1 [[!meta title="Freely rotating chains"]]
3 [[Velocity clamp force spectroscopy|Force_spectroscopy]] pulls are
4 [often fit to polymer models][carrionvazquez99] such as the worm-like
5 chain (WLC). However, [Puchner et al.][puchner08] had the bright idea
6 that, rather than fitting each loading region with a polymer model, it
7 is easier to calculate the change in contour length by converting the
8 abscissa to contour-length space. While the WLC is commonly used,
9 Puchner gets better fits using the freely rotating chain (FRC) model.
11 Computing force-extension curves for either the WLC or FJC is
12 complicated, and it is common to use interpolation formulas to
13 estimate the curves. For the WLC, we use [Bustamante's
14 formula][bustamante94]:
17 F_WLC(x) = \frac{k_B T}{p} \left[
18 \frac{1}{4}\left(\frac{1}{\left(1-\frac{x}{L}\right)^2} - 1\right)
22 For the FRC, Puchner uses [Livadaru][livadaru03]'s equation 46.
27 \frac{fa}{3k_B T} & \text{for } \frac{fb}{k_B T} \lt \frac{b}{l} \\
28 1-\left(\frac{fl}{4k_B T}\right)^{-\frac{1}{2}}
29 & \text{for } \frac{b}{l} \lt \frac{fb}{k_B T} \lt \frac{l}{b} \\
30 1-\left(\frac{fb}{ck_B T}\right)^{-1}
31 & \text{for } \frac{l}{b} \lt \frac{fb}{k_B T}
35 Unfortunately, there are two typos in Livadaru's equation 46. It
36 should read (confirmed by private communication with Roland Netz).
41 \frac{fa}{3k_B T} & \text{for } \frac{fb}{k_B T} \lt \frac{b}{l} \\
42 1-\left(\frac{4fl}{k_B T}\right)^{-\frac{1}{2}}
43 & \text{for } \frac{b}{l} \lt \frac{fb}{k_B T} \lt \frac{l}{b} \\
44 1-\left(\frac{cfb}{k_B T}\right)^{-1}
45 & \text{for } \frac{l}{b} \lt \frac{fb}{k_B T}
49 Regardless of the form of Livadaru's equation 46, the suggested FRC
50 interpolation formula is Livadaru's equation 49, which has continuous
51 cross-overs between the various regimes and adds the possibility of
52 elastic backbone extension.
55 \frac{R_z}{L} = 1 - \left\{
56 \left(F_\text{WLC}^{-1}\left[\frac{fl}{k_BT}\right]\right)^\beta
57 + \left(\frac{cfb}{k_BT}\right)^\beta\right\}^{\frac{-1}{\beta}}
58 + \frac{f}{\tilde{\gamma}} \;,
61 where $l=b\frac{\cos(\gamma/2)}{|\ln(\cos\gamma)|}$ (Livadaru's
62 equation 22) is the effective persistence length, $\beta$ determines
63 the crossover sharpness, $\tilde{\gamma}$ is the backbone stretching
64 modulus, and $F_\text{WLC}^{-1}[x]$ is related to the inverse of
65 Bustamante's interpolation formula,
68 F_\text{WLC}[x] = \frac{3}{4} - \frac{1}{x} + \frac{x^2}{4} \;.
71 By matching their interpolation formula with simlated FRCs, Livadaru
72 suggests using $\beta=2$, $\tilde{\gamma}=\infty$, and $c=2$. In his
73 paper, Puchner suggests using $b=0.4$ nm and $\gamma=22^{\circ}$.
74 However, when I contacted him and pointed out the typos in Livadaru's
75 equation 46, he reran his analysis and got similar results using the
76 corrected formula with $b=0.11$ nm and $\gamma=41^{\circ}$. This
77 makes more sense because it gives a WLC persistence length similar to
78 the one he used when fitting the WLC model:
81 l = b\frac{\cos(\gamma/2)}{|\ln(\cos\gamma)|} = 0.366\text{ nm}
84 (vs. his WLC persistence length of $p=0.4$ nm).
86 In any event, the two models (WLC and FRC) give similar results for
87 low to moderate forces, with the differences kicking in as $fb/k_B T$
88 moves above $l/b$. For Puchner's revised numbers, this corresponds to
91 f \gt \frac{l}{b} \cdot \frac{k_B T}{b}
92 = \frac{\cos(\gamma/2)}{|\ln(\cos\gamma)|} \cdot \frac{k_B T}{b}
93 \approx 122 \text{ pN} \;,
96 assuming a temperature in the range of 300 K.
98 I've written an `inverse_frc` implementation in
99 [[crunch.py|Comparing_velocity_clamp_experiments/crunch.py]] for
100 [[comparing velocity clamp experiments]]. I test the implementation
101 with [[frc.py|Comparing_velocity_clamp_experiments/frc.py]] by
102 regenerating [Livadaru et al.'s figure 14][livadaru03].
104 [[!img Comparing_velocity_clamp_experiments/figure-14.png
105 alt="Inverse FRC test matching Livadaru et al.'s figure 14"
106 title="Inverse FRC test matching Livadaru et al.'s figure 14"]]
109 [carrionvazquez99]: http://dx.doi.org/10.1073/pnas.96.20.11288
110 [puchner08]: http://dx.doi.org/10.1529/biophysj.108.129999
111 [bustamante94]: http://dx.doi.org/10.1126/science.8079175
112 [livadaru03]: http://dx.doi.org/10.1021/ma020751g