Photochromic nucleobase photoisomerized by visible light

Article abstract of DOI:10.1246/cl.2010.956

The compound 8-phenylazoguanosine (^8PAG), a new type of photochromic nucleobase (PCN) that can be photoisomerized by visible light irradiation, was developed. ^8PAG shows rapid and efficient reversible cis-trans isomerization and can

Full text of DOI:10.1246/cl.2010.956

doi:10.1246/cl.2010.956
956
Published on the web July 31, 2010
Photochromic Nucleobase Photoisomerized by Visible Light
Shinzi Ogasawara,*1,2 Syoji Ito,1,3 Hiroshi Miyasaka,3 and Mizuo Maeda2
1PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012
2Bioengineering Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198
3Division of Frontier Materials Science, Graduate School of Engineering Science and Center for Quantum Science
and Technology under Extreme Conditions, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531
(Received June 22, 2010; CL-100578; E-mail: o_shinji@riken.jp)
The compound 8-phenylazoguanosine (8PAG), a new type of
photochromic nucleobase (PCN) that can be photoisomerized by
visible light irradiation, was developed. 8PAG shows rapid and
efficient reversible cis-trans isomerization and can be iteratively
isomerized by alternate irradiation with 420 and 550 nm without
photolysis.
For reversible photoregulation of nucleic acid structure and
self-assembly of nucleoside derivatives,1 we previously devel-
oped photochromic nucleobases (PCNs) that can reversibly
change their photochemical and physical properties, such as
absorption and fluorescence, upon photoisomerization by ex-
ternal light stimuli.2 We have shown their application in the
photoregulation of duplex2 and G-quadruplex3 formation.
Recently, Spada et al. demonstrated the potential of our
molecules for photoregulation of supramolecular architectures
by controlling PCN self-assembly.4 Although PCNs may be
applied to photoregulation of important biological events such as
RNA interference,5 blood clotting,6 cellular senescence,7 tran-
scription,8 and translation9 through duplex or G-quadruplex
regulation, there are disadvantages in that the photoisomeriza-
tion requires ultraviolet light, which causes critical damage to
nucleic acids including formation of cyclobutane dimer and
(6-4) photoproducts.10 Therefore, PCNs that can be photo-
isomerized by longer wavelengths are needed for biological
applications. Herein, we report the synthesis and photoisome-
rization properties of a new type of PCN, 8-phenylazoguanosine
Scheme 1. Synthesis of 8PAG. Reagents and conditions: (a)
aniline, sodium nitrite, water, 0 °C, 2 h, 98%; (b) isobutyric
anhydride, dry DMA, 150 °C, 2 h, 72%; (c) i: bis(trimethylsi-
lyl)acetamide, 1,2-dichloroethane, 80 °C, 1 h, ii: 1,2,3,5-tetra-O-
acetyl-¢-D-ribofuranose, trimethylsilyl trifluoromethanesulfo-
nate, anhydrous toluene, 80 °C, 2 h, 66%; (d) NH3/H2O/MeOH,
60 °C, 4 h, 79%.
(
8PAG), which can be reversibly photoisomerized using visible
light (Figure 1).
For synthesis of 8PAG we employed the stepwise procedure
phenylazo-2-N-isobutylguanine (3) in 72% yield. Treatment of a
suspension of 3 in 1,2-dichloroethane with bis(trimethylsilyl)-
acetamide (BSA) at 80 °C for 1 h gave trimethylsilyl-protected 3,
which was then coupled with 1,2,3,5-tetra-O-acetyl-¢-D-ribo-
furanose in anhydrous toluene at 80 °C for 2 h in the presence of
trimethylsilyl trifluoromethanesulfonate (TMSOTf) as a catalyst.
Purification by flash column chromatography give the product 4
as an orange foam in 66% yield. Deprotection of 4 with NH3/
H2O/MeOH at 60 °C for 4 h gave a 79% yield of 8PAG (5). As is
typical for purine ribonucleoside synthesis, a mixture of N7 and
N9 isomers was expected. However, the main product was only
the N9 isomer, the structure of which was confirmed by
2D NMR (HMBC). In the HMBC analysis, we observed a
correlation between H1¤ and C4 (see Supporting Information).13
The absorption spectrum of trans-8PAG showed a peak at
420 nm and a shoulder around 470 nm. By analogy with the
absorption spectra of azobenzene derivatives such as 4-N,N-
dimethylaminoazobenzene,12 it is likely that the large absorption
band at 420 nm corresponds to the ³³* transition, while the
shoulder corresponds to the n³* transition. We demonstrated the
reversible photoisomerization of 8PAG in ethanol using a 300 W
shown in Scheme 1 because the direct reaction of guanosine and
benzenediazonium ion yielded 8-phenylguanosine.11 Guanine (1)
reacted with the benzenediazonium ion rapidly in cold aqueous
solution at pH 10 to give 8-phenylazoguanine (2) in 98% yield.
A suspension of 2 in dry DMA and isobutyric anhydride was
stirred at 150 °C for 2 h. The crystalline ocher product of this
reaction was filtered and washed twice with EtOH to yield 8-
Figure 1. Schematic illustration of cis-trans photoisomeriza-
tion of 8-phenylazoguanosine (8PAG).
Chem. Lett. 2010, 39, 956-957
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

