Synthesis and reversible photoisomerization of photoswitchable nucleoside, 8-styryl-2′-deoxyguanosine

Article abstract of DOI:10.1016/j.tetlet.2008.01.124

A novel guanosine derivative with a photoswitching property, 8-styryl-2′-deoxyguanosine was synthesized, and showed very rapid and efficient reversible E-Z photoisomerization upon illumination at specific wavelength. In addition, E-Z photoisomerization can be iteratively performed by alternate illumination with monochroic 254 nm and 370 nm light without any side reactions.

Full text of DOI:10.1016/j.tetlet.2008.01.124

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Tetrahedron Letters 49 (2008) 2479–2482
Synthesis and reversible photoisomerization of
photoswitchable nucleoside, 8-styryl-20-deoxyguanosine
a,
b
a
*
Shinzi Ogasawara , Isao Saito , Mizuo Maeda
a Bioengineering Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
b School of Engineering, NEWCAT Institute, Nihon University, Tamura, Koriyama, Fukushima 963-8642, Japan
Received 29 November 2007; revised 21 January 2008; accepted 22 January 2008
Available online 1 February 2008
Abstract
A novel guanosine derivative with a photoswitching property, 8-styryl-20-deoxyguanosine was synthesized, and showed very rapid
and efficient reversible E–Z photoisomerization upon illumination at specific wavelength. In addition, E–Z photoisomerization can
be iteratively performed by alternate illumination with monochroic 254 nm and 370 nm light without any side reactions.
Ó 2008 Elsevier Ltd. All rights reserved.
In the past decade there has been great interest in photo-
chromic compounds including stilbene1 and azobenzene2
derivatives, which reversibly change the photochemical and
physical properties upon E–Z photoisomerization by the
external light stimuli. This photoswitching property has
found numerous applications in bionanotechnology
because light-triggered methods allow accurate and easy
control of the location, dose, and time when an event
occurs. For example, azobenzene derivatives have been
incorporated into peptide,3 protein,4 double-stranded
DNA,5 single-stranded RNA,6 RNAzyme,7 and DNA-
zyme8 to control activities by light illumination. Although
such photoactivation systems have shown reversibility, the
active/inactive ratio was not sufficiently high, due to the
installation of the bulky azobenzene chromophore which
may influence the active-state conformation. Therefore, if
the photoisomerization function was directly installed to
a biomolecule instead of attaching a photochromic com-
pound, it would lead to a more powerful photoswitching
method. Majima and co-workers have reported
(deoxy)ribofuranosyl 2-phenylazoimidazole and observed
that these compounds undertake E–Z photoisomerization.9
We describe herein the synthesis and photochemical pro-
perties of a novel guanosine derivative, 8-styryl-20-deoxy-
guanosine (8STG), which reversibly photoisomerized by
illuminating at specific wavelength (Fig. 1). Compared with
azobenzene derivatives, stilbene derivatives have several
advantages such as thermal stability and fluorescence gen-
eration. Such photoswitchable guanine base would be very
useful, since guanine base is concerned with unique confor-
mation of nucleic acid including Z-form,10 G-quartet,11
and quadruplex found in triplet repeat sequence.12
The E-isomer 8STG(E) (2) and the Z-isomer 8STG(Z) (4)
were prepared individually from 8-bromo-20-deoxyguazno-
sine (1),13 as described in Scheme 1. For 8STG(E), Suzuki–
Miyaura cross-coupling of (E)-2-phenylvinylboronic acid
and 1 was performed in DMF at 110 °C for 16 h, by using
tetrakis(triphenylphosphine)palladium(0) as the catalyst.
After purification by column chromatography on silica
*
Fig. 1. Schematic illustration of E–Z photoisomerization of the 8-styryl-
20-deoxyguanosine (8STG).
Corresponding author. Tel.: +81 48 467 9312; fax: +81 48 462 4658.
E-mail address: o_shinji@riken.jp (S. Ogasawara).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.01.124

