Transport of Ions, DNA Polymers, and Microtubules in the Nanofluidic Regime - Nanofluidics: Nanoscience and Nanotechnology - page 23

leads to the expected decrease of the measured persistence length upon decreasing the
deformation length.
An open question remained what happens for shorter lengths L of microtubules
that could not be probed in the experiments in reference [119], because the observed ∝L -2
decrease in persistence length predicts a vanishingly small persistence length on the L ≈
0.1 µm length scale that we probe in our experiments. In contrast, a recently proposed
theoretical model, describing the bending of microtubules in terms of bundles of worm-like
chains, 127 predicts a saturation of the persistence length upon a further decrease of the
deformation length. Our method contributes a measurement of the persistence length of
~0.1 µm long tips, which is one order-of-magnitude smaller than the 2.6 µm length that
was previously probed. Nevertheless, the value of p = 0.08 ± 0.02 mm that we find is
similar to the 0.11 mm that was measured for the 2.6 µm long microtubules. Moreover, in
separate control experiments we established a tip persistence length of 0.24 ± 0.03 mm. 128
Thus our data indicate a lower bound on the persistence length of short lengths of
microtubules which is consistent with a recently proposed theory describing the mechanics
of wormlike bundles.
In conclusion, we have shown that electric forces in nanofabricated structures are
an excellent tool for the study of the mechanical properties of individual biomolecules. We
have measured the stiffness of short microtubule ends, which contributes to a better
understanding of the mechanical properties of these macromolecules on short length scales.
1.4.3 Electrophoresis of Individual Microtubules in Microfluidic
Channels
Finally, we describe the use of micron-size fluidic channels to confine and
measure the electrophoresis of freely suspended individual microtubules. 122 Initially, these
experiments were performed to measure the mobility of microtubules needed to calibrate
the electric field-induced forces in steering experiments mentioned in the previous section.
In addition, the high stiffness of microtubules makes them an excellent model system for
rod-like particles, which provides an opportunity to measure and test the predicted
anisotropy in the electrophoretic mobility for rod-like particles. 129 These experiments also
allow us to measure the electrical properties of microtubules, such as the effective charge
per tubulin dimer.
We observe the electrophoretic motion of fluorescently labelled microtubules
inside 50 x 1 µm 2 slit-like channels that are fabricated between the entrance reservoirs that
are separated by 5 mm. The experimental geometry is shown in Figure 1.11(c), with
however one important difference: the omission of the kinesin molecules. Upon application
of a voltage difference between the electrodes at either end of the channel, we observe that
the freely suspended microtubules move in the direction opposite to the electric field. Note
that the motion of the negatively charged microtubules in our channels is a superposition of
their electrophoretic velocity and any fluid velocity inside the channel due to electro-
osmotic flow.
In Figure 1.15(a) we show representative time-lapse images of two microtubules
that are driven by an electric field E = 4 kV/m. The displacements of these microtubules,
which are oriented with their axes approximately equal but in opposite directions to the
field, are not collinear with the electric field. Instead, the velocity is slightly directed
toward the axis of each microtubule. This orientation-dependent velocity is a hallmark of
the anisotropic mobility of a cylindrical particle, Figure 1.15(b). The mobility µ of a
microtubule is different for the electric field components perpendicular ( µ ⊥ , E ⊥ ) and