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Mechanically Induced Chemistry New Perspectives on the Nanoscale.

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Meeting Reviews
Mechanically Induced Chemistry:
New Perspectives on the Nanoscale**
Irmgard Frank*
The idea of inducing chemical reactions
by exerting mechanical force dates back
to the beginning of the twentieth
century, when Ostwald developed the
concept
of
mechanochemistry.[1]
Mechanically induced chemistry thus
has a long history and it continues to
be of high importance for industrial
applications such as the preparation of
electrode materials.[2] New aspects of
this field have emerged from recent
developments in the nanosciences.
With atomic force and scanning
tunneling microscopy (AFM and STM,
respectively) techniques, it is meanwhile
possible to manipulate single molecules
and to eventually induce chemical reactions. Of particular importance in this
context is the work done by the group of
Hermann
Gaub
(Munich),
who
succeeded in detecting the breaking of
a single chemical bond and measuring
the corresponding force. In parallel to
this experimental progress, molecular
dynamics methods have been developed
that allow such processes to be
explained in full detail. Besides classical
molecular dynamics approaches, Car–
Parrinello molecular dynamics has
become very important and allows the
quantum mechanical nature of chemical
bonds to be taken into account during a
molecular dynamics simulation. At the
87th International Bunsen Discussion
Meeting that took place in Tutzing near
[*] Dr. I. Frank
Ludwig-Maximilians-Universitt Mnchen
Department Chemie
Butenandtstrasse 5–13, Haus E
81377 Mnchen (Germany)
Fax: (+ 49) 89-2180-77568
E-mail: frank@cup.uni-muenchen.de
[**] 87th International Bunsen Discussion
Meeting, October 3–6, 2005, Tutzing
(Germany).
852
Munich during October 3–6, 2005, the
whole
spectrum
of
theoretical
approaches were brought together with
recent experimental developments.
Michael Klein (Philadelphia) gave
an impressive overview of the different
aspects of this topic in the introductory
talk. Klein:s own contributions to the
field cover a broad range: He performed
the first Car–Parrinello molecular
dynamics simulations in the field of
mechanically induced chemistry. This
work showed the behavior of a molecular knot under strong tension up to
bond breaking.[3] Additionally, consecutive reactions were observed, such as
the formation of a ring from the fragments. Classical molecular dynamics has
been used for the investigation of phenomena on larger scales, while for the
investigation of significantly larger
molecular systems, Klein and co-workers have developed coarse-grain methods. Here, the atomic approach is abandoned and thus it becomes possible to
simulate, for example, the intrusion of
macromolecules into biological membranes.[4]
In general, the simulation of biosystems under mechanical load is particularly
important.
Klaus
Schulten
(Urbana) presented simulations of
highly complex biological systems. His
group uses a broad range of methods
depending on the nature of the problem,
and of special importance is NAMD or
NAMD2 (“not (just) another molecular
dynamics” program), the code developed by Schulten for classical molecular
dynamics simulations of extremely large
systems. With this approach, systems
such as a-hemolysin and its membrane
environment (about 3 A 105 atoms) was
studied.[5] In connection with the SMD
(steered molecular dynamics) approach
that was also developed by Schulten and
co-workers, and endorsed by the versatile graphics program VMD, a mighty
package is now available for the simulation of large systems under mechanical
load.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
A general problem not only to the
investigation of the dynamics of proteins
but also especially to the simulation of
chemical reactions is the limited timescale that is accessible to molecular
dynamics simulations. Hence, several
groups are currently developing
approaches to accelerate rearrangements or chemical reactions in a simulation. In several of the studies presented, metadynamics was applied, a
method that was recently developed by
Alessandro Laio and Michele Parrinello.[6] Metadynamics represents an
alternative approach to the “chemical
flooding” method by Helmut GrubmDller (GEttingen), who impressively demonstrated the efficiency of his approach
with simulations of chemical reactions
of strained ring systems. “Chemical
flooding” thus complements “conformational flooding”, which allows the study
of rearrangements in larger systems. In
part with Paul Tavan (Munich), GrubmDller also conducted simulations of
strained systems with classical molecular dynamics and characterized the fundamental phenomena many years ago.
Applications ranged from simple polymers to the simulation of the unbinding
of the streptavidin–biotin complex.[7]
Since many years, the work of GrubmDller is closely connected to the
experiments performed by Matthias
Rief (Munich), who has performed
strain experiments for diverse systems
such as polysaccharides and DNA.[8]
Together with Hermann Gaub, he
answered the question how strong are
the forces that arise from the breaking
of a single covalent bond. Rief presented current results on the unfolding
of green fluorescent protein (GFP).
