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Organic Chemistry · Biomolecules: Amino Acids, Peptides, and Proteins

Structures of Amino Acids

8 min read
6 key termsSourcesReport an error
Numerical values (molar masses, pKa values) are standard reference values; verify against current sources before relying on them in assessments.
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On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Check yourself
  8. Study tools
  9. Sources & references

In 30 seconds

Amino acids are the monomer units of peptides and proteins. All twenty standard amino acids share one framework: an α-carbon bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen, and a variable — the side chain alone distinguishes one amino acid from another, by its size, charge, polarity, and hydrogen-bonding ability. Nineteen of the twenty are chiral (glycine, R = H, is not); nearly all natural amino acids are L. At physiological pH, amino acids are zwitterions: carboxyl deprotonated (-COO-), amino protonated (-NH3+), net charge zero. This topic covers the general structure, the side-chain classification that drives protein folding, and the three-letter/one-letter shorthand used across biochemistry.

Why this matters

  • Proteins are amino-acid polymers. Every enzyme, antibody, and structural protein is an amino-acid chain; side chains determine folding and function.
  • Side-chain chemistry drives medicine. Drug–receptor binding, enzyme active sites, and antigen–antibody recognition depend on side-chain polarity, charge, and size.
  • Nutrition depends on the essential amino acids. Humans synthesize only about ten of the twenty; the rest are dietary.
  • Chirality matters for biology. Proteins use only L-amino acids; D forms are rare (some bacterial cell walls, some antibiotics).
  • Charge explains behavior. formation and pH-dependent side-chain ionization explain protein solubility and electrophoretic migration.
  • Exam relevance. Drawing the general structure, classifying side chains, assigning D/L or R/S, and one-letter codes are standard questions.

The college version

Core Concepts

The common framework

Every standard amino acid is H2N-CH(R)-COOH, with the central carbon the α-carbon (next to the carboxyl). It carries four different substituents (except glycine): amino group, carboxyl, hydrogen, and R — which makes it a chiral center, so each amino acid except glycine exists as a pair of enantiomers.

Chirality and the L convention

Absolute configuration uses the D/L convention based on glyceraldehyde's Fischer projection. With carboxyl at top and R at bottom, L-amino acids put the amino group on the left; D-amino acids on the right. Almost all protein amino acids are L. In R/S terms, the L form of most amino acids is S, but L-cysteine is R (sulfur changes the priority order) — a classic trap. Glycine has no chiral center and is achiral.

Classifying the twenty side chains

The twenty standard amino acids fall into four side-chain classes:

  • Nonpolar (hydrophobic): Gly (G), Ala (A), Val (V), Leu (L), Ile (I), Met (M), Phe (F), Trp (W), Pro (P). These cluster in protein interiors and membrane-spanning regions.
  • Polar, uncharged: Ser (S), Thr (T), Cys (C), Tyr (Y), Asn (N), Gln (Q). They hydrogen-bond with water and each other; cysteine can form disulfide bonds.
  • Acidic (negative at pH 7): Asp (D) and Glu (E), whose -COOH side chains ionize to -COO-.
  • Basic (positive at pH 7): Lys (K, -NH2 side chain), Arg (R, guanidinium group), and His (H, imidazole ring).

Zwitterions and ionization

In the solid state and neutral water, an amino acid is a zwitterion (internal salt): H3N+-CH(R)-COO-. The -COOH proton transfers to -NH2, giving separated charges but zero net charge. In acid, the carboxylate is protonated (net +1); in base, the ammonium is deprotonated (net -1). The pH of zero net charge is the isoelectric point (pI) — where the amino acid does not migrate in an electric field, the basis of electrophoresis.

Three-letter and one-letter codes

Each amino acid has a three-letter abbreviation (Ala) and a one-letter symbol (A), used to write protein sequences compactly (insulin's A chain begins GIVEQ...). The one-letter code is usually the name's first letter, with collisions resolved: arginine R, asparagine N, aspartate D, glutamine Q, glutamate E, phenylalanine F, tryptophan W, tyrosine Y, lysine K.

Common Confusions

Do Not ConfuseWithDifference
L-amino acidS-configurationMost L-amino acids are S, but L-cysteine is R — its sulfur side chain flips the priority.
GlycineA chiral amino acidGlycine has two H on the α-carbon: no chiral center, achiral, no D/L forms.
Acidic vs. basic side chainsPolar unchargedAsp/Glu are negative at pH 7; Lys/Arg/His positive; Ser/Thr/Tyr polar but neutral.
Amino acidAmineAn amino acid has both -NH2 and -COOH on the same carbon; a simple amine has only the nitrogen function.
ZwitterionUncharged moleculeA zwitterion has separated + and - charges (net zero), unlike a molecule with no charges at all.
One-letter code first letterAlways the name's first letterExceptions abound: R = arginine, N = asparagine, D = aspartate, E = glutamate, Q = glutamine, F = phenylalanine, W = tryptophan, Y = tyrosine, K = lysine.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine twenty kinds of beads with the same hole in the middle but different outside shapes and colors — some greasy and clingy, some wet and sticky, some with a plus sign, some with a minus. Amino acids are those beads. String them in a line and they fold into a protein; the bead shapes decide how it folds, like a necklace that curls on its own because of what its beads are made of. Nearly all beads in living things are "left-handed," and glycine is the one bead with no left or right at all.