957
Figure 3. Switching cycles between trans and cis by alternate
illumination with 420 and 550 nm light. The illumination
periods are 20 s for both wavelengths.
photoregulation of important biological events is opened by this
type of PCN and is now in progress.
The authors thank Prof. H. Koshino of RIKEN for the
2D NMR measurements. The present work was supported by
Precursory Research for Embryonic Science and Technology
(PRESTO), Japan Science and Technology Agency (JST).
Figure 2. (a) Absorption spectra of 8PAG for trans-to-cis
photoisomerization with illumination at 420 nm, and (b) cis-to-
trans photoisomerization with illumination at 550 nm. Arrows
indicate increases and decreases of the absorption peaks. (c)
Time profile of transient absorbance for thermal cis-to-trans
isomerization of 8PAG monitored at 430 nm. Solid black line is
the best calculated curve.
References and Notes
1
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xenon lamp (MAX-302, Asahi Spectra Co., Ltd.) which can
provide a specific wavelength with a 10-nm peak width at half
height by employing an adequate bandpass filter. As shown in
Figures 2a and 2b, upon irradiation of a solution of 8PAG with
light at 420 nm (corresponding to the ³³* absorption), the
absorption spectra changes in a way similar to that observed for
the trans-cis photoisomerization of azobenzene derivatives. The
intense ³³* peak decreases and is accompanied by a slight
increase at the tail portion (500-600 nm) of the n³* absorption
until a photostationary state is reached. Subsequent irradiation at
the longer 550 nm wavelength reverses the course of the reaction
and the original spectrum is completely recovered in 30 s.
The thermal cis-to-trans isomerization rate of 8PAG was
measured in ethanol at 21 °C. Figure 2c shows the time profiles
of transient absorbance monitored at 430 nm after excitation
with a femtosecond 400 nm laser pulse with an output energy of
8.2 mW for 30 s. Thermal cis-to-trans isomerizaiton of 8PAG was
observed with a time constant of 109 s. Finally, the reversible
switching was repeated a dozen times by alternate illumination
with 420 and 550 nm light, and good reversibility of the cis-
trans photoisomerization was observed without any side
reactions (Figure 3).
6
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In summary, we have successfully developed a new type of
photochromic nucleobase, 8-phenylazoguanosine (8PAG), that
can be photoisomerized by visible light irradiation. This
compound shows a very rapid and highly efficient reversible
cis-trans photoisomerization upon illumination at specific
wavelengths. In addition, photoisomerization can be iteratively
performed by alternate illumination with monochroic 420 and
550 nm light without any side reactions. 8PAG can be widely
used for the reversible photoregulation of nucleic acid structures
and supramolecular architectures. An application to in vivo
13 Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.
html.
Chem. Lett. 2010, 39, 956-957
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/