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- Primary ($$1^\circ$$): Specific linear sequence of amino acids in a polypeptide chain.
- Secondary ($$2^\circ$$): Local folding patterns held by hydrogen bonds ($$\alpha$$\text{-helix} and $$\beta$$ \text{-pleated sheet}).
- Tertiary ($$3^\circ$$): Overall 3D folding held by H-bonds, disulfide bridges, ionic bonds, and van der Waals interactions.
- Quaternary ($$4^\circ$$): Spatial arrangement of multiple sub-units.
