Biomolecules · Lesson 6 of 12
Amino Acids: Side Chains, Charges and Nutritional Classes
“Read amino-acid structures and codes, explain zwitterions and amphoteric behaviour, and distinguish chemical classification from nutritional requirement.”
• Identify the α-amino-acid framework and interpret side-chain structures. • Write the major charge changes on addition of acid or base. • Explain the high melting behaviour of amino-acid solids. • Distinguish side-chain classes, essentiality and stereochemistry. • Use the twenty common amino-acid names and codes.
One framework, many side chains
An amino acid contains amino and carboxyl functionality. In an α-amino acid, the amino nitrogen is attached to the carbon immediately next to the carboxyl carbon. The usual neutral structural shorthand is H₂N–CH(R)–COOH, where R is the side chain. The α-carbon also carries H; it is not the carbon of the carboxyl group. In β- or γ-amino acids the amino group lies farther away. The standard protein building blocks considered here are α-amino acids.
Complete hydrolysis of ordinary proteins yields amino acids, revealing the building blocks that peptide bonds had connected. Protein-rich foods include pulses, dairy products, eggs, fish and meat. Amino acids serve as precursors for structural and functional proteins, while their different R groups help determine protein folding and chemistry. The common names are retained and abbreviated using three-letter and one-letter codes; for example glycine is Gly or G and alanine Ala or A.
| Amino acid | Codes | Side chain R or special connectivity | Chemical feature |
|---|---|---|---|
| Glycine | Gly / G | H | Small, achiral |
| Alanine | Ala / A | CH₃ | Nonpolar |
| Valine | Val / V | CH(CH₃)₂ | Nonpolar; essential |
| Leucine | Leu / L | CH₂CH(CH₃)₂ | Nonpolar; essential |
| Isoleucine | Ile / I | CH(CH₃)CH₂CH₃ | Nonpolar; essential |
| Arginine | Arg / R | (CH₂)₃NH–C(=NH)NH₂ | Basic; conditionally essential |
| Lysine | Lys / K | (CH₂)₄NH₂ | Basic; essential |
| Glutamic acid | Glu / E | CH₂CH₂COOH | Acidic |
| Aspartic acid | Asp / D | CH₂COOH | Acidic |
| Glutamine | Gln / Q | CH₂CH₂CONH₂ | Polar amide |
| Asparagine | Asn / N | CH₂CONH₂ | Polar amide |
| Threonine | Thr / T | CH(OH)CH₃ | Polar; essential |
| Serine | Ser / S | CH₂OH | Polar |
| Cysteine | Cys / C | CH₂SH | Thiol; can form disulfide links |
| Methionine | Met / M | CH₂CH₂SCH₃ | Thioether; essential |
| Phenylalanine | Phe / F | CH₂C₆H₅ | Aromatic; essential |
| Tyrosine | Tyr / Y | CH₂–C₆H₄–OH (para) | Phenolic aromatic |
| Tryptophan | Trp / W | CH₂–(indol-3-yl) | Aromatic; essential |
| Histidine | His / H | CH₂–(imidazol-4-yl) | Ionisable ring; essential |
| Proline | Pro / P | (CH₂)₃ links α-carbon back to N | Cyclic secondary amino group |
Read each side chain from its point of attachment to the α-carbon. Leucine has –CH₂–CH(CH₃)₂, whereas isoleucine has –CH(CH₃)–CH₂–CH₃. Both have the same side-chain atom count, but different branching next to the backbone. Asparagine and glutamine have amides, –CONH₂, which should not be counted as ordinary basic amine groups. In proline the side chain closes back onto the amino nitrogen, so its free amino group is secondary rather than H₂N–.
Acidic, basic and neutral refer to side-chain behaviour
Every ordinary α-amino acid has an acidic carboxyl and a basic amino group. Extra ionisable side-chain groups distinguish acidic aspartic and glutamic acids from basic lysine and arginine; histidine has an ionisable imidazole ring. At a near-neutral pH, aspartate and glutamate usually have an additional negative side-chain charge, lysine and arginine usually an additional positive one, and histidine’s protonation depends sensitively on its environment.
Neutral in this classification does not mean that the molecule has no internal charges or that its solution must have pH exactly seven. It means the side chain is not being classified as an additional acidic or basic group in the simple scheme. Counting written NH groups without considering their chemistry would incorrectly classify amides such as glutamine.
Zwitterions explain salt-like properties
The carboxyl group can lose a proton and the amino group can accept one. In the common dipolar form of a simple amino acid, nitrogen is ⁺H₃N– and the carboxyl is –COO⁻: ⁺H₃N–CH(R)–COO⁻. A molecule bearing both positive and negative formal charges is a zwitterion. For a side chain with no extra charge, these balance to zero net charge, although the molecule is not uncharged internally.
