CH(R)-COO⁻
The pH at which the amino acid exists predominantly as a zwitterion is called the isoelectric point (pI). At a pH below the pI, the solution is acidic, and the zwitterion acts as a base, accepting a proton on its –COO⁻ group to form a cation. At a pH above the pI, the solution is alkaline, and the zwitterion acts as an acid, donating a proton from its –NH₃⁺ group to form an anion.
Formation of Peptide Bonds
Amino acids can join together to form polymers called polypeptides, which fold to become proteins. This polymerisation occurs through a series of condensation reactions. When two amino acids react, the carboxyl group of one reacts with the amine group of the other. A molecule of water is eliminated, and a new covalent bond, known as a peptide bond, is formed.
Peptide bond: –CO–NH–
The resulting molecule is a dipeptide. This process can be repeated, adding more amino acids to form a long polypeptide chain. The sequence of amino acids in this chain is known as the primary structure of the protein. The peptide bond can be broken by hydrolysis (the addition of water), which is typically carried out by heating with strong acid or alkali.
Worked examples
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Alanine (R = –CH₃) has an isoelectric point (pI) of 6.0. Draw the structure of the species present in a solution of alanine at: (a) pH 1.0 (b) pH 6.0 (c) pH 11.0
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(a) At pH 1.0 (strongly acidic, pH < pI): The molecule will be fully protonated and have a net positive charge. The carboxylate group accepts a proton. Structure: H₃N⁺–CH(CH₃)–COOH
Draw the structure of the two possible dipeptides that can be formed from one molecule of glycine (Gly, R = H) and one molecule of alanine (Ala, R = CH₃). In one of your structures, circle the peptide bond.
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The two amino acids can join in two different orders: Ala-Gly or Gly-Ala.
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What is the general structure of an α-amino acid?
A central carbon atom (the α-carbon) bonded to an amine group (–NH₂), a carboxylic acid group (–COOH), a hydrogen atom (–H), and a variable side chain (–R group).
Key takeaways
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The α-carbon is a chiral centre for 19 of the 20 common amino acids, as it is bonded to four different groups (–H, –NH₂, –COOH, and –R).
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This chirality means that amino acids (except glycine) can exist as two non-superimposable mirror images, or enantiomers (L- and D-forms).
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The exception is glycine, where the R group is a hydrogen atom. Since the α-carbon is bonded to two hydrogen atoms, it is achiral and does not exhibit optical isomerism.
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Practice Questions: Amino Acids
Practice Questions: Amino Acids
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