A combined optical and EPR spectroscopy study: Azobenzene-based biradicals as reversible molecular photoswitches

Article abstract of DOI:10.1039/c7cp02088k

Azobenzene compounds are known as versatile examples for photoswitchable systems because of their isomeric cis- and trans-configurations. The switching between these isomers can be reversibly controlled by light excitation. In this study we characterize t

Full text of DOI:10.1039/c7cp02088k

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Steinwand, S. Klenik, J. Plackmeyer, J. Bats, J. Wachtveitl and T. F. F. Prisner, Phys. Chem. Chem. Phys.,
2017, DOI: 10.1039/C7CP02088K.
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ARTICLE
A Combined Optical and EPR Spectroscopy Study: Azobenzene-
Based Biradicals as Reversible Molecular Photoswitches
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
a
a
a,b
c
Eva A. Jaumann,‡ Sabrina Steinwand,‡ Szabina Klenik, Jörn Plackmeyer, Jan W. Bats, Josef
a
a,b
Wachtveitl,* and Thomas F. Prisner*
DOI: 10.1039/x0xx00000x
Azobenzene compounds are known as versatile examples for photoswitchable systems because of their isomeric cis- and
trans-configurations. The switching between these isomers can be reversibly controlled by light excitation. In this study we
characterize two members of this class by joining the azobenzene moiety with each two paramagnetic nitroxide spin
labels. Two different linkers were chosen to tune the molecular properties. The combined approach using optical and EPR
spectroscopy proved the reversibility of the photoexcitation and the high fatigue resistance. Furthermore, depending on
the nature of the linker, PELDOR distance measurements monitored clearly the photo-induced structural changes of the
azobenzene unit. Thus, a powerful concept is presented resulting from the combination of these two complementary
www.rsc.org/
spectroscopic
techniques.
moieties is one of the first examples showing a combination of
two different molecular characteristics like photochromism
Introduction
2
9
and magnetic properties.
responsive spin systems were reported:
isomerization of a fullerene azobenzene derivate was studied
Since then, different light-
Light is the most powerful non-invasive tool to control the
structure and function of biological and non-biological
3
0–33
The photo-
1
–5
molecules. Introduction of small light-responsive molecules,
e.g. photoswitches, into macromolecules enables
3
1
by spin relaxation times and continuous wave (cw-EPR).
UV/vis and cw-EPR were used to characterize azobenzene
a
conformational transduction from the small organic molecule
3
0
6
–11 derivatives carrying one nitroxide group. Inducing not only
one, but two nitroxide groups enables following the structural
changes by pulsed electron-electron double resonance
to the larger systems like supramolecular self-assemblies.
The well-established photoswitch azobenzene can be
converted from the energetically favorable trans-isomer into
the cis-isomer upon photo-excitation with UV-light, leading to
3
4–36
spectroscopy (PELDOR, also called DEER
).
1
2
Here, we present a combined optical and EPR (cw-EPR and
PELDOR) study on the two azobenzene-based biradicals
strongly pronounced structural and electronic changes, while
the reverse reaction is enabled photo-chemically as well as
thermally. This thermal back rate is highly dependent on the
substitution pattern. Due to the high flexibility in the choice of
functional groups as substituents of azobenzene, a wide field
of application is obtained, which ranges from the
compound
1 and 2 (Scheme 1). These molecules allow a
modulation of the spin properties, as dipolar and exchange
coupling between the two paramagnetic centers by photo-
isomerization. The benefit of using PELDOR is clearly
1
3–16 demonstrated, allowing to monitor directly the distance
conformational control of biological systems, like peptides,
19,20
1
7,18
21
and RNA, to non-biological systems,
change in spin-bearing nitroxide moieties induced by
proteins,
like liquid crystals,
DNA
2
2,23
24,25
26
polymers,
metal-organic materials
2
7,28
and foldamers.
The attachment of two nitroxide spin-labels to the azobenzene
induces magnetic properties to the molecule. A trans-
azobenzene derivate with two attached nitronyl nitroxide
photoswitching.
Scheme 1: Chemical structures of two nitroxide-labeled azobenzenes, connecting the
nitroxide units to the azobenzene moiety via carboxylate groups (compound 1) or
ethynyl groups (compound 2).
