Models for the 310‐helix/coil, π‐helix/coil, and α‐helix/310‐helix/coil transitions in isolated peptides

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Abstract
Models for the 310-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 310-helix formation have also been combined to describe a three-state equilibrium in which α-helical, 310-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 310-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 310-helices and mixed 310-/α-helices.