Worm-like chain

Worm-like chain

The worm-like chain (WLC) model in polymer physics is used to describe the behavior of semi-flexible polymers; it is sometimes referred to as the "Kratky-Porod" worm-like chain model.

Theoretical Considerations

The WLC model envisions an isotropic rod that is continuously flexible; this is in contrast to the freely-jointed chain model that is flexible only between discrete segments. The worm-like chain model is particularly suited for describing stiffer polymers, with successive segments displaying a sort of cooperativity: all pointing in roughly the same direction. At room temperature, the polymer adopts a conformational ensemble that is smoothly curved; at T = 0 K, the polymer adopts a rigid rod conformation.

For a polymer of length l, parametrize the path of the polymer as s subseteq (0,l), allow hat t(s) to be the unit tangent vector to the chain at s, and vec r(s) to be the position vector along the chain. Then

:hat t(s) equiv frac {partial vec r(s) }{partial s} and the end-to-end distance vec R = int_{0}^{l}hat t(s) ds .

It can be shown that the orientation correlation function for a worm-like chain follows an exponential decay:

:langlehat t(s) cdot hat t(0) angle=langle cos ; heta (s) angle = e^{-s/P},,

where P is by definition the polymer's characteristic persistence length. A useful value is the mean square end-to-end distance of the polymer:

egin{matrix}langle R^{2} angle & = & langle vec R cdot vec R angle \ \ & = & langle int_{0}^{l} hat t(s) ds cdot int_{0}^{l} hat t(s') ds' angle \ \ & = & int_{0}^{l} ds int_{0}^{l} langle hat t(s) cdot hat t(s') angle ds' \ \ & = & int_{0}^{l} ds int_{0}^{l} e^{-left | s - s' ight | / P} ds' \ \ langle R^{2} angle & = & 2 Pl left [ 1 - frac {P}{l} left ( 1 - e^{-l/P} ight ) ight ] end{matrix}

* Note that in the limit of l >! > P, then langle R^{2} angle = 2Pl. This can be used to show that a Kuhn segment is equal to twice the persistence length of a worm-like chain.


= Biological Relevance = Several biologically important polymers can be effectively modeled as worm-like chains, including:
* double-stranded DNA;
* unstructured RNA; and
* unstructured polypeptides (proteins).


= Stretching Worm-like Chain Polymers = Laboratory tools such as atomic force microscopy (AFM) and optical tweezers have been used to characterize the force-dependent stretching behavior of the polymers listed above. An interpolation formula that describes the extension x of a WLC with contour length L_0 and persistence length P in response to a stretching force F is :frac {FP} {k_{B}T} = frac {1}{4} left ( 1 - frac {x} {L_0} ight )^{-2} - frac {1}{4} + frac {x}{L_0}

where k_B is the Boltzmann constant and T is the absolute temperature (Bustamante, "et al.", 1994; Marko "et al.", 1995).

In the particular case of stretching DNA in physiological buffer (near neutral pH, ionic strength approximately 100 mM) at room temperature, the compliance of the DNA along the contour must be accounted for. This enthalpic compliance is accounted for by adding a stretch modulus K_0 to the above relation:

:frac {FP} {k_{B}T} = frac {1}{4} left ( 1 - frac {x} {L_0} + frac {F}{K_0} ight )^{-2} - frac {1}{4} + frac {x}{L_0} - frac {F}{K_0}

where a typical value for the stretch modulus of double-stranded DNA is around 1000 pN and 45 nm for the persistence length (Wang, "et al.", 1997).

See also

* Polymer
* Polymer physics
* Ideal chain

References

* O. Kratky, G. Porod (1949), "Röntgenuntersuchung gelöster Fadenmoleküle." "Rec. Trav. Chim. Pays-Bas." 68: 1106-1123.
* J. F. Marko, E. D. Siggia (1995), "Stretching DNA." "Macromolecules", 28: p. 8759.
* C. Bustamante, J. F. Marko, E. D. Siggia, and S. Smith (1994), "Entropic elasticity of lambda-phage DNA." "Science", 265: 1599-1600. PMID 8079175
* M. D. Wang, H. Yin, R. Landick, J. Gelles, and S. M. Block (1997), "Stretching DNA with optical tweezers." "Biophys. J.", 72:1335-1346. PMID 9138579
* C. Bouchiat et al., [http://www.biophysj.org/cgi/content/abstract/76/1/409 "Estimating the Persistence Length of a Worm-Like Chain Molecule from Force-Extension Measurements"] , Biophys J, January 1999, p. 409-413, Vol. 76, No. 1


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