Results and discussion
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The incorporation of photoswitches, e.g. the established is, with a value of about 47 nm, clearly shifted to higher
azobenzene, into macromolecules allows a conformational wavelengths with respect to compound (Figure 1). This is the
1
transfer from the photoswitch to larger molecules by photo- result of the increased delocalization of the electron density
isomerization of the two isomeric species. Good photo-
chemical properties are a prerequisite for the application of
azobenzenes as a trigger (part I). Besides a high switching
amplitude and efficiency, a slow thermal relaxation from the
energetically unfavorable cis- to the trans-isomer is essential
for applications, which require a sufficient thermal stability of
both isomers.
The photo-induced structural changes modulate the dipolar
coupling between the unpaired electrons, which can be used
to extract distance information by the PELDOR technique (part
II).
Due to their different conjugation properties, two different
groups - either carboxylates (compound
1) or ethynyls
(compound 2) - were chosen for the connection of the
nitroxide spin labels to the azobenzene moiety. While ethynyl due to the structural differences (Scheme 1).
groups can support the formation of spin exchange coupling, However, this shift is only apparent for the ππ*-band, but not
carboxylate groups are expected to suppress this interaction.
for the nπ*-band, which can be seen as a contribution at about
70 nm. Interestingly, the extinction coefficients of both the
4
I. Optical spectroscopy
Figure 1: UV/vis-absorption spectra of compound 1 (solid lines) and 2 (dashed lines) in
toluene as trans-isomer (black line) as well as after photo-excitation at 365 nm and
The photo-chemical characterization of compound
toluene was performed by UV/vis-absorbance experiments.
In the steady-state UV/vis-spectra of the trans-isomer of
compound
in toluene an intense highly structured ππ*- and the nπ*-band of compound
absorption band of the ππ*-band can be observed at 333 nm with respect to compound as well as pure azobenzene. Upon
with a weak absorption band at about 465 nm, which refers to excitation at 385 nm, a trans cis-isomerization occurs, which
1 and 2 in
3
85 nm, respectively (dark green). Pure cis-spectra were calculated from the
absorbance change upon excitation (light green).
1
2 are clearly increased
1
ꢀ
the nπ*-band (Figure 1). Excitation at 365 nm leads to a leads to an increase of the ππ*-band of the cis-isomer at about
conversion of the trans- to the cis-isomer, which can be seen in 290 nm as well as of the nπ*-band at 457 nm. The cis-amount
the decrease of the ππ*-band of the trans-isomer at 333 nm of the photo-stationary state (PSS385nm) is estimated to 67%.
and an increase of the nπ*-band of the cis-isomer at 438 nm. A The quantum yields of the isomerization of compound
clear isosbestic point can be detected at 387 nm. By twice as high as the values for compound (Table 1), but
extrapolation of a pure cis-spectrum of compound
, the cis- lower than for the unsubstituted azobenzene.
amount in the photo-stationary state (PSS365nm) can be
estimated to about 62%. Cis trans-isomerization is enabled
2 are
1
1
ꢀ
by excitation at 420 nm as well as by thermal relaxation of the
cis-isomer. However, a clearly lower quantum yield, especially
for the trans
isomerization directions of compound
unsubstituted azobenzenes (Table 1).
temperature, the thermal relaxation proceeds with a reaction
ꢀcis-isomerization, is obtained for both
1
with respect to
1
2,37
At
room
-6 -1
rate of 8.2·10 s (Figure 2). Furthermore, the temperature-
dependence of the thermal cis
ꢀ
trans-isomerization was Figure 2: Temperature-dependent thermal relaxation of the cis-isomers of compound 1
filled symbols) and 2 (open symbols) to the trans-isomers in toluene (left) and the
(
recorded (Figure 2). Based on Arrhenius’ law, the energy
barrier for the thermal relaxation in the ground state was
determined (87 kJ/mol), which is only slightly lower than for
corresponding Arrhenius-plots (right).
The thermal relaxation rate of the cis-isomer of
-4 -1
temperature is 1.7·10 s and thus clearly accelerated
2 at room
3
8
the pure azobenzene in toluene.
Under the assumption that the structured ππ*-band results
compared to
1, which is consistent with the lower energy
from the vibrational structure of the trans-isomer of
barrier between the ground states of the cis- and the trans-
isomer of 81 kJ/mol (Figure 2).
compound
1
, these modulations would refer to a frequency of
429 cm (see ESI Figure S3), which is in good agreement with
the totally symmetric N=N stretching vibration of the trans-
-1
1
Both compounds show a good fatigue resistance in toluene
(Figure 3). After each cycle, compound 1 exhibits a constant
3
9–41
isomer.