Amino acids often form ionic crystal lattices, which require substantial energy to disrupt. They are therefore commonly high-melting, often decomposing solids. Their polar ionic groups interact strongly with water, although solubility varies greatly with side chain, pH and crystal packing. The useful comparison with corresponding halo acids concerns the additional zwitterionic interactions, not a universal guarantee that every amino acid is highly soluble in every condition.
Amphoteric behaviour: follow the proton
An amphoteric substance can react with both acids and bases. Add acid to a simple amino-acid zwitterion: the carboxylate accepts H⁺, giving ⁺H₃N–CH(R)–COOH with net charge +1. Add base: OH⁻ removes a proton from ⁺H₃N–, giving H₂N–CH(R)–COO⁻ and H₂O, with net charge −1. Do not remove a carbon atom or change the R group when drawing a simple acid–base reaction.
Problem
Write glycine forms favoured in strongly acidic, intermediate and strongly basic conditions, ignoring counterions.
- 1.Glycine has R = H. Strong acid favours ⁺H₃N–CH₂–COOH: the carboxylate is protonated and net charge is +1.
- 2.Between the principal ionisation steps, ⁺H₃N–CH₂–COO⁻ predominates: internal charges sum to zero.
- 3.Strong base favours H₂N–CH₂–COO⁻, net −1. The pH changes protonation state, not the carbon skeleton.
Essentiality is a nutritional classification
An essential amino acid must be supplied adequately by diet because the body cannot synthesise enough of it. A non-essential amino acid can ordinarily be synthesised; the word does not mean that the amino acid is dispensable for protein function. For healthy adults, the nine indispensable amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Arginine is conditionally essential, particularly when growth or physiological demands exceed synthesis.
Lysine and tryptophan are examples of essential amino acids; alanine and aspartic acid are examples normally classed as non-essential. Chemical and nutritional classifications answer different questions: lysine is both basic and essential, whereas glutamic acid is acidic and usually non-essential. Side-chain polarity alone does not determine whether the body can synthesise an amino acid.
Stereochemistry and the glycine exception
For most standard α-amino acids, the α-carbon has four different substituents: amino group, carboxyl group, H and R. It is therefore a stereocentre. Glycine has R = H, giving two identical H substituents, so it is achiral. Protein biosynthesis generally incorporates L-amino acids; D amino acids occur in other biological contexts. L does not mean negative optical rotation.
For the conventional amino-acid Fischer projection with COOH at the top and R at the bottom, NH₂ on the left identifies the L series. This configuration label is distinct from the carbohydrate D/L application and from the measured rotation sign, even though both systems are historically related to glyceraldehyde. Some amino acids also contain a side-chain stereocentre; the basic α-carbon discussion does not erase that additional information.
Problem
Classify lysine and alanine by side-chain behaviour and nutritional requirement, then decide whether glycine is chiral.
- 1.Lysine has an extra basic amino side chain and is essential for adults. Alanine has a nonpolar methyl side chain and is usually non-essential.
- 2.These facts do not follow from the number of carbons alone. Biosynthetic ability determines essentiality, while ionisable groups determine acid–base class.
- 3.Glycine is achiral because its α-carbon has two H substituents. A zwitterionic charge arrangement does not by itself create a stereocentre.
Quick Quiz
Quiz
Which atom bears the amino group in an α-amino acid?
Explanation: The α-carbon is adjacent to the carboxyl carbon.
Why is glycine achiral?
Explanation: Four different substituents are required for an ordinary tetrahedral stereocentre.
What is the net charge of ⁺H₃N–CH₂–COO⁻?
Explanation: The +1 nitrogen and −1 carboxylate charges sum to zero.
Which pair contains two ordinarily essential adult amino acids?
Explanation: Diet must adequately supply lysine and tryptophan.
Why is glutamine not classified as a basic amino acid simply by counting NH₂ groups?
Explanation: Functional-group context matters; an amide is not an ordinary amine.
Practice Problems
- Draw the predominant simple forms of alanine in strongly acidic and strongly basic media.
- Explain why amino acids often melt or decompose at higher temperatures than analogous simple halo acids.
- Give two essential and two non-essential amino acids and explain what the terms mean.
- Compare glycine, alanine and proline structurally.
- Show how a zwitterion reacts with acid and with base, checking net charge.
- Decode Glu, Gln, Asp and Asn and explain the important chemical difference.
Alanine has R = CH₃. In strong acid, ⁺H₃N–CH(CH₃)–COOH has net +1. In strong base, H₂N–CH(CH₃)–COO⁻ has net −1. Protonation changes occur on the amino/carboxyl groups; the methyl side chain and carbon skeleton remain unchanged.
Key Takeaways
• Standard protein amino acids use the α-amino-acid framework. • Side chains determine many chemical differences and have established codes. • Zwitterions contain opposite formal charges within one molecule. • Amino acids can accept and donate protons and are amphoteric. • Healthy adults require nine indispensable amino acids; other requirements can be conditional. • Essentiality does not mean biological importance or side-chain polarity. • Glycine is achiral; standard protein amino acids otherwise generally have L configuration.