Detailed Explanation
The alpha helix is the most common secondary structure in peptides and proteins. It forms when the peptide backbone spirals into a right-handed coil with 3.6 amino acid residues per complete turn and a rise of 5.4 Å per turn. Each backbone N–H group hydrogen-bonds to the C=O group four residues earlier (the i→i+4 pattern), creating a remarkably stable and regular structure.
Not all amino acids are equally comfortable in alpha helices. Alanine, leucine, methionine, and glutamate are strong helix formers. Proline is a helix breaker because its cyclic side chain can't donate the required hydrogen bond and introduces a rigid kink. Glycine is a weak helix former because its conformational flexibility makes the ordered helical state entropically unfavorable.
Alpha helices are functionally critical in many peptides. Antimicrobial peptides like LL-37 and magainin adopt amphipathic alpha helices — one face hydrophobic, one face hydrophilic — that insert into and disrupt bacterial membranes. In drug design, stapled peptides use hydrocarbon cross-links to lock an alpha-helical conformation, dramatically improving cell penetration and stability.
Key Facts
- 3.6 residues per turn, 5.4 Å rise per turn
- Stabilized by i→i+4 backbone hydrogen bonds
- Discovered by Pauling and Corey (1951)
- Strong helix formers: Ala, Leu, Met, Glu
- Helix breaker: proline
- Amphipathic helices are key to antimicrobial peptide function
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