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Organic Chemistry 2 · Biological Molecules

Amino Acids, Peptides, and Proteins

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Study tools

In 30 seconds

Amino acids are molecules bearing both an amine and a carboxylic acid; in the 20 proteinogenic alpha amino acids these groups sit on the same carbon, making that carbon a stereocenter (except glycine). In water they exist as zwitterions, and their net charge depends on pH, crossing zero at the isoelectric point. Amino acids link by amide (peptide) bonds into peptides and proteins, whose sequence — the — can be read by .

Why this matters

Protein primary structure underlies conditions such as sickle-cell disease, where a single substitution (glutamate → valine in hemoglobin) changes protein behavior. Clinically, amino acids are measured in newborn screening for metabolic disorders, and peptide drugs (e.g., insulin) are produced by controlled synthesis or recombinant methods. This material is conceptual only — it does not constitute dosing, treatment, or patient-specific guidance, which remain the responsibility of licensed clinicians.

The college version

1. Structure, Chirality, and Zwitterions

An has –NH₂ and –COOH on the alpha carbon; the variable R group determines identity. With four different groups, the alpha carbon is a stereocenter, so all proteinogenic amino acids except glycine are chiral and exist as L-enantiomers in proteins. In neutral water the carboxyl transfers a proton to the amine, producing a (⁺H₃N–CHR–COO⁻), which explains their high melting points and water solubility.

2. Acid-Base Behavior and the Isoelectric Point

Amino acids are polyprotic: the –COOH deprotonates (pKₐ ≈ 2) and the –NH₃⁺ deprotonates (pKₐ ≈ 9–10), while acidic or basic side chains add more ionizable groups. The is the pH at which the net charge is zero (a zwitterion). Below pI the molecule is net positive; above pI it is net negative. For a simple amino acid, pI = (pKₐ1 + pKₐ2)/2.

3. Peptide Bonds and Primary Structure

The is the formed by condensation of one amino acid's carboxyl with another's amine. The repeating –NH–CHR–CO– backbone defines the primary structure (linear sequence). The amide bond has partial double-bond character, restricting rotation and shaping protein geometry. (conceptually) builds the chain in a chosen order; because amino acids carry two reactive ends, protecting groups (e.g., Boc, Fmoc on N; esters on C) temporarily block the ends not meant to react, then are removed. Edman degradation labels and clips the N-terminal residue step by step, identifying it to read the sequence.

How it works

  1. In solution, an amino acid self-neutralizes to a zwitterion whose charge shifts with pH around its pI.
  2. The N-terminus of one amino acid and the C-terminus of another condense to form a peptide bond.
  3. Protecting groups mask the ends that should not react, enabling controlled, directional chain growth.
  4. Repetition yields a specific sequence (primary structure), which then folds into secondary, tertiary, and quaternary structure.
  5. Edman degradation removes and identifies residues one at a time from the N-terminus to determine that sequence.

Common confusions

Do not confuseWithDifference
Alpha amino acidAny amine/acid moleculeAlpha amino acids place both groups on one carbon
ZwitterionNeutral moleculeA zwitterion has separated + and − charges but net zero
Isoelectric pointpKₐpI is a specific pH of zero net charge; pKₐ is an acid-dissociation constant
Peptide bondGlycosidic bondPeptide bonds link amino acids via amide; glycosidic bonds link sugars via acetal
Primary structureSecondary structurePrimary is the sequence; secondary is local H-bonded folding (helices/sheets)

Memory aids

"N to C, cut from N" — peptides are written and synthesized from N-terminus to C-terminus, and Edman degradation sequences from the N-terminus. For pI: "the pH where the plus and minus cancel."

Quick review

Topic Recap

Alpha amino acids carry amine and carboxyl on the same chiral carbon and exist as zwitterions whose charge depends on pH, reaching zero at the isoelectric point. Condensation forms planar amide (peptide) bonds that make up the primary structure; protecting groups enable controlled stepwise peptide synthesis, and Edman degradation reads that sequence from the N-terminus. Side-chain chemistry then drives folding into functional proteins of profound biological relevance to health and medicine.

Knowledge Check

  1. Why is glycine the only achiral proteinogenic amino acid?
  2. What is a zwitterion, and at what pH does an amino acid carry zero net charge?
  3. What functional group links two amino acids in a protein?
  4. Why are protecting groups necessary in peptide synthesis?
  5. Which end of a peptide does Edman degradation read from?

