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Amino acids are organic compounds whose molecules contain both a basic amino group (–NH₂) and an acidic carboxyl group (–COOH). Because of these two groups with opposite properties, amino acids are amphoteric: they react with both acids and alkalis. They are the "building blocks" of proteins: joined by peptide bonds into long chains, amino acids form proteins. Proteins are the basis of life: enzymes, hormones, antibodies, transport molecules (haemoglobin), and the structures of muscle, hair and skin. This topic combines what you know about amines and carboxylic acids and links directly to biology.
General form of α-amino acids H₂N–CHR–COOH; saturated amino acids with one amino and one carboxyl group: CₙH₂ₙ₊₁NO₂
Depending on where the amino group is relative to the carboxyl group, there are α-, β- and γ-amino acids: in an α-amino acid the –NH₂ is on the carbon next to the carboxyl group. Only α-amino acids are found in proteins.
The simplest examples: glycine (aminoethanoic acid) H₂N–CH₂–COOH (M = 75) and alanine (2-aminopropanoic acid) CH₃–CH(NH₂)–COOH (M = 89). Many amino acids are known by traditional names and written with three-letter codes: Gly, Ala, Val, Ser, Cys and so on.
Proteins contain 20 kinds of α-amino acid. They are classified by side chain: neutral (one NH₂, one COOH — glycine, alanine), acidic (two COOH — glutamic acid, aspartic acid), basic (two amino groups — lysine), sulfur-containing (cysteine, methionine) and aromatic (phenylalanine, tyrosine, tryptophan). Essential amino acids (valine, leucine, lysine, tryptophan and others) cannot be made in the human body and must come from food.
In crystals and in neutral solution an amino acid exists as an internal salt (a dipolar ion, or zwitterion): the carboxyl group passes its proton to the amino group.
H₂N–CH₂–COOH ⇌ ⁺H₃N–CH₂–COO⁻
So, unlike most organic substances, amino acids behave like ionic crystals: they are colourless crystalline solids, very soluble in water, poorly soluble in organic solvents, and they melt with decomposition at high temperatures. Many taste slightly sweet (the name glycine comes from the Greek word for "sweet").
Kinds of isomerism: chain isomerism, position isomerism of the amino group (α, β, γ), optical isomerism (every α-amino acid except glycine has an asymmetric carbon) and functional-group isomerism with nitroalkanes: C₂H₅NO₂ fits both glycine and nitroethane.
The mass fraction of nitrogen in a saturated amino acid with one amino group is 15.73 %. Identify the amino acid.
H₂N–CH₂–COOH + HCl → [H₃N–CH₂–COOH]Cl
H₂N–CH₂–COOH + NaOH → H₂N–CH₂–COONa + H₂O
H₂N–CH₂–COOH + H₂N–CH(CH₃)–COOH → H₂N–CH₂–CO–NH–CH(CH₃)–COOH + H₂O
Peptide (amide) bond –CO–NH–; forming a peptide from n amino acids releases n − 1 molecules of water
If each of two different amino acids is used once, 2 dipeptides form (Gly-Ala and Ala-Gly); from three different ones, 3! = 6 tripeptides. A peptide is written with the free amino group on the left (N-terminus) and the free carboxyl group on the right (C-terminus).
Find the molar mass of the tripeptide formed from three glycine molecules.
Proteins are natural polymers (polypeptides) made of hundreds or thousands of amino acid residues. Their structure is described at four levels:
A protein’s biological function depends on its three-dimensional shape: an enzyme’s active site exists only in a correctly folded chain.
Proteins have many roles: catalytic (enzymes such as pepsin and amylase), transport (haemoglobin carries oxygen), protective (antibodies), structural (collagen, keratin), contractile (actin, myosin), regulatory (the hormone insulin) and energetic (breaking down 1 g of protein releases about 17 kJ). By composition, simple proteins consist of amino acids only, while conjugated proteins have a metal ion, carbohydrate or lipid attached.
Denaturation is the destruction of a protein’s natural three-dimensional structure (secondary, tertiary, quaternary) by heat, strong acids or alkalis, heavy-metal salts, alcohol or radiation; the primary structure is kept. Egg white setting when boiled is an example. Hydrolysis breaks the peptide bonds and gives amino acids (with enzymes, acids or alkalis); digestion is protein hydrolysis.
Biuret test: CuSO₄ in alkaline solution gives a violet colour — detects peptide bonds
Xanthoproteic test: conc. HNO₃ gives a yellow colour — detects amino acids with aromatic rings
Sulfur-containing amino acids: heating with alkali and Pb(CH₃COO)₂ gives a black PbS precipitate
Burning protein smells of burnt feathers (wool), another simple way to detect it.
13.2 g of glycylglycine (M = 132) was completely hydrolysed. What mass of glycine forms and what mass of water is used?
Test tubes contain: 1) a protein; 2) methylamine; 3) glucose; 4) ethanoic acid. Which reagents identify them?