For compound
isomer can be seen in the steady-state UV/vis-spectrum, which
absorbance of the trans-isomer at 333 nm in 25 cycles of
2
, an unstructured ππ*-band of the trans-
excitation with alternating UV- and visible light. Less than
2
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0
.03% per cycle degrades, moreover, UV/vis-spectra after the hyperfine line splitting of a fast tumbling nitroxide radical a
th
first and the 25 cycle match completely.
Compound shows only a slight decrease during 25 cycles, The excitation at 365 nm leads to a slight broadening of the
which refers to a degradation of about 0.4% per cycle. spectral lines. This small, but visible change in the spectrum
Therefore, more than 50% of compound are present after reflects the conversion of the trans- to the cis-isomer. Since
80 cycles. No absorption of potential degradation products the reverse reaction (cis trans-isomerization) can be carried
resolved hyperfine coupling to hydrogens (Figure 4a).
2
2
1
ꢀ
between 300 and 600 nm are observed during the cycles, only out by photo-excitation too, illumination of the PSS365nm with
a UV/vis-absorbance decrease in the spectrum of the 420 nm yields the same spectrum as before any excitation.
azobenzene derivative.
This is best seen in the difference spectra (Figure 4b). The
experimental spectra are simulated assuming an isotropic
rotational correlation time of 80 ps for the trans-isomer and
Table 1: Spectroscopic properties of compounds 1 and 2. Quantum yields for the
transꢀcis- and the cisꢀtrans-isomerization, the reaction rate, half-life and energy
barrier for the thermal relaxation from the cis- to the trans-isomer.
4
2
3
00 ps for the cis-isomer using the EasySpin toolbox (see ESI).
On the contrary, compound exhibits more complex spectra
with a typical signature of an exchange coupled biradical
2
1
1.6 ± 0.4
14 ± 3
2
4
3–47
QYππ* (transꢀcis)
3.1 ± 0.7
31 ± 15
(Figure 4c).
A transꢀcis-isomerization is achieved by
1
)
QYnπ* (cisꢀtrans)
excitation with 385 nm and causes a change in the cw-EPR
spectrum. The conformational change of the azobenzene
moiety from trans to cis leads to an attenuation of the
exchange coupling, because it is very sensitive to the extent of
-
1
-6
-4
k
c
t, RT [s ]
(8.2 ± 1.1)·10
23 ± 4
(1.7 ± 0.2)·10
1.13 ± 0.16
81 ± 3
ꢀ
2
)
t
1/2, RT [h]
E
A
[kJ/mol]
87 ± 9
4
8
orbital overlap in the mediating azobenzene system.
1
)
Based on the assumption that the product absorbance is negligible at the
Accordingly, the experimental spectra can be simulated with a
rotational correlation time of 100 ps and an exchange coupling
of 19.3 MHz for the trans-isomer and 16.0 MHz for the cis-
excitation wavelength. Product absorbance is considered in the corresponding
uncertainty analysis.
2
)
Obtained by extrapolation of the Arrhenius-plot.
Figure 3: Left: Fatigue resistance of the compounds 1 (squares) and 2 (circles) in
toluene. Normalized absorbance is shown after excitation with UV-light (dark green)
and visible light (black). Right: UV/vis-spectra of the respective trans-isomers after
cycle 1 (black) and cycle 25 (orange).
isomer (see ESI). After irradiating the PSS385nm with 455 nm,
the spectrum resembles again the spectrum before excitation.
Figure 4: Area-normalized cw-EPR spectra at room temperature in deuterated toluene
solution. a) Comparison of the spectra before and after excitation with 365 nm
Consequently, compounds
1 and 2 show good photo-chemical
behavior in toluene, which enables a photo-control of (PSS365nm) and 420 nm (PSS420nm) of compound 1 and b) the corresponding difference
azobenzene-isomerization for a sufficiently long time. These spectra between the spectra before and after (PSS365nm (black) and PSS420nm (red),
respectively) excitation. c) Comparison of the spectra before and after excitation with
properties would allow an incorporation in a larger system and
3
85 nm (PSS385nm) and 455 nm (PSS455nm) of compound 2 and d) the corresponding
difference spectra between the spectra before and after (PSS385nm (black) and PSS455nm
red), respectively) excitation.
a precise conformational control.
