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| '''Helix–coil transition models''' are formalized techniques in [[statistical mechanics]] developed to describe [[conformation (chemistry)|conformation]]s of linear [[polymer]]s in solution. The models are usually but not exclusively applied to [[polypeptide]]s as a measure of the relative fraction of the molecule in an [[alpha helix]] conformation versus [[turn (biochemistry)|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.<ref name="Doig2008">{{cite book
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| |author=Doig AJ
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| |year=2008
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| |chapter=The alpha-Helix as the Simplest Protein Model: Helix–Coil Theory, Stability, and Design
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| |title=Protein Folding, Misfolding and Aggregation: Classical Themes and Novel Approaches
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| |editor= Muñoz V
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| |publisher=Royal Society of Chemistry
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| }}</ref> 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-terminus|N-terminal]] and [[C-terminus|C-terminal]] ends, propagation parameters may differ in each direction.
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| The two states are
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| * helix state: characterized by a common rotating pattern kept together by [[hydrogen bond]]s, (see [[alpha-helix]]).
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| * coil state: conglomerate of randomly ordered sequence of atoms (see [[random coil]]).
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| Common transition models include the [[Zimm-Bragg model]] and the [[Lifson-Roig model]], and their extensions and variations.
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| Energy of host poly-alanine helix in aqueous solution:
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| :<math>
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| \Delta G_{folding} = (m-2)\Delta H_\alpha - m T \Delta S
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| </math> | |
| where ''m'' is number of residues in the helix.<ref name="Chakrabartty1995">{{cite journal
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| |doi=10.1016/S0065-3233(08)60334-4
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| |author=Chakrabartty A, Baldwin RL
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| |year=1995
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| |title=Stability of alpha-helices
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| |journal=Adv Protein Chem
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| |volume=46
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| |pages=141–176
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| |pmid=7771317
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| }}</ref>
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| ==References==
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| {{reflist}}
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| {{DEFAULTSORT:Helix-coil transition model}}
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| [[Category:Protein structure]]
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| [[Category:Statistical mechanics]]
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| [[Category:Thermodynamics]]
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| {{physics-stub}}
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