Biomedical Engineering Reference
In-Depth Information
kinesin were elaborated (Endow and Higuchi
2000
; Kikkawa et al.
2001
; Svoboda
et al.
1993
; Visscher et al.
1999
;
2003
; Schliwa and Woehlke
2003
) and the move-
ment of single-kinesin molecules on microtubule tracks under variable loads and
ATP concentrations was studied. A puzzling problem is the mechanism of directed
motion along tracks that do not exhibit any chemical or electrical gradient. As for
myosin-V, a hotly debated question (Hua et al.
2002
) is whether the two heads of
kinesin move in a hand-over-hand or in an inchworm manner. High-resolution fl uo-
rescence microscopy experiments (Yidiz et al.
2004
) provided strong evidence in
favor of the hand-over-hand model. Even if the intimate mechanism of the move-
ment is not yet known, the fact remains that kinesin molecules generate force and
transport cargo unidirectionally.
3
Artifi cial Systems
3.1
Rotary Motors
Artifi cial molecular rotary motors are systems capable of undergoing unidirectional
and repetitive rotations under the action of external energy inputs. The construction
of molecular rotary motors poses several challenges particularly because it is diffi -
cult to satisfy the unidirectional rotation requirement. At present, artifi cial rotary
motors have only been obtained by exploiting a C=C isomerization reaction in care-
fully designed hindered chiral alkenes (Sect.
3.1.1
), but we also include the descrip-
tion of the chemically driven unidirectional rotation in helicene-triptycene
compounds and of surface-mounted molecular rotors.
3.1.1
Rotary Motors Powered by Light Energy
Photochemical
trans-cis
isomerization around a carbon-carbon double bond is a
well-know reversible reaction (Balzani and Scandola
1991
) . Exploitation of this
photochemical reaction in a suitably designed compound which contains hindering
substituents and stereogenic centers led to a photoinduced unidirectional rotary
motion (Koumura et al.
1999
) .
In the alkene
1
shown in Fig.
3
, each one of the two helical subunits linked by a
double bond can adopt a right-handed (
P
) or a left-handed (
M
) helicity. As a result,
a total of four stereoisomers are possible for this compound. The
cis
-
trans
photoi-
somerization reactions are reversible and occur upon irradiation at appropriate
wavelengths. By contrast, the inversions of helicities while maintaining a
cis
or a
trans
confi guration occur irreversibly under the infl uence of thermal energy. Upon
irradiation (³280 nm, 218 K) of a solution of (
P
,
P
)-
trans
-
1
(Fig.
3a
), a mixture of
(
P
,
P
)-
trans
-
1
and (
M
,
M
)-
cis
-
1
is obtained in a ratio of 5:95 (Fig.
3b
). By warming
the solution up to 293 K, (
M
,
M
)-
cis
-
1
interconverts irreversibly to (
P
,
P
)-
cis
-
1