Answers and Rationales

  1. Glycine's side chain is a hydrogen atom, so its alpha carbon carries two identical H substituents and is not a stereocenter.
  2. A zwitterion is a molecule bearing both a positive (–NH₃⁺) and a negative (–COO⁻) charge; net charge is zero at the isoelectric point (pI).
  3. An amide linkage called the peptide bond, formed between one residue's carboxyl carbon and the next residue's nitrogen.
  4. Because each amino acid has two reactive ends (amine and carboxyl); protecting groups block the end that should not react, enforcing the desired N-to-C direction and preventing unwanted couplings.
  5. The N-terminus — Edman reagents label and remove the N-terminal residue one at a time.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Think of an amino acid as a bead with two snap connectors: a blue "base" snap (–NH₂) and a red "acid" snap (–COOH), plus a colored side tag that makes each bead unique. In water the bead donates a proton from its red snap to its blue snap, so it carries both a + and a − — like a battery with two opposite ends. Stringing beads together snaps the red end of one bead to the blue end of the next, making a long chain; the order of the colored tags is the "recipe" for the whole necklace.

This stops being exact because the "snaps" are real functional groups undergoing proton transfer and condensation, the side chains range from nonpolar to charged and can themselves gain or lose protons, and the chain folds into specific three-dimensional shapes governed by the sequence — not a passive string of beads.

Simple Example

Glycine (R = H) and alanine (R = CH₃) condense with loss of water to give the dipeptide glycylalanine, linked by an amide (peptide) bond between glycine's carboxyl carbon and alanine's nitrogen.

Worked example

Peptide-bond formation:

  1. Activate the carboxyl carbon of amino acid 1 (conceptually, convert –COOH to a better electrophile, e.g., an activated ester).
  2. The nitrogen of amino acid 2 (nucleophile) attacks this carbonyl carbon; a lone pair moves to form the new C–N bond.
  3. The C=O pi electrons move onto oxygen to give a tetrahedral intermediate, then collapse, ejecting the leaving group and reforming the carbonyl to yield the amide (peptide) bond with loss of water overall.
  4. Electron accounting: two electron-pair arrows (N→C and C=O→O, then O→C) conserve charge; the product amide is planar with partial C–N double-bond character from nitrogen lone-pair donation.
  5. In stepwise synthesis, protecting groups keep the wrong ends unreactive so the desired N-to-C direction is enforced.

Key takeaways

  • High yield: All proteinogenic amino acids except glycine are chiral and occur as L-enantiomers.
  • High yield: At physiological pH an amino acid is a zwitterion; the pI is the pH where net charge is zero.
  • High yield: For simple amino acids, pI = (pKₐ of COOH + pKₐ of NH₃⁺)/2; acidic/basic side chains shift pI.
  • High yield: A peptide bond is an amide with resonance, so the C–N bond is planar and rotationally restricted.
  • High yield: Protecting groups are essential because every amino acid has two reactive ends; they direct which end couples.
  • High yield: Edman degradation reads sequence from the N-terminus, one residue at a time.
  • Cysteine can form disulfide cross-links; proline's ring constrains backbone flexibility.

Keep learning

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

Study toolsYou’ll learn to · Key vocabulary

You’ll learn to

  • Describe the structure and chirality of alpha amino acids and explain their zwitterionic and acid-base behavior.
  • Define the isoelectric point and predict how amino acid charge changes with pH.
  • Explain peptide-bond (amide) formation, the role of protecting groups in peptide synthesis, and how primary structure is sequenced.
  • Connect amino acid side-chain properties to protein structure and function, within medical-safety boundaries.

Key vocabulary

Amino acid
Molecule with both –NH₂ and –COOH
Alpha amino acid
Amino acid with both groups on the same (alpha) carbon
Chirality
Alpha carbon with four different substituents
Zwitterion
Internal salt carrying both + and − charges
Isoelectric point (pI)
pH of zero net charge
Peptide bond
Amide linkage between amino acids
Amide linkage
C(=O)–N bond with partial double-bond character
Peptide synthesis
Stepwise amide-bond formation in defined order
Protecting group
Reversible block on an amine or carboxyl
Edman degradation
Sequential N-terminal labeling and cleavage
Primary structure
The linear amino acid sequence

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