Secondary structure কী?

The secondary structure of a protein is the local, regular folding of its backbone, held by hydrogen bonds between backbone C=O and N–H groups. Two patterns dominate. The $\alpha$ helix: a right-handed coil of 3.6 residues per turn, rising $0.15\,\mathrm{nm}$ per residue ($0.54\,\mathrm{nm}$ per turn), each C=O bonded to the N–H four residues ahead, with the side chains pointing outward. The $\beta$ sheet: chains stretched almost fully ($0.35\,\mathrm{nm}$ per residue) and laid side by side, parallel or antiparallel, bonded between neighbouring strands, with side chains alternating above and below the sheet. Between them, turns and loops reverse the chain’s direction. Proline, whose ring locks $\phi$, breaks helices; glycine, with no side chain, allows turns that no other residue can make.