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S. Ogasawara et al. / Tetrahedron Letters 49 (2008) 2479–2482
Fig. 2. Wavelength-dependence of the E-isomer ratio in the photo-
stationary state (PSS) as determined by the HPLC profiles.
Based on the above results, we demonstrated the rever-
sible photoisomerization of 8STG using monochroic 370 nm
and 254 nm light for E?Z and Z?E photoisomerization,
respectively.18 Illumination of 8STG(E) at 370 nm using a
100 W Xenon lamp generated 8STG(Z), which led to a
rapid decrease in the absorption at kmax = 340 nm and an
increase in the absorption at 249 nm (Fig. 3a). In this reac-
tion, the PSS was achieved within 5 s at 96% conversion, as
determined by the peak area in HPLC analysis (Fig. 3c). In
contrast, when 8STG(Z) was illuminated at 254 nm, the
intensity of the peak at kmax = 249 nm decreased, while
the intensity of the peak at 340 nm increased, indicating
Z?E photoisomerization (Fig. 3b). This reaction reached
the PSS within 40 s and had 67% conversion (Fig. 3d).
Conversion for E?Z and Z?E photoisomerization at
the PSS correspond to the ratio of absorbance coefficient
at 370 nm and 254 nm, respectively.
Scheme 1. Synthesis of 8-styryl-20-deoxyguanosine. Reagents and
conditions: (a) (E)-2-phenylvinylboronic acid (1.2 equiv), Pd(PPh3)4
(0.05 equiv), Et3N (4.0 equiv), 110 °C, DMF, 64%; (b) ethynylbenzene
(1.2 equiv), Pd(PPh3)4 (0.025 equiv), CuI (0.05 equiv), Et3N (3.0 equiv),
110 °C, DMF, 59%; (c) 5% Pd/C, H2, EtOH, 40%.
gel, 2 was obtained in 64% yield.14 Sonogashira cross-
coupling reaction of 1 and ethynylbenzene was employed
to give 3. Hydrogenation of 3 on 5% Pd/C at room temper-
ature for 16 h gave 4 in 40% yield.15 Both purified 8STG(E)
and 8STG(Z) were characterized by different spectroscopic
measurements, including FAB mass spectrometry and
NMR experiments. It is known that substitution at C8 of
20-deoxyguanosine causes an anti to syn conformational
change with respect to the relevant N9–C10 glycosidic
bond, which can be confirmed by NMR spectroscopy.
An anti to syn conformational change results in a downfield
shift of the C10, C30, C40, and H20 signals, but an upfield
shift of the C20 signals as compared to that of 20-deoxygua-
nosine.16 Such shifts were not observed for 8STG(E), while
downfield shift of the C10and H20, and upfield shift of the
C20 signals were observed for 8STG(Z). Furthermore,
NOE between 2-amino proton and the 50-hydroxy proton,
and H10 and vinyl proton have been observed only in
8STG(Z). These results indicate that 8STG(Z) prefers syn
conformation relative to 8STG(E).
The photoisomerization of 8STG was examined in
aqueous solution containing 2.0% acetonitrile.17 The wave-
length dependence of the E:Z ratio in the photostationary
state (PSS) was investigated within the range of 240 nm to
390 nm at 10 nm intervals by illuminating with a mono-
chroic light. The PSS was obtained after 2 h irradiation.
As seen in Figure 2, the photostationary equilibrium tends
to be E < Z by illumination at more than 300 nm, whereas
illumination at below 300 nm tends to be E > Z. The most
lopsided E:Z ratio was 6:94 at 370 nm and 80:20 at 250 nm.
These results indicate that highly reversible E–Z photos-
witching of 8STG can be realized by alternate illumination
with 250 nm and 370 nm light.
Fig. 3. (a) Absorption spectra for photoisomerization of 8STG(E) with
illumination at 370 nm, and (b) photoisomerization of 8STG(Z) with
illumination at 254 nm. Arrows indicate increase and decrease of the
absorption peaks. (c) Time course for photoisomerization of 8STG(E) and
(d) 8STG(Z).