(
II. EPR spectroscopy
EPR spectroscopy is a valuable tool to complement the photo-
chemical characterization by monitoring the spin states of the
two unpaired electron spins of the biradical. The two attached
nitroxide labels act as probes to follow structural changes
caused by photo-isomerization. Whereas cw-EPR is able to
detect altered mobility and exchange coupling, PELDOR is used
to measure the different interspin distances. Therefore, cw-
EPR and PELDOR measurements were performed before and
after photo-excitation.
PELDOR experiments A conformational change during the
trans cis-isomerization leads to a different distance between
ꢀ
the two attached spin-labels. The PELDOR experiments were
performed at 50 K, which prevents the thermal relaxation from
the cis- to the trans-isomer. Therefore, the obtained distance
distributions reflect the states purely obtained by light
excitation. Figure 5a) shows the background corrected PELDOR
time traces together with the Fourier transformed dipolar
spectra and the distance distributions obtained by Tikhonov
regularization for compound 1. The distance of 2.2 nm
cw-EPR spectroscopy The room temperature cw-EPR spectra
measured for the trans-isomer agrees very well with the
of compound
1 show in addition to the classical three
distance of 2.24 nm calculated from the X-ray structure for this
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Figure 5:
4
9
isomer. The trans
65 nm leads to a damping of the oscillations in the PELDOR exchange couplings from the cw-EPR spectra simulations for
signal and therefore a broadening in the dipolar spectrum. A both cis- and trans-isomer allows to predict the Pake pattern
shorter distance around 1.6 nm becomes visible, which can be for ferromagnetic and antiferromagnetic exchange
assigned to the cis-isomer. The trans cis-isomerization is not interaction. The simulations shown in the Supporting
ꢀcis-isomerization by excitation with couplings from the MD distance distributions and the
3
a
ꢀ
complete in the PSS365nm and the distance of the trans-isomer Information indicate a ferromagnetic coupling (Figure S7).
at 2.2 nm is still visible in the distance distribution, which is in
agreement with an incomplete isomerization as seen in the
EXPERIMENTAL
optical absorption measurements. Since the short distance for
the cis-isomer is at the lower limit of the PELDOR method, a
Synthesis
quantitative determination of this distance and consequently
Chemical synthesis was performed under argon atmosphere in
the ratio between the two isomers is not possible. However,
dried and degassed solvents, employing standard Schlenk
compound
1 in PSS365nm can be switched back to the trans-
techniques. TEMPOL, azobenzene-4,4’-dicarbonyl dichloride,
isomer after irradiation with 420 nm, which is reflected both in
the dipolar spectrum and distance distribution.
4
-iodoaniline,
sodiumperborate
tetrahydrate
PdCl
and
dichlorobis(triphenylphosphane)palladium ((PPh
3
)
2
2
) were
The PELDOR time traces for compound
2 are shown in 5b).
commercial materials and used as received. Pyridine was pre-
dried over KOH and distilled from CaH . Diethylamine was
dried with CaH and distilled. TPA (2,2,5,5-tetramethyl-3-
pyrroline-1-oxyl-3-acetylene) was synthesized as described
Here, the direct conversion to a distance distribution is not
possible, since not only the dipolar, but also the exchange
coupling contributes to the time trace.
2
2
Nevertheless, a clear change can be seen by excitation with
5
3
earlier.
,4’-Diiodo-azobenzene was obtained by mixing 3 g
15.07 mmol) 4-iodoaniline and 3.065 g (19.92 mmol)
385 nm in the time trace and in the frequency domain.
4
Furthermore, after irradiation with 455 nm the measured
signal indicates a successful switching back to the trans-
isomer. In favorable cases where the two halves of the Pake
pattern in the frequency domain are separated enough, the
exchange and the dipolar coupling constants can be separately
(
sodiumperborate tetrahydrate in glacial acetic acid. The
reaction mixture was stirred for 30 minutes at room
temperature and then heated to 50° C for 3 hours. After
cooling to room temperature, the reaction mixture was
filtrated and the solid residue washed with water and dried
over KOH. 4,4’-diiodo-azobenzene was obtained as orange
solid in a yield of 1.585 g (3.65 mmol, 48%).
5
0–52
determined from the peaks and edges.
In the case of
compound 2, they are not easily detectable, probably because
of a distribution of not only the dipolar coupling, but also of
the exchange coupling. Nevertheless, calculation of the dipolar
4
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Compound
ARTICLE
1
was synthesized by addition of azobenzene-4,4’- were geometrically positioned and treated as riding atoms.
