Peptide bond: -CO-NH- formed by condensation. Has partial double bond character (resonance between C=O and C-N) → planar, restricted rotation. The trans configuration is preferred (bulky R groups on opposite sides). A dipeptide has 1 peptide bond, tripeptide has 2, and an n-residue protein has (n-1) peptide bonds.

Primary structure: Linear sequence of amino acids. Determined by gene sequence. Even one amino acid change can be critical (sickle cell anemia: Glu → Val at position 6 of beta-hemoglobin).

Secondary structure: Alpha-helix (right-handed coil stabilized by i to i+4 intramolecular H-bonds along backbone) and beta-pleated sheet (intermolecular H-bonds between adjacent chains, parallel or antiparallel).

Tertiary structure: Overall 3D folding driven by hydrophobic interactions (nonpolar side chains cluster in interior), disulfide bonds (Cys-S-S-Cys), ionic bonds (Asp COO- to Lys NH3+), and H-bonds.

Quaternary structure: Assembly of multiple polypeptide subunits. Hemoglobin: 2 alpha + 2 beta subunits.

Denaturation: Loss of secondary/tertiary/quaternary structure (primary sequence intact) by heat, pH extremes, heavy metals, urea. Coagulation of egg white on boiling is denaturation.