Photochromism of diarylethene-functionalized 7-deazaguanosines

Article abstract of DOI:10.1002/ejoc.201300261

In a prior report we introduced a novel class of photochromic nucleosides (PCNs) that combine the structural features of adenine with the photochromic properties of diarylethenes. Herein, we translated this concept to the nucleoside guanosine, generating

Full text of DOI:10.1002/ejoc.201300261

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201300261
Photochromism of Diarylethene-Functionalized 7-Deazaguanosines
Marco Singer,[a] Alexander Nierth,[a][‡] and Andres Jäschke*[a]
Keywords: Photochromism / Nucleosides / Biosensors / Cyclization
In a prior report we introduced a novel class of photochromic
nucleosides (PCNs) that combine the structural features of
adenine with the photochromic properties of diarylethenes.
Herein, we translated this concept to the nucleoside
guanosine, generating reversibly switching guanosine-like
PCNs. These switches consist of a 7-deazaguanosine unit
and a second aryl functionality, which are linked through a
cyclopentene unit. Irradiation of the open-form isomer with
light at 300 nm induces a pericyclic reaction that can be re-
versed with visible light. In addition to optical stimulation,
these switches respond to light-independent stimuli, such as
the presence of acid or complexation with metal ions.
switches with distinct optical properties for the generation
of multimodal systems with two or more independently
Introduction
The incorporation of light-responsive modules into bio- switchable units.
polymers has become a powerful strategy for the investiga-
We addressed these issues by designing novel photochro-
tion and control of biological processes in vitro and in liv- mic compounds that mimic natural nucleosides.[9] These
ing cells. Distinct alterations of the chemical and photo- photochromic nucleosides (PCNs) were structurally related
physical properties of the photosensitive moiety can be trig- to adenosine. We continued to investigate the generality of
gered at the molecular level in a spatially and temporally this concept, with the goal of preparing PCNs of the other
resolved fashion by optical, and thus noninvasive, stimula- natural nucleosides. Herein, we present reversibly switching
tion.[1] In the field of nucleic acids the photocontrol of photochromic guanosine derivatives 1a and 1b that are
DNA or RNA functions, such as transcription,[2] gene ex- based on a diarylethene scaffold (Scheme 1). The purine
pression,[3] aptamer folding[4] or catalytic activity[5] has been moiety of guanosine was derivatized with a cyclopentene-
demonstrated with photolabile protecting groups. However, thiophene unit to form a functional diarylethene. These
the inherent limitation of this approach is irreversible cleav- compounds undergo a reversible pericyclic reaction, gener-
age of the “caged nucleosides”. In contrast, bistable photo- ating closed and opened isomers by irradiation with light of
chromic compounds such as azobenzenes and spiropyranes different wavelengths. Furthermore, they provide hydrogen
can be switched reversibly and, with their incorporation donor and acceptor positions for Watson–Crick base pair-
into nucleic acids, reversible modulation of very basic func- ing, as well as key structural features of nucleosides like
tions such as duplex[6] and triplex[7] stability have been dem- ribose and purine moieties. Apart from the diarylethene-
onstrated. However, these photochromic subunits do not specific photoisomerization, these switches respond to light-
share most structural motifs of the canonical nucleotides. independent stimuli, such as the presence of acid or com-
Their application in highly structured nucleic acids, such as plexation with metal ions.
(desoxy)ribozymes, is constricted because their non-nucleo-
sidic nature can lead to detrimental effects on the correct
folding required for activity.[8] Furthermore, it would be de-
sirable – from a spectroscopic point of view – to generate
[a] Institute of Pharmacy and Molecular Biotechnology (IPMB),
Heidelberg University,
Im Neuenheimer Feld 364, 69120 Heidelberg, Germany
Fax: +49-6221-54-6430
E-mail: jaeschke@uni-hd.de
Homepage: http://www.jaeschke.uni-hd.de
[‡] Current address: The Scripps Research Institute (TSRI),
Department of Chemistry, Beckman Center for Chemical
Sciences,
10550 N. Torrey Pines Road, La Jolla, CA 92037, USA
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300261.
Scheme 1. Photoisomerization of diarylethene-functionalized
guanosine derivatives 1a and 1b.
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Eur. J. Org. Chem. 2013, 2766–2769