2
dicarbonyl dichloride to TEMPOL. Under an argon atmosphere Structure refinement on F values was performed using 2399
2
to a solution of 0.352 g (1.97 mmol) TEMPOL in 15 mL of dry reflections and 177 parameters. Final wR(F ) was 0.100 and
pyridine 0.308 g (1 mmol) azobenzene-4,4’-dicarbonyl R(F) was 0.059 (for I > 2σ(I)). Largest residual density was
3
dichloride was added. The resulting red solution was stirred at between -0.19 and +0.55 e Å . Crystal data for C32
H
34
N
4
O
2
: M
r
room temperature overnight and afterwards transferred to an = 506.63, triclinic, space group P -1 (Nr. 2), a = 5.9834(8),
Erlenmeyer flask containing ice/water. HCl (conc.) was added b = 6.4373(8), c = 18.610(2) Å , = 89.153(2), = 87.364(2),
= 78.992(2)°, V = 702.85(16) Å , Z = 1, dcalc = 1.197 Mg m ,
α
β
3
-3
dropwise until a pH of 2 was reached and the mixture was
γ
-1
extracted with dichloromethane until the organic phase stayed absorption coefficient 0.076 mm , F(000) = 270, T = 171(2) K,
θ
colourless (6 times). The combined organic phases were range 2.19-25.15°, index ranges -6 ≤ h ≤ 7, -7 ≤ k ≤ 7,
-
washed with water, 1 M NH
over Na SO the solvent was evaporated and the crude reflexes (Rint = 0.058), largest difference peak and hole 0.551
product was recrystallized from ethanol affording 0.160 g and -0.185 e Å , goodness of fit on F 0.939, R index (all data)
0.28 mmol, 28 %) of pure as an orange powder, suitable for wR2 = 0.1179, R index conventional (I > 2σ(I)) R1 = 0.0591.
X-ray powder diffraction. Elemental analysis calculated for CCDC-851945 contains the supplementary crystallographic
C H N O (578.71) is C 66.39, H 7.31, N 9.72; experimental data of compound . These data can be obtained via
www.ccdc.cam.ac.uk/data_request/cif.
4
Cl (aq) and brine. After drying 22 ≤ l ≤ 22, 4303 reflections collected, 2400 independent
2
4
3
2
(
1
2
3
2 42 4 6
found is C 66.17, H 7.42, N 9.47.
For the synthesis of compound
2 via Sonogashira cross-
coupling reaction 0.396 g (0.91 mmol) 4,4’-diiodoazobenzene, Steady-state absorption measurements
0
.137 g (0.19 mmol) (PPh ) PdCl and 0.041 g (0.22 mmol) CuI
3 2 2
UV/vis-absorption spectra were recorded in a 10x10 mm
were dissolved in diethylamine and repeatedly evacuated and
loaded with argon. A mixture of 0.300 g (1.83 mmol) TPA and
quartz glass cuvette using a Specord S600 UV/vis spectrometer
-5
Analytik Jena AG) (concentrations in Figure 1: 2.8·10 M for
(
0.100 g (3.8 mmol) PPh3 in diethylamine was added under
-5
1 and 1.7·10 M for compound 2). Measurements
compound
argon. The mixture was heated for 4 hours. After cooling to
room temperature volatile compounds were removed under
reduced pressure. The residue was dissolved in
dichloromethane and washed several times with water and
finally brine. After drying with Na SO the solvent was
on the temperature-dependent thermal relaxation of the cis-
isomer were performed by using a Peltier cooled cell holder
with external heat exchanging (concentrations in Figure 2:
-5
-5
2.6·10 M for compound 1 and 2.2·10 M for compound 2).
2
4
Investigations on the fatigue resistance of azobenzenes were
performed by cyclic measurements with simultaneous
removed in vacuo and the remaining solid was purified by
means of column chromatography using mixture
CH Cl :EtOAc (20:1 v/v) as the eluent. The crude product was
a
illumination with alternating LEDs in the UV- and the visible
-5
range (concentrations in Figure 3: 2.9·10 M for compound
2
2
1
recrystallized from toluene and afforded pure
2
as orange
-5
and 1.0·10 M for compound 2). The illumination time of the
crystals with were suitable for X-ray diffraction. Elemental
analysis calculated for C H N O (506.65) is C 77.86, H 6.76, N
two LEDs was chosen to nearly achieve a photo-stationary
state. Quantum yields were obtained by determining the
3
2 34 4 2
11.06; experimentally found were C 75.74, H 6.94, N 10.95.
absorbance change of compound
1 and 2 upon excitation at a
defined wavelength with a known excitation power and
sample volume (see ESI).
