Biomedical Engineering Reference
In-Depth Information
macrocycles in catenanes can be obtained (see Sect.
3.3
). Other candidates for the
construction of artifi cial rotary motors are double-decker compounds (Shinkai et al.
2001
) and the surface-mounted molecular rotors whose rotational motion can be
controlled with the tip of a scanning tunneling microscope (Zheng et al.
2004
) .
3.2
Molecular Shuttles
Rotaxanes are minimally made of an axle-type molecule surrounded by a macrocy-
clic (ring) component (Sauvage and Dietrich-Buchecker
1999
) . Bulky groups (stop-
pers) placed at the ends of the axle prevent the disassembly of the components that
are therefore interlocked during the synthesis. Because of their peculiar structure,
at least two interesting molecular motions can be envisaged in rotaxanes, namely,
(1) rotation of the macrocyclic ring around the axle and (2) translation, i.e., shuttling
(Anelli et al.
1991
), of the ring along the axle. Hence, rotaxanes are good prototypes
for the construction of both rotary and linear molecular motors.
The axle and ring components usually exhibit some kind of interaction originat-
ing from complementary chemical properties, which is also exploited in the
template-directed synthesis of such systems. In rotaxanes containing two different
recognition sites in the axle component, it is possible to switch the position of the
ring between the two “stations” by an external stimulus. Systems of this type,
termed molecular shuttles, constitute indeed the most common implementation of
the molecular motor concept with rotaxanes.
3.2.1
Molecular Shuttles Powered by Chemical Reactions
The fi rst example of a chemically driven molecular shuttle was reported in 1994
(Bissell et al.
1994
). Since then, many molecular shuttles relying on chemical stim-
ulation have been described in the literature (Balzani et al.
2000a
;
2003
;
2001a,
b
;
Elizarov et al.
2002
; Tseng et al.
2003
; Keaveney and Leigh
2004
; Leigh and Perez
2004
). One of the best systems in terms of switching and stability is compound
7
3+
shown in Fig.
5
(Ashton et al.
1998
). It is made of an axle component containing an
ammonium (AMH) and an electron acceptor bipyridinium (BPM) unit that can
establish hydrogen-bonding and electron donor-acceptor interactions, respectively,
with the ring component, which is a crown ether with electron-donor properties. An
anthracene moiety is used as a stopper because its absorption, luminescence, and
redox properties are useful to monitor the state of the system. Since the N
+
-H···O
hydrogen bonding interactions between the macrocycle and the ammonium center
are much stronger than the electron donor-acceptor interactions of the macrocycle
with the BPM, the rotaxane exists as only one of the two possible translational iso-
mers (Fig.
5a
, state 0). Deprotonation of the ammonium center with a base (Fig.
5b
)
causes 100% displacement of the macrocycle by Brownian motion to the BPM unit
(Fig.
5c
, state 1); reprotonation with an acid (Fig.
5d
) directs the macrocycle back
