4. 2: Secondary Structure and Loops - Biology LibreTexts Describe the idealized geometries of different helical types—alpha (α), 3₁₀, and pi (π) helices—including parameters such as residues per turn (n), pitch, and the characteristic phi psi angles Compare and contrast the alpha helix with the 3₁₀ and pi helices in terms of stability, dimensions, and side chain orientation