Illumination procedure
A Thorlabs LED Driver DC4100 and a DC4100-HUB illuminated
the samples. LEDs at 365 nm and 385 nm induced the
trans
→
cis photo-isomerization and LEDs at 420 nm and
55 nm the cis trans photo-isomerization for compound
, respectively (Thorlabs M365L2, M385L2, M420L3 and
4
→
1
and
2
M455L3).
Scheme 2: Synthesis of the nitroxide-labeled azobenzene compounds 1 and 2.
cw-EPR spectroscopy
Field-swept cw-EPR spectra of both compounds in degassed
toluene-D
8
solution (100 µM) at X-band (9.4 GHz) were
X-ray crystallography
measured on a Bruker ELEXSYS E500 spectrometer at room
temperature with a microwave power of 2.00 mW. The
modulation amplitude was set to 0.01 mT. Photo-isomerization
was achieved by illuminating the samples between the
measurements. Simulations of the experimental spectra were
performed on the basis of the EasySpin toolbox for the Matlab
X-ray structural information on compound
1
has already been
4
published. Orange rod-shaped crystals of compound were
9
2
obtained by recrystallization from toluene. A single crystal was
measured on a Siemens SMART 1K CCD diffractometer at
α
171 K with Mo-K radiation and a graphite monochromator.
The structure was determined by direct methods; H atoms
42
program package (see ESI).
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Furthermore, the photo-reversibility of the isomerization can
be proven by cw-EPR.
PELDOR experiments
Additionally, PELDOR sensitively records interspin distance
changes. Since these experiments are performed at low
temperatures (50 K), the thermal relaxation is negligible. For
Experiments were recorded in toluene-D
8
solution with
1
00 µM sample concentration at X-band (9.4 GHz) and 50 K on
Bruker ELEXSYS E580 spectrometer equipped with
a
a
compound 1, the measured distances of 2.2 nm for the trans-
continuous-flow helium cryostat (CF935, Oxford Instruments)
using a 3 mm split-ring resonator (EN 4118X-MS3, Bruker).
Microwave pulses were amplified by a 1 kW TWT amplifier
isomer and a shorter distance around 1.6 nm for the cis-isomer
are in agreement with the predicted distances based on the X-
ray structure and MD-simulations. Although the inherent spin
exchange coupling prevents a distance determination for
3
6
(ASE 117X). The dead-time free four-pulse PELDOR sequence
was used for all experiments with pulse lengths of 16 ns for
the probe pulses (π/2 and π) and 12 ns for the pump pulse (π).
The pump pulse frequency was chosen to be resonant with the
center of the nitroxide powder spectrum to obtain maximum
pumping efficiency and the probe frequency was set 70 MHz
above this frequency. For illumination, the samples were
thawed, illuminated and frozen again in liquid nitrogen before
inserting into the spectrometer. The freezing time is on a much
shorter timescale than the thermal relaxation, the latter can
be therefore neglected.
compound 2, not only the reversibility of the photo-
isomerization, but also a putative ferromagnetic coupling
mechanism of the spin exchange through the conjugated
system is shown.
Consequently, we demonstrated the - to the best of our
knowledge - first molecular systems, which allow a photo-
responsive distance determination by the PELDOR technique.
Acknowledgements
Molecular Dynamic Simulations
We thank Dmitry Akhmetzyanov for helpful discussions and Dr.
Ivan Krstić and Dr. Heike Staudt for preliminary work.
Generous support by the German Research Foundation (SFB
The MD simulations based on the force field MM2 were
4,55
carried out using Chem3D version 16.
5
The parameters used
9
02 “Molecular Principles of RNA-based Regulation” and DFG
were 2 fs step interval, 10 fs frame interval, 20,000 iterations,
a heating/cooling rate of 1 kcal/atom/ps, and a target
temperature of 300 K.
via WA 1850/4-2) as well as the International Max Planck
Research School (IMPRS) of the MPI of Biophysics in Frankfurt
is gratefully acknowledged.
Conclusions
Notes and references
In this work, the integrated approach combining two
spectroscopic methods allows a detailed description of
photoswitchable and spin-labeled azobenzene compounds.
The concept of photoswitches is a powerful tool for the spatio-
temporal control of conformations or even reactions. The
combination of an azobenzene, as an optical trigger, and
nitroxide spin labels enables a photo-responsive determination
of molecular distances by using PELDOR.
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