Biomedical Engineering Reference
In-Depth Information
Much of the inspiration to construct molecular devices and machines comes
from the outstanding progress of molecular biology that has begun to reveal the
secrets of the natural nanodevices which constitute the material base of life (Goodsell
2004
). The bottom-up construction of devices as complex as those present in nature
is, of course, an impossible task. Therefore, chemists have tried to construct much
simpler systems without mimicking the complexity of the biological structures. In
the last few years, synthetic talent that has always been the most distinctive feature
of chemists, combined with a device-driven ingenuity evolved from chemists' atten-
tion to functions and reactivity, has led to outstanding achievements in this fi eld
( Balzani et al.
2000a
,
2003
;
2001a,
b
) .
1.2
Characteristics of Molecular Motors and Machines
A
molecular motor
can be defi ned as an assembly of a discrete number of molecular
components designed to perform mechanical-like movements under the control of
appropriate energy inputs (Akkerman and Coops
1967
; Cozzi et al.
1981
; Bedard
and Moore
1995
; Dominguez et al.
2003
; Shima et al.
2004
) . The words
motor
and
machine
are often used interchangeably when referred to molecular systems. It
should be recalled, however, that a motor converts energy into mechanical work
while a machine is a device, usually containing a motor component, designed to
accomplish a function. Molecular motors and machines operate via electronic and/
or nuclear rearrangements and make use of thermal fl uctuations (Brownian motion)
(Astumian and Hänggi
2002
; Parisi
2005
). Like the macroscopic counterparts, they
are characterized by (1) the kind of energy input supplied to make them work;
(2) the type of motion (linear, rotatory, oscillatory, etc.) performed by their compo-
nents; (3) the way in which their operation can be monitored; (4) the possibility to
repeat the operation at will (cyclic process); and (5) the timescale needed to com-
plete a cycle. According to the view described above, an additional and very impor-
tant distinctive feature of a molecular machine with respect to a molecular motor is
(6) the function performed (
2001a
) .
1.2.1
Energy Supply
The problem of the energy supply to make artifi cial molecular motors work [point
(1)] is of the greatest importance (Ballardini et al.
2001a,
b
). The most obvious way
to supply energy to a chemical system is through an exergonic chemical reaction. In
the previously mentioned address (Feynman
1960a,
b
) to the American Physical
Society, Richard Feynman observed: “An internal combustion engine of molecular
size is impossible. Other chemical reactions, liberating energy when cold, can be
used instead.” This is exactly what happens in our body, where the chemical energy
supplied by food is used in long series of slightly exergonic reactions to power the
biological machines that sustain life.
