Biomolecular Machines - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 184

quantum dots, for example. It was found that kinesin binding, needed to transport
the cargo-microtubule system, is inhibited if the microtubule filaments are uniformly
functionalized with quantum dots, whereas for microtubules functionalized only in
the central region, the translational velocities attain 0:3ms 1 .
Because of the glue-like character of the biotin-streptavidin bond, efficient cargo
attachment occurs for optimized gliding velocities of microtubules, which can be
controlled between 50 and 450 nm s 1 by modifying the concentration of kinesin
substrate ATP ( Agarwal et al. 2009 ). For biotinylated 4-nm polystyrene nanospheres
that attach to streptavidin-coated microtubules, which move on a surface coated with
kinesin, the optimum gliding velocity of microtubules for loading was found to be
200 nm s 1 .
There is also an inverse configuration, in which kinesin motors walk along
microtubules, preferred in some applications because the microtubules can be
oriented along certain directions. In addition, cargoes can be attached to kinesin
motors in the same way as to microtubules. In this inverse configuration, an array of
long and properly aligned microtubules is required for motor displacements along
desired directions. Such an array can be fabricated by immobilizing first microtubule
seeds, then polymerizing the microtubules only from the plus ends of the seeds
to obtain long structures of about 50m, and, finally, attaching the microtubule
filaments to the surface ( Brown and Hancock 2002 ). Microtubule alignment along a
certain direction was achieved by allowing a solution of buffer to flow, which causes
microtubules to align downstream, while microtubule attachment to the surface by
a solution of gluteraldehyde or by binding to mutant kinesins cannot harness ATP
hydrolysis.
Bifunctional DNA molecules with a biotinylated end attached to a microtubule
that glides over kinesin immobilized on a surface and with a thiolated end to a
patterned gold pad can be (over)stretched between the motile microtubule and the
pads ( Dinu et al. 2006 ). This stretching method manipulates many DNA molecules
in parallel and can help construct dynamic networks. In particular, œ-phage DNA
can be stretched to more than 18:6m, which is the full contour length of DNA.
However, as yet, the stretching process can be only observed but not controlled.
Control of microtubule/kinesin interactions has been reported in Ionov et al.
( 2006 ). In this work, as shown in Fig. 6.12 , kinesin molecules were adsorbed on
Si between chains of the poly(N-isopropylacrylamide) thermoresponsive polymer,

a b

microtubule

polymer

kinesin

Si

Fig. 6.12 Controlled microtubule/kinesin interactions via conformation changes of a thermore-
sponsive polymer: ( a ) inhibited interaction and ( b ) allowed interactions