Models for the 3 10 ‐helix/coil, π‐helix/coil, and α‐helix/3 10 ‐helix/coil transitions in isolated peptides

Models for the 3 10 ‐helix/coil and π‐helix/coil equilibria have been derived. The theory is based on classifying residues into helical or nonhelical (coil) conformations. Statistical weights are assigned to residues in a helical conformation with an associated helical hydrogen bond, a helical conformation with no hydrogen bond, an N‐cap position, a C‐cap position, or the reference coil conformation. The models for α‐helix formation and 3 10 ‐helix formation have also been combined to describe a three‐state equilibrium in which α‐helical, 3 10 ‐helical, and coil conformations are populated. The results are compared with the modified Lifson‐Roig theory for the α‐helix/coil equilibrium. The comparison accounts for the experimental observations that 3 10 ‐helices tend to be short and π‐helices are not favored for any length. This work may provide a framework for quantitatively rationalizing experimental work on isolated 3 10 ‐helices and mixed 3 10 ‐/α‐helices.