One of the first experiments where the chemical
reaction could be followed directly in the time
domain has been performed in 1987 in our research
group (at that time at the TUM)
[1],
[2].
In these experiments we followed the initial reaction
dynamics of the photosynthetic protein
Bacteriorhodopsin (BR). The initial reaction of BR
is a light induced photoisomerisation of the chromophore
retinal from the all-trans to the 13 - cis form
(See
Fig.1).
The development of femtosecond light pulses in the
early 1980's
[3]
allowed for the first time to study fast
photochemical reactions in real time. In 1985,
the latest part of the initial photoisomerisation
reaction - the transfer to the ground state photo
product - could be detected by optical techniques
[4],
[5],
[6].
The most important question at that time treated the
nature of the reactive motions preceding this transfer
to the groundstate photoproduct. These reactive motions
can be discussed within the frame of a potential energy
diagram of the system where the energy of the different
electronic states (in our case of the retinal molecule)
is plotted as a function of a nuclear coordinate
(see
Fig.2).
For the photoisomerisation of retinal, the reaction
coordinate could be assigned - at least in part - to
the rotation around the C13 - C14 double bond. The
presented picture is a cut through the multidimensional
energy landscape of the polyatomic molecule where we
present only one of the many degrees of freedom
(the minimum energy path). In this picture the
excitation of bacteriorhodopsin by light promotes the
molecule to the excited electronic state potential
surface without any change of nuclear geometry. From
this so-called Franck-Condon region the system moves
along the S1 potential curve to the minimum. From this
part of the potential curve the back reaction to the
ground state takes place.
Fig.1:
The relevant configurations of
the retinal molecule
During the motion on the potential surface the distance
between S1 and S0 surface is strongly changing. As
a consequence one would expect a continuous change
of the frequency of emission from the excited electronic
state. These spectral changes can be well observed
experimentally.
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Fig.2:
Potential energy diagram explaining
the femtochemical S1-reaction of
bacteriorhodopsin
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Fig.3:
Schematic of the set-up used for
the femtosecond pump-probe experiment.
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In the experiments be performed in 1987, we studied
absorption changes and stimulated emission of bacteriorhodopsin
on the 100 fs time scale. We excited the sample at 620 nm
(Fig.3)
and probed the transmission changes at different wavelengths
(photon energies) throughout the near infrared (see arrows
in Fig.4).
In these experiments, Fig.5,
we could find a fast decay of
stimulated emission with 200 fs at high energies of the
probing photons. At lower photon energies (larger wavelengths)
we observed predominantly the decay of the S1 state with
500 fs. Within the model of the potential surfaces
(see Fig.2)
we can relate the 200 fs reaction to a motion
of the retinal molecule away from the Franck-Condon region
to a geometry from where the remaining isomerizational motion
and the transition to the ground state can take place. The
experimental data show clearly that the important initial part
of the femtochemical reaction of the retinal molecule could be
followed in time by the "slow motion" technique of ultrafast
spectroscopy. In parallel to our experiments published in
Chemical Physics Letters
[1],
other investigations had been
performed by the group of R. Mathies and C. Shank
[7].
These experiments were focused to the Franck-Condon region
and the motion of the retinal out of this region. The data
gave very interesting information complementary to our
experiment, which presents the motion towards the minimum
of the potential surface.
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Fig.4:
Absorption and fluorescence
spectrum of bacteriorhodopsin.
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Fig.5:
Absorption changes recorded during
the S1-reaction of bacteriorhodopsin.
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More recent investigations by different research groups confirmed
the basic results of the early papers. They also gave interesting
additional information for a better understanding of the molecular
processes during the S1 motion as a multidimensional process.
In conclusion: The experiments on bacteriorhodopsin have
been the first where photochemical reactions could be observed
in 'real time' on the excited state potential energy surface.
Later on, fascinating experiments on photo dissociation of
small molecules have displayed the dissociative motion on the
S1 potential surface and oscillatory features during ultrafast
chemical reactions (see:
[8], [9]
and The Nobel Price for Chemistry 1999,
www.nobel.se/announcement-99/chemistry99.html )
If you are interested in doing research in the field of femtochemistry and
bacteriorhodopsin please contact
Wolfgang Zinth.
Currently we are expanding this research area and can offer different
diploma thesis (Diplomarbeiten)
on ultrafast spectroscopy on
bacteriorhodopsin and other retinal containing proteins. We are also
offering positions for
students (Werkstudenten)
for IR-spectroscopy on
these proteins.
References
[1]
J. Dobler, W. Zinth, W. Kaiser, D. Oesterhelt
Excited-state reaction dynamics of Bacteriorhodopsin
studied by femtosecond spectroscopy.
Chem. Phys. Lett. 144 (Feb. 1988) 215
[2]
W. Zinth
Die schnellsten molekularen Vorgänge bei der
Photosynthese von Bakteriorhodopsin.
Naturwiss. 75 (Apr. 1988) 173
[3]
R.L. Fork, B.I. Greene, C.V. Shank,
Appl. Phys. Lett. 38 (1981) 671.
[4]
H.-J. Polland, M.A. Franz, W. Zinth, W. Kaiser,
E. Kölling, D. Oesterhelt
The early picosecond events in the photocycle of Bacteriorhodopsin.
Biophys. J. 49 (1986) 651
[5]
A.V. Sharkov, A.V. Pakulev, S.V. Chekalin, Y.A. Matveetz,
Biochim. Biophys. Acta 808 (1985) 94
[6]
M.C. Nuss, W. Zinth, W. Kaiser, E. Kölling, D. Oesterhelt
Femtosecond spectroscopy of the first events
of the photochemical cycle in Bacteriorhodopsin.
Chem. Phys. Lett. 117 (1985) 1
[7]
R.A. Mathies, C.H. Brito Cruz, W.T. Pollard, C.V. Shank
Direct Observation of the Femtosecond Excited
State cis-trans Isomerization in Bacteriorhodopsin.
Science, 240 (May 1988) 777.
[8]
T. S. Rose, M. J. Rosker, A. H. Zewail
Femtosecond real-time observation of wave packet
oscillations (resonance) in dissociation reactions.
J. Chem. Phys. 88 (May 1988) 6672.
[9]
M. J. Rosker, M. Dantus, A. H. Zewail,
Science, 241 (1988) 1200.
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