Here, two metastable states have been
found as intermediates of the unfolding
process. On the basis of the experiments,
the free-energy landscape could be
characterized in full detail.[9]
A further highlight of the meeting
was the talk by Peter Hamm (ZDrich)
about photoswitchable peptides. By
means of a photoreaction, it is possible
Angew. Chem. Int. Ed. 2006, 45, 852 – 854
Angewandte
Chemie
in these systems to induce mechanical
tension in a peptide chain. By using
infrared spectroscopy with femtosecond
time resolution, the consecutive fast
dynamics in the peptide backbone can
be studied on the timescale of a few
picoseconds. Particularly prominent is
the experiment using azobenzene as a
photoswitch[10] which has also been
investigated theoretically by the groups
of Gerhard Stock and Paul Tavan.
Hamm also discussed the use of alternative chromophores.
At present, several groups are working on the spectroscopic investigation of
single dye molecules under tensile stress,
the results of which may lead to new
possibilities in the development of local
stress sensors. The studies presented by
the groups of Claus Seidel (DDsseldorf)
and Christoph BrIuchle (Munich) lead
one to expect that this aim will be
reached in the near future.
The meeting closed with two more
theoretical sessions, starting with a lecture by Dominik Marx (Bochum),
whose
Car–Parrinello
molecular
dynamics simulations on the formation
of gold nanowires under tensile stress
are known beyond the theoretical
chemistry community[11] (Figure 1).
These systems, which have been investigated experimentally by the group of
Harald Fuchs, demonstrate most
impressively that remarkable phenomena can be triggered by mechanical
stress on the nanoscale.
Gotthard Seifert (Dresden) then
presented the results of simulations
with tight-binding density functional
theory (DFT). His investigations of, for
Figure 1. Formation of a gold nanowire under
tensile stress, modeled with Car–Parrinello
molecular dynamics. A sufficiently strong
covalent attachment to the gold surface is
achieved with sulfur (light yellow; courtesy of
D. Marx).
Angew. Chem. Int. Ed. 2006, 45, 852 – 854
example, molybdenum sulfide nanotubes,[12] convincingly demonstrated the
high potential of the method in the field
of inorganic systems (Figure 2). The
simulations showed in detail which processes occur if nanotubes are broken
under extreme tensile stress.
In the concluding talk, Schulten
pointed out how multifaceted and
broad the field of mechanically induced
chemistry is, even if one restricts the
discussion to phenomena on the atomic
scale. With the range of different molecular dynamics methods (Car–Parrinello,
tight-binding,
force-field,
coarsegrained) and with methods from statistical mechanics, it is nowadays possible
to explain highly complex processes on
different scales from a theoretical point
of view. In combination with exciting
Figure 2. Tight-binding DFT simulation of the breaking of MoS2 nanotubes under mechanical
load (courtesy of I. Ivanovskaya, T. Heine, and G. Seifert).
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
853
Meeting Reviews
developments concerning single-molecule methods and time-resolved spectroscopy, novel insights will be gained in
the near future into the processes that
occur in materials under extreme conditions on the shortest length and timescales.
[1] W. Ostwald, Handbuch der allgemeinen
Chemie, Bd. 1, Akademische Verlagsgesellschaft mbH, Leipzig, 1919, 70.
[2] S. Kipp, V. Sepelak, K. D. Becker, Chem.
Unserer Zeit 2005, 39, 384.
854
www.angewandte.org
[3] A. M. Saitta, P. D. Soper, E. Wasserman,
M. L. Klein, Nature 1999, 399, 46.
[4] G. Srinivas, D. E. Discher, M. L. Klein,
Nat. Mater. 2004, 3, 638.
[5] A. Aksimentiev, K. Schulten, Biophys. J.
2005, 88, 3745.
[6] A. Laio, M. Parrinello, Proc. Natl. Acad.
Sci. USA 2002, 99, 12 562.
[7] H. GrubmDller, B. Heymann, P. Tavan,
Science 1996, 271, 997.
[8] M. Rief, H. GrubmDller, ChemPhysChem 2002, 3, 255.
[9] H. Dietz, M. Rief, Proc. Natl. Acad. Sci.
USA 2004, 101, 16 192.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
[10] J. Bredenbeck, J. Helbing, A. Sieg, T.
Schrader, W. Zinth, C. Renner, R.
Behrendt, L. Moroder, J. Wachtveitl, P.
Hamm, Proc. Natl. Acad. Sci. USA 2003,
100, 6452.
[11] D. KrDger, H. Fuchs, R. Rousseau, D.
Marx, Angew. Chem. 2003, 115, 2353;
Angew. Chem. Int. Ed. 2003, 42, 2251.
[12] G. Seifert, H. Terrones, M. Terrones, G.
Jungnickel, T. Frauenheim, Phys. Rev.
Lett. 2000, 85, 146.
DOI: 10.1002/anie.200504567
Angew. Chem. Int. Ed. 2006, 45, 852 – 854
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