# Helix-coil transition model

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Helix-coil transition model

Helix-coil transition models are formalized techniques in statistical mechanics developed to describe conformations of linear polymers in solution. The models are usually but not exclusively applied to polypeptides as a measure of the relative fraction of the molecule in an alpha helix conformation versus turn or random coil. The main attraction in investigating alpha helix formation is that one encounters many of the features of protein folding but in their simplest version [ Doig, A.J. (2008) "The alpha-Helix as the Simplest Protein Model: Helix–Coil Theory, Stability, and Design", in "Protein Folding, Misfolding and Aggregation: Classical Themes and Novel Approaches", V. Muñoz, Ed. Royal Society of Chemistry. ] . Most of the helix-coil models contain parameters for the likelihood of helix nucleation from a coil region, and helix propagation along the sequence once nucleated; because polypeptides are directional and have distinct N-terminal and C-terminal ends, propagation parameters may differ in each direction.

Common transition models include the Zimm-Bragg model and the Lifson-Roig model, and their extensions and variations.

Energy of host poly-alanine helix in the aqueous solution::$Delta G_\left\{folding\right\} = \left(m-2\right)Delta H_alpha - m T Delta S$where "m" - is number of residues in the helix [ Chakrabartty, A., Baldwin, R.L. 1995. Stability of alpha-helices. Adv. Protein Chem.,46:141-176.]

References

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