Photochromism of Diarylethene-Functionalized 7-Deazaguanosines
The photophysical and photochemical properties of di- spectrum in the visible range. The photoisomerization reac-
arylethene switches are known to depend strongly on the tion was monitored by UV/Vis and HPLC analysis (see the
electronic properties of the associated heteroaryl units.[10] Supporting Information).
As a consequence, substitutions at the thiophene moiety
Upon irradiation with UV-light, the initially colorless
that increase or decrease the electron density allow the spec- solutions of compounds 1a and 1b display a strong color-
tral properties of the resulting diarylethenes to be con- ation (Figure 1), which is characteristic for diarylethenes
trolled. With appropriate functionalization of these that undergo ring closure. The color rapidly fades upon ir-
switches, this feature can be used for reversible in situ alter- radiation with visible light – due to cycloreversion – demon-
ation of spectral properties, e.g., by protonation or com- strating that both compounds are fully functional diaryleth-
plexation, thus allowing these switches to be addressed in enes. Figure 2 (a and b) show the evolution of absorbance
a light-independent manner. Here, we demonstrate spectral spectra during photoisomerization. In both cases, irradia-
tuning through the introduction of a pyridyl substituent, tion with UV-light triggers the emergence of a broad ab-
as well as acidichromism (i.e., pH-dependent switching) of sorption band in the visible range, with 1a having its maxi-
diarylethene-functionalized nucleosides. Finally, we report mum at 505 nm, whereas the maximum of 1b is located at
the effect of metal ion complexation on the spectral proper- 536 nm. The difference in their spectral properties is attrib-
ties of such switches.
uted to the thiophene substituents, which alter the elec-
tronic properties of the switches. The bathochromic shift of
1b is due to a relative decrease of electron density in the
thiophene moiety caused by the electron-deficient pyridyl
substituent, as compared with the electron-rich phenyl sub-
stituent of 1a. This illustrates the possibility of fine-tuning
the spectral profile of the switches by changing the elec-
tronic properties of the heteroaryl moieties.
Results and Discussion
The general synthetic approach to the guanosine-like
PCNs 1a and 1b is depicted in Scheme 2. The basic scaffold
of diarylethenes requires two – usually identical – hetero-
aryl units that are linked through a 1,2-cyclopentenyl linker
to form the photoreactive hexatriene core. To obtain photo-
chromic nucleosides, we took advantage of the variability of
these heteroaryl units and introduced a novel glycosylated
deazapurine derivative 2, which is structurally related to the
natural nucleoside guanosine. This fragment was linked to
cyclopentene-thiophene conjugates 3a and 3b to resemble
the photochromic structure. The eight-step synthesis started
from 2,6-diaminopyrimidin-4-one and led to title com-
pounds 1a and 1b. A detailed reaction scheme (Scheme S1)
and the experimental procedures can be found in the Sup-
porting Information.
Figure 1. Solutions of 1a (left cuvette) and 1b (right cuvette) in
acetonitrile after irradiation with UV-light.
Comparison of these data with our previous results on
adenosine-like PCNs reveal that switches with identical
thiophene substituents but different deazapurines are also
subjected to a shift in their respective absorbance maxima.
In conjunction with the phenylthiophene partner, exchange
of the deazaadenosine by deazaguanosine leads to a ba-
thochromic shift of 20 nm, whereas the pyridylthiophene-
bearing PCNs display a difference of 31 nm.[9]
Upon irradiation of switches 1a and 1b with UV-light,
the kinetic plots presented in Figure 2 (a and b) reach pla-
teaus that reflect the photostationary states (PSS) of both
compounds. The ratio between opened and closed form was
investigated by HPLC, detecting both isomers at their
isosbestic points for quantification (see the Supporting In-
formation). After irradiation at 300 nm, the chromatograms
of 1a and 1b show the formation of a novel peak with spec-
Scheme 2. Retrosynthetic approach to guanosine-like PCNs.
The photochromic properties of compounds 1a and 1b tral properties that are identical to those of the closed-ring
were assessed in acetonitrile solutions at a concentration of species. Ring closure at the PSS is achieved to 86 and 81%
30 μm.[11] For ring closure, irradiation was performed with for 1a and 1b, respectively (see the Supporting Infor-
a 100 W Xenon lamp equipped with a monochromator at mation). Exposure of the cyclized PCNs to visible light ef-
300 nm or with a hand-held UV-lamp at 366 nm. The cyclo- fectively induced cycloreversion, depicted in the kinetic plot
reversion was triggered by a LED lamp emitting a broad by a rapid decrease of the absorbance at the maximum in
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M. Singer, A. Nierth, A. Jäschke
SHORT COMMUNICATION
Figure 2. Absorption spectra evolution and time course of photoisomerization of compounds 1a (a) and 1b (b). Arrows indicate changes