S. Ogasawara et al. / Tetrahedron Letters 49 (2008) 2479–2482
2481
Table 1
Photophysical propertiesa
this article can be found in the online version, at
doi:10.1016/j.tetlet.2008.01.124.
Entry
Uiso
kabs (nm)
kem (nm)
8STG(E)
8STG(Z)
a
0.35
0.15
340
249
450
450
References and notes
1. (a) Meier, H. Angew. Chem., Int. Ed. 2001, 40, 1851–1853; (b) Guaten,
E.; Klasinc, L. Tetrahedron Lett. 1968, 26, 3097–3101; (c) Gegiou, D.;
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1996, 100, 13338–13341; (d) Rau, H.; Luddecke, E. J. Am. Chem. Soc.
1982, 104, 1616–1620.
In nitrogen-saturated aqueous solutions (concentration: 5 lM). Uiso
represents UE?Z and UZ?E for 8STG(E) and 8STG(Z), respectively.
3. (a) Hayashi, G.; Hagihara, M.; Dohno, C.; Nakatani, K. J. Am.
Chem. Soc. 2007, 129, 8678–8679; (b) Zhang, Z. H.; Burns, D. C.;
Kumita, J. R.; Smart, O. S.; Woolley, G. A. Bioconjugate Chem. 2003,
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Commun. 1995, 8466–8467.
4. (a) Volgraf, M.; Gorostiza, P.; Numano, R.; Kramer, R. H.; Isacoff,
E. Y.; Trauner, D. Nat. Chem. Biol. 2006, 2, 47–52; (b) Loudwing, S.;
Bayley, H. J. Am. Chem. Soc. 2006, 128, 12404–12405; (c) Nakayama,
K.; Endo, M.; Majima, T. Chem. Commun. 2004, 2386–2387.
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128, 1009–1015.
6. Roy, S.; Nagarajan, R.; Wu, P. C.; Yang, K.; Bruno, F. F.; Parmer,
V. S.; Tripathy, S. K.; Kumar, J.; Samuelson, L. A. J. Macromol. Sci.,
Pre Appl. Chem. 2001, 38, 1383–1392.
Fig. 4. Switching cycles between E and Z by alternate illumination with
254 nm and 370 nm light. The illumination periods are 50 s and 7 s for
254 nm and 370 nm, respectively.
7. Liu, Y.; Sen, D. J. Mol. Biol. 2004, 341, 887–892.
We next investigated the quantum yield for E–Z photo-
isomerization and the fluorescence spectrum in an aqueous
solution at room temperature. As summarized in Table 1,
the quantum yields were UE?Z = 0.35 and UZ?E = 0.15,
which were obtained by comparing the initial rates of
photoisomerization to that of E-stilbene.19 Fluorescence
emission was observed for both 8STG(E) and 8STG(Z),
which had a similar fluorescence maximum at 450 nm,
but the intensity for 8STG(Z) was only one-sixth of
8STG(E). Finally, the reversible switching was repeated
several times by alternate illumination with 254 nm and
370 nm light, and good reversibility of E–Z photoisomer-
ization was observed without any side reactions, as shown
in Figure 4.
8. Keiper, S.; Vyle, J. S. Angew. Chem., Int. Ed. 2006, 45, 3306–3309.
9. Endo, M.; Nakayame, K.; Kaida, Y.; Majima, T. Tetrahedron Lett.
2003, 44, 6903.
10. Ha, S. C.; Lowenhaupt, K.; Rich, A.; Kim, Y.-G.; Kim, K. K. Nature
2005, 437, 1183–1186.
11. (a) Bertrand, H.; Monchaud, D.; Cian, A. D.; Guillot, R.; Mergny, J.-
L.; Fichou, M.-P. T. Org. Biol. Chem. 2007, 5, 2555–2559; (b)
Paeschke, K.; Simonsson, T.; Postberg, J.; Rhodes, D.; Lipps, H. J.
Nat. Struct. Mol. Biol. 2005, 12, 847–854; (c) Parkinson, G. N.; Lee,
M. P. H.; Neidle, S. Nature 2002, 417, 876–880; (d) Arnal-Herault, C.;
Banu, A.; Barboiu, M.; Michau, M.; Lee, A. Angew. Chem., Int. Ed.
2007, 46, 4268–4272; (e) Wong, A.; Ida, R.; Spindler, L.; Wu, G. J.
Am. Chem. Soc. 2005, 127, 6990–6998.
12. Sotoya, H.; Matsugami, A.; Ikeda, T.; Ouhashi, K.; Uesugi, S.;
Katahira, M. Nucleic Acids Res. 2004, 32, 5113–5118.
13. Gannett, P. M.; Sura, T. P. Synth. Commun. 1993, 23, 1611–1615.
14. 1H NMR (DMSO-d6) d: 10.7 (s, 1H), 7.68 (d, J = 7.3, 2H) 7.53 (d,
J = 16.1, 1H) 7.47 (d, J = 16.1, 1H), 7.37 (m, 2H), 7.29 (m, 1H), 6.47
(s, 2H), 6.37 (dd, J = 8.6, 6.2, 1H), 5.26 (d, J = 4.2, 1H), 5.17 (t,
J = 5.1, 1H), 4.45 (m, 1H), 3.82 (m, 1H), 3.71 (m, 1H), 3.65 (m, 1H),
2.60 (m, 1H), 2.09 (m, 1H). 13C NMR (DMSO-d6) d: 156.2, 153.1,
151.5, 144.1, 136.0, 132.8, 128.6, 128.2, 127.0, 116.4, 115.7, 87.1, 82.3,
70.2, 61.2. HR-FAB (M+H)+ for C18H19N5O4. Calcd: 370.1515;
found: 370.1571.
15. 1H NMR (DMSO-d6) d: 10.8 (s, 1H), 7.56 (m, 2H), 7.29 (m, 3H), 6.83
(d, J = 13.0, 1H), 6.64 (d, J = 13.0, 1H), 6.47 (s, 2H), 6.15 (dd,
J = 8.5, 6.6, 1H), 5.18 (s, 1H), 5.00 (s, 1H), 4.33 (s, 1H), 3.76 (dd,
J = 7.8, 4.6, 1H), 3.61 (m, 1H), 3.53 (m, 1H), 2.71 (m, 1H), 1.83 (m,
1H). 13C NMR (DMSO-d6) d: 156.4, 153.3, 151.2, 142.6, 135.4, 135.3,
129.0, 128.1, 127.9, 117.4, 116.3, 87.3, 83.1, 70.6, 61.7, 38.0. HR-FAB
(M+H)+ for C18H19N5O4. Calcd: 370.1515; found: 370.1564.
16. (a) Greene, K. L.; Wang, Y.; Live, D. J. Biomol. NMR 1995, 5, 333–
338; (b) Uesugi, S.; Ikehara, M. J. Am. Chem. Soc. 1977, 99, 3250–
3253.
In summary, we have successfully developed a photo-
chromic 20-deoxyguanosine, 8-styryl-20-deoxyguanosine
(
8STG). This compound shows a very rapid and highly effi-
cient reversible E–Z photoisomerization upon illumination
at specific wavelength. In addition, E–Z photoisomeriza-
tion can be iteratively performed by alternate illumination
with monochroic 254 nm and 370 nm light without any side
reactions.20 Therefore, 8STG may be widely used for the
photochemical control of nucleic acid structure. To the best
of our knowledge, this is the first example for the nucleo-
base carrying photoswitching property.
Supplementary data
17. A photostationary state (PSS) was achieved by illumination for 2 h at
room temperature using a spectrofluorometer (FP-6500, JASCO),
which can modulate monochromic light with a 1 nm peak width at
half height.
Detailed synthesis and fluorescence spectra for both of
1
8STG(E) and 8STG(Z), H and 13C NMR spectra of com-
pounds 2, 3, and 4. Supplementary data associated with

