- Zimm-Bragg model
In

statistical mechanics , the**Zimm-Bragg model**is ahelix-coil transition model that describes helix-coil transitions ofmacromolecule s, usuallypolymer chains. Most models provide a reasonable approximation of the fractional helicity of a givenpolypeptide ; the Zimm-Bragg model differs by incorporating the ease ofpropagation with respect tonucleation .**Helix-coil transition models**Helix-coil transition models assume that

polypeptides are linear chains composed of interconnected segments. Further, models group these sections into two broad categories: "coils", random conglomerations of disparate unbound pieces, are represented by the letter 'C', and "helices", ordered states where the chain has assumed a structure stabilized byhydrogen bonding , are represented by the letter 'H'.cite journal|author=Samuel Kutter|coauthors=Eugene M. Terentjev|journal=The European Physical Journal E - Soft Matter|date=16 October 2002|title=Networks of helix-forming polymers|volume=8|issue=5|pages=539–47|pmid=15015126|publisher=EDP Sciences|doi=10.1140/epje/i2002-10044-x]Thus, it is possible to loosely represent a macromolecule as a string such as CCCCHCCHCHHHHHCHCCC and so forth. The number of coils and helices factors into the calculation of fractional helicity, $heta$, defined as :$heta\; =\; frac\{left\; langle\; i\; ight\; angle\}\{N\}$where:$left\; langle\; i\; ight\; angle$ is the average helicity and:$N$ is the number of helix or coil units.

**Zimm-Bragg**The Zimm-Bragg model takes the cooperativity of each segment into consideration when calculating fractional helicity. The probability of any given

monomer being a helix or coil is affected by which the previous monomer is; that is, whether the new site is a nucleation or propagation.By convention, a coil unit ('C') is always of statistical weight 1. Addition of a helix state ('H') to a previously coiled state (nucleation) is assigned a statistical weight $sigma\; s$, where $sigma$ is the nucleation parameter and:$s\; =\; frac\{\; [H]\; \}\{\; [C]\; \}$.Adding a helix state to a site that is already a helix (propagation) has a statistical weight of $s$. For most

protein s,:$sigma\; ll\; 1\; <\; s$which makes the propagation of a helix more favorable than nucleation of a helix from coil state.cite book|author=Ken A. Dill|coauthors=Sarina Bromberg|title=Molecular Driving Forces - Statistical Thermodynamics in Chemistry and Biology|pages=505|publisher=Garland Publishing, Inc.|date=2002]From these parameters, it is possible to compute the fractional helicity $heta$. The average helicity $left\; langle\; i\; ight\; angle$ is given by:$left\; langle\; i\; ight\; angle\; =\; left(frac\{s\}\{q\}\; ight)frac\{dq\}\{ds\}$where $s$ is the statistical weight and $q$ is the partition function given by the sum of the probabilities of each site on the polypeptide. The fractional helicity is thus given by the equation:$heta\; =\; frac\{1\}\{N\}left(frac\{s\}\{q\}\; ight)frac\{dq\}\{ds\}$.

**tatistical mechanics**The Zimm-Bragg model is equivalent to a one-dimensional

Ising model and has no long-range interactions, i.e., interactions between residues well separated along the backbone; therefore, by the famous argument ofRudolf Peierls , it cannot undergo aphase transition .The statistical mechanics of the Zimm-Bragg model [

*cite journal | last = Zimm | first = BH | author link = Bruno Zimm | coauthors = Bragg JK | year = 1959 | title = Theory of the Phase Transition between Helix and Random Coil in Polypeptide Chains | journal = Journal of Chemical Physics | volume = 31 | pages = 526–531 | doi = 10.1063/1.1730390*] may be solved exactly using the transfer-matrix method. The two parameters of the Zimm-Bragg model are σ, thestatistical weight for nucleating a helix and "s", the statistical weight for propagating a helix. These parameters may depend on the residue "j"; for example, aproline residue may easily nucleate a helix but not propagate one; aleucine residue may nucleate and propagate a helix easily; whereas glycine may disfavor both the nucleation and propagation of a helix. Since only nearest-neighbour interactions are considered in the Zimm-Bragg model, the fullpartition function for a chain of "N" residues can be written as follows:$mathcal\{Z\}\; =\; left(\; 0,\; 1\; ight)\; cdot\; left\{\; prod\_\{j=1\}^\{N\}\; mathbf\{W\}\_\{j\}\; ight\}\; cdot\; left(\; 1\; ,\; 1\; ight)$

where the 2x2 transfer matrix

**W**_{"j"}of the "j"th residue equals the matrix of statistical weights for the state transitions:$mathbf\{W\}\_\{j\}\; =\; egin\{bmatrix\}s\_\{j\}\; 1\; \backslash $

sigma_{j} s_{j} & 1end{bmatrix}

The "row-column" entry in the transfer matrix equals the statistical weight for making a transition from state "row" in residue "j-1" to state "column" in residue "j". The two states here are "helix" (the first) and "coil" (the second). Thus, the upper left entry "s" is the statistical weight for transitioning from helix to helix, whereas the lower left entry "σs" is that for transitioning from coil to helix.

**ee also***

Alpha helix

*Lifson-Roig model

*Random coil

*Statistical mechanics **References**

*Wikimedia Foundation.
2010.*