in the absorbance upon irradiation with UV-light (300 nm). Cycling between the opened and closed form (c, d) was performed by
alternating illumination with 366 nm (UV-handheld lamp) and visible light. The decline in absorbance of 1a (c) and, to a lesser extent,
in 1b (d) is attributed to degradation during the switching process.
the visible range. This demonstrates that the photoisomer- or complexed with an appropriate metal ion. Figure 3
ization is reversible and can be repeated several times by shows the impact of protonation by trifluoroacetic acid
altering the illumination wavelength. Monitoring several cy- (TFA) and complexation with Cu2+ on the spectral proper-
cles by UV/Vis spectrophotometry, however, reveals a con- ties of the closed isomer. In both cases, the absorption
tinuous decrease in the maximal absorbance intensity of the maximum is significantly redshifted, with the copper-in-
closed state (Figure 2, c and d), indicating that during these duced shift being more pronounced (165 nm) than that for
cycles the guanosine-like PCNs slowly degrade. Because the protonation by TFA (89 nm). However, both protonation
yellow colored by-product(s) formed in that process are not and complexation reduce the thermostability of the closed
photochromic, the switching behavior is gradually lost. The form, causing it to relax into its opened form at ambient
photodegradation was not analyzed further. However, temperature. Again, this property could be exploited to or-
based on previous studies, it can be assumed that a stable thogonally address several PCNs.
conjugated, possibly aromatic compound, is generated that
does not react back to the open form.[12] Notably, by-prod-
uct formation could not be suppressed by oxygen depletion
and the stability of the PCNs was dependent on the thio-
phene as well as the deazapurine hetereoaryl component.
Direct comparison with adenosine-like PCNs shows that
the deazaadenosine moiety confers a much higher photo-
stability to the resulting PCNs, irrespective of the thiophene
unit.[9]
In addition to light, the cycloreversion of diarylethenes
can be triggered thermally. We therefore tested the thermal
stability of closed-ring isomers and monitored the ab-
sorbance changes at different temperatures (Figure S4). The
closed isomers of both switches are stable at 20 °C for
hours, whereas quantitative light-independent isomerization
occurs at 60 °C within 20 min (see the Supporting Infor-
mation). Even though thermal relaxation is generally an un-
Figure 3. Bathochromic shift of the absorbance maximum of closed
1b in acetonitrile (30 μm) upon addition of TFA or CuSO4.
desired feature of diarylethenes, it could be beneficial in the
case of PCNs because a set of switches with different ther-
mal stabilities could be addressed in an orthogonal light-
independent way, simply by changing the temperature.
Conclusions
The photochromic and photophysical properties of
We have developed photochromic nucleosides based on
PCNs strongly depend on the heteroaryl moieties, as dem- glycosylated deazaguanine in combination with substituted
onstrated here and in our earlier report.[9] Alteration of the thiophenes as heteroarylic subunits. These compounds form
electronic properties of the thiophene unit in situ would switches that undergo diarylethene-specific light-induced
provide a unique additional feature because this would al- cyclization and cycloreversion reactions. Overall, their pho-
low these switches to be addressed with alternative stimuli tochromic properties are defined by the electronic features
in addition to light or thermal energy. The pyridyl substitu- of their heteroaryl subunits. We have shown that this can
ent of compound 1b is an example of a multi-addressable be exploited to generate switches with distinct spectral
switch because it can be protonated in the presence of acid properties by changing the thiophene subunit. Alternatively,
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Photochromism of Diarylethene-Functionalized 7-Deazaguanosines
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Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures, chemical synthesis (Scheme S1),
HPLC analysis of purity and photostationary states (Figures S1
and S2), thermal relaxation of closed isomers (Figure S3), and
NMR spectra.
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Acknowledgments
This work was supported by the Deutsche Forschungsgemeinschaft
(DFG) (Ja 794/3). M. S. acknowledges a LGFG fellowship from
the Federate State of Baden-Württemberg. A. N. acknowledges a
research fellowship from the DFG (NI 1341/1-1). The authors
thank Heiko Rudy and Tobias Timmermann for technical support.
[10] a) H. Tian, S. J. Yang, Chem. Soc. Rev. 2004, 33, 85–97; b) M.
Irie, Chem. Rev. 2000, 100, 1685–1716.
[11] Spectroscopic analysis of these compounds was performed in
acetonitrile due to their poor solubility in water. Acetonitrile
is the most polar reference solvent and commonly used to study
photoisomerization of diarylethenes.
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Received: February 19, 2013
Published Online: April 15, 2013
Eur. J. Org. Chem. 2013, 2766–2769
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