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S. Ogasawara et al. / Tetrahedron Letters 49 (2008) 2479–2482
18. E–Z photoisomerization was performed in an aqueous solution
containing 25 lM 8STG and 2% acetonitrile at room temperature
using a 100 W Xenon lamp (LAX-Cute, Asahi Spectra Co., Ltd),
which can extract a specific wavelength with a 10 nm peak width at
half height by employing an adequate bandpass filter (HQBP254-UV
for Z?E photoisomerization and HQBP370-UV for E?Z photoiso-
merization, Asahi Spectra Co., Ltd). After illumination, the E-isomer
ratio was determined by the peak area in HPLC analysis with UV
detection at the isosbestic point (268 nm) in the E–Z
photoisomerization.
19. Values of the UE?Z and UZ ?E were obtained by comparing the initial
rates of photoisomerization to that of E-stilbene during irradiation in
nitrogen-saturated methanol solution (UE?Z = 0.48); Gorner, H.;
Kuhn, H. J. Adv. Photochem. 1995, 19, 1–117.
20. A reviewer comment that most of DNA especially d(TT)
should undergo the dimerization to produce thymine dimer under
photoirradiation of short wavelength light (254 nm). We agree
with the reviewer and will effort to develop
a novel photo-
switchable nucleoside, which can be photoisomerized by longer
wavelength.