Page 100 - Science at the nanoscale

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RPS: PSP0007 - Science-at-Nanoscale
June 12, 2009
From Atoms and Molecules to Nanoscale Materials
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of a “dumbbell shaped axle” (blue) and a macrocyclic “wheel”
(green). The macrocyclic wheel can rotate around and slide along
the axis of the dumbbell. The rotaxane can function as molecular
switch by controlling the position of the ring from one position to
another on the axis.
Another proposed component of molecular machines is the
catenane family of which one example is shown in Fig. 4.28(b).
This is again an interlocking architecture consisting of two inter-
locked macrocyclic rings, which are not separable unless one of
the covalent bonds is broken. There are many different designs of
catenanes, all of them work on the basis that the rings can rotate
with respect to each other, with weak interactions between spe-
cially encoded motifs on the rings that determine their preferred
configurations.
Complex molecular structures have been built in nature to carry
An example is ATP syn-
out specific physiological functions.
thase, which is an enzyme that synthesises adenosine triphosphate
(ATP) from adenosine diphosphate (ADP):
+
ATP Synthase, H
ATP
ADP + Phosphate
−−−−−−−−−−−−−→
Energy is required for this reaction and this is often driven by
protons moving down an electrochemical gradient. The enzyme
has a large mushroom-shaped structure ∼10nm across and ∼8 nm
The
high, consisting of two segments F 0 and F 1 (Fig. 4.29).
hydrophobic F 0 segment is embedded in the membrane and per-
forms proton translocation, while the hydrophillic F 1 segment
protrudes into the aqueous phase to perform ATP synthesis. Dur-
ing the reaction, conformational changes in some segments of the
enzyme generate a rotation, making ATP synthase the smallest ch04
rotary machine known in nature. 16
In summary, scientists are looking into the nano-world of
biological molecules for inspiration in designing molecular elec-
tronic and mechanical machines. One major question is how
to provide suitable energy inputs to drive these nano-machines.
While temperature gradient is difficult to maintain over such
small dimensions, chemical reactions will produce side-products
that need to be transported away efficiently. One promising
solution for powering man-made nano-machines is the use of
16 P. D. Boyer, J. E. Walker and J. C. Skou shared the 1997 Nobel Prize in Chemistry
for their independent work on ATP synthase and the other ion-transporting en-
+
+
zyme, Na , K -ATPase.