Worked example

Example 1: Molar mass of glycine and alanine

Calculate the molar masses of glycine, C2H5NO2, and alanine, C3H7NO2 (C = 12.011, H = 1.008, N = 14.007, O = 15.999 g/mol).

Formula first:

M = nC M(C) + nH M(H) + nN M(N) + nO M(O)

Substitute for glycine:

M = 2(12.011) + 5(1.008) + 14.007 + 2(15.999) = 24.022 + 5.040 + 14.007 + 31.998 = 75.07 g/mol

Substitute for alanine:

M = 3(12.011) + 7(1.008) + 14.007 + 2(15.999) = 36.033 + 7.056 + 14.007 + 31.998 = 89.09 g/mol

Answer: Glycine is 75.07 g/mol, alanine 89.09 g/mol — exactly one CH2 (14.03 g/mol) apart, the same step seen throughout homologous series.

Example 2: Is glycine chiral? Assign R/S to L-alanine

(a) Why is glycine achiral? (b) L-Alanine has the amino group on the left in its Fischer projection. Is L-alanine R or S?

Reasoning (a): chirality needs four different substituents. Glycine's α-carbon has two identical hydrogens (R = H), so only three distinct groups — no chiral center, no enantiomers.

Reasoning (b): assign priorities at the α-carbon: NH2 (highest), COOH, CH3, H (lowest). Rotate so H points away; the sequence NH2 → COOH → CH3 traces counterclockwise, which is S.

Answer: Glycine is achiral; L-alanine is S. But L-cysteine is R — its sulfur-bearing side chain outranks the carboxyl in Cahn–Ingold–Prelog priority, so the L form maps to R there. Never assume "L = S."

Example 3: Predicting net charge from pH

What is the net charge of alanine at pH 1, pH 7, and pH 12?

Reasoning walkthrough: alanine's ionizable groups are the α-carboxyl (pKa ≈ 2.3) and α-ammonium (pKa ≈ 9.7); the CH3 side chain is not ionizable.

  • pH 1 (below both pKa): carboxyl protonated, amino protonated  →  net +1.
  • pH 7 (between pKa): carboxyl deprotonated, amino protonated  →  zwitterion, net 0.
  • pH 12 (above both): both deprotonated  →  net -1.

Answer: +1, 0, -1. Alanine's pI is (2.3 + 9.7)/2 = 6.0, where the zwitterion dominates and alanine does not migrate in an electric field.

Key takeaways

  • General structure: H2N-CH(R)-COOH; the α-carbon is chiral except in glycine (R = H).
  • Natural amino acids are L (amino group left in Fischer projection); most are S, but L-cysteine is R.
  • Four classes of side chains: nonpolar, polar uncharged, acidic (Asp, Glu), basic (Lys, Arg, His).
  • At pH 7 amino acids are zwitterions, H3N+-CH(R)-COO-, with zero net charge.
  • pI = pH of zero net charge; basis of electrophoresis.
  • Essential amino acids (humans, ~10): Val, Leu, Ile, Met, Phe, Trp, Thr, Lys, His, and Arg (conditionally).
  • One-letter codes: G A V L I M F W P / S T C Y N Q / D E / K R H.
  • Cysteine's thiol side chain forms disulfide bonds that lock protein folds.

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

  1. Draw the general structure of an amino acid and label the α-carbon, amino group, carboxyl group, and side chain.

    Show answer

    H2N-CH(R)-COOH: the central α-carbon bonds to NH2, COOH, H, and R.

  2. Why is glycine achiral?

    Show answer

    Its R group is H, so the α-carbon carries two identical hydrogens — only three distinct substituents, no chirality.

  3. List the four side-chain classes and give two examples of each.

    Show answer

    Nonpolar (Ala, Val, Leu, Ile, Met, Phe, Trp, Pro, Gly); polar uncharged (Ser, Thr, Cys, Tyr, Asn, Gln); acidic (Asp, Glu); basic (Lys, Arg, His).

  4. What is a zwitterion, and why does it form at neutral pH?

    Show answer

    A zwitterion is an internal salt, H3N+-CH(R)-COO-: the carboxyl proton transfers to the amino group, giving separated charges but zero net charge.

  5. What is the net charge of lysine at pH 1 (side-chain -NH2, pKa ≈ 10.5)?

    Show answer

    At pH 1 the carboxyl and both amino groups are protonated (including side-chain -NH3+)  →  net +2.

  6. What is the molar mass of valine, C5H11NO2?

    Show answer

    5(12.011) + 11(1.008) + 14.007 + 2(15.999) = 60.055 + 11.088 + 14.007 + 31.998 = 117.15 g/mol.

Keep learning

Ready to build on this? Continue to the next lesson.

Study tools & related lessonsKey vocabulary · Related

Key vocabulary

α-Carbon
Central carbon of an amino acid bearing NH2, COOH, H, and R
Side chain (R)
Variable group that distinguishes the twenty amino acids
Zwitterion
Internal salt with separated + and - charges, net zero
pI (isoelectric point)
pH at which net charge is zero
Essential amino acid
Amino acid the body cannot synthesize in adequate amounts
L/D convention
Stereochemical label from the Fischer projection of glyceraldehyde

Sources & references

  1. openstax.org — Organic Chemistry

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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