A G-octamer scaffold via self-assembly of a guanosine-based Au(I) isonitrile complex for Au(I)-Au(I) interaction

Article abstract of DOI:10.1039/c1cc10774g

A guanosine-based Au(I) isonitrile complex was demonstrated to serve as the reliable scaffold via self-assembly, wherein the quartet and octamer were formed in the absence and presence of a potassium ion, respectively, exhibiting a switchable emission bas

Full text of DOI:10.1039/c1cc10774g

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2011, 47, 4682–4684
www.rsc.org/chemcomm
COMMUNICATION
A G-octamer scaffold via self-assembly of a guanosine-based Au(I)
isonitrile complex for Au(I)–Au(I) interactionw
Xiangtai Meng,a Toshiyuki Moriuchi,*a Masatoshi Kawahata,b Kentaro Yamaguchib and
Toshikazu Hirao*a
Received 10th February 2011, Accepted 4th March 2011
DOI: 10.1039/c1cc10774g
A guanosine-based Au(I) isonitrile complex was demonstrated to
serve as the reliable scaffold via self-assembly, wherein the
quartet and octamer were formed in the absence and presence
of a potassium ion, respectively, exhibiting a switchable emission
based on Au(I)–Au(I) interaction.
presence of a cation. From this point of view, a variety of
guanosine derivatives have been reported with porphyrin-G,
pyrene-G, oligothiophene-G, and OPV-G.4 On the other hand,
the supramolecular architecture in gold(I) chemistry has attracted
increasing attention, in particular with regard to the pheno-
menon of aurophilicity.5 A number of di-, tri-, and polynuclear
gold(I) complexes are known to exhibit specific luminescence
properties based on Au(I)–Au(I) interaction.6 Although numerous
functionalized guanosine derivatives were developed, to the best of
our knowledge, the utilization of self-assembly properties of
guanosine derivatives to study the Au(I)–Au(I) interaction has
not been reported so far. Herein, we report an Au(I) complex 1
possessing the guanosine moiety.
Recently, the research field of bioorganometallic chemistry,
which is a hybrid area between organometallic chemistry and
biochemistry, has drawn much attention.1 Conjugation of
organometallic compounds with biomolecules such as nucleo-
bases, amino acids, and peptides is envisioned to provide novel
systems depending on both properties.1,2 Highly-ordered
molecular assemblies are constructed in bio-systems to fulfill
unique functions as observed in enzymes and receptors.
Introduction of functional complexes into highly-ordered
biomolecules is considered to be a convenient approach to the
corresponding biomaterials and bio-inspired systems. Guanosine
(G) and its derivatives have a high potential for self-assembly
(Fig. 1).3 These properties have been investigated in detail with
the goal to synthesize new guanosine derivatives and to explore
their electronic and optical properties. Moreover, functionaliza-
tion of the guanosines at the C8 position or the sugar hydroxyl
groups is allowed to construct functionalized assemblies in the
The Au(I) complex 1 was obtained in a four-step synthesis
from 8-bromoguanosine (Scheme 1). The Stille cross-coupling
reaction of the 8-bromoguanosine 2 with the tin compound 3
afforded the protected guanosine 4,7 followed by deprotection
with K2CO3 to give the 8-ethynylguanosine 5.8 Reaction of 5
with Au(tht)Cl (tht = tetrahydrothiophene) in the presence of
NaOAc resulted in the neutral oligomer 6 as an orange solid in a
high yield.9 Upon addition of phenyl isocyanide to the CH2Cl2
suspension of 6, the neutral Au(I) compound 1 was obtained. All
new compounds were fully characterized by 1H, 13C NMR, IR,
and HRMS techniques (see ESIw). A strong band due to
n(NRC) was observed at 2209 cmÀ1 in the IR spectrum of 1.
Fig. 1 Assembly types of guanosine derivatives.
a Department of Applied Chemistry, Graduate School of Engineering,
Osaka University, Yamada-oka, Suita, Osaka 565-0871, Japan.
E-mail: moriuchi@chem.eng.osaka-u.ac.jp,,
hirao@chem.eng.osaka-u.ac.jp; Fax: +81-6-6879-7415;
Tel: +81-6-6879-7413
b Pharmaceutical Sciences at Kagawa Campus Tokushima Bunri
University, 1314-1 Shido, Sanuki, Kagawa 769-2193, Japan.
E-mail: yamaguchi@kph.bunri-u.ac.jp; Fax: +81-87-894-0181;
Tel: +81-87-894-511
w Electronic supplementary information (ESI) available: Experimental
procedures and characterization data for all new compounds, and
excitation spectra of 1. See DOI: 10.1039/c1cc10774g
Scheme 1 Synthesis of 1.
c
4682 Chem. Commun., 2011, 47, 4682–4684
This journal is The Royal Society of Chemistry 2011

View Article Online
amide proton signal became rather sharp and a new signal
appeared at about 10.12 ppm. The latter is considered to be
attributable to the hydrogen-bonded amino protons. Moreover,
only one set of signals appears in the 1H NMR spectrum. With
reference to previous studies on the assembly properties of
guanosine derivatives,11 a D4-octamer (all of the sugar moieties
of guanosine are syn) appeared to be formed.12 The amino and
amide signals are broad at 25 1C, but become sharper at low
temperatures probably because the octamer structure becomes
more rigid. Furthermore, at a low temperature, the chemical
shifts of the hydrogen-bonded amide (12.75 and 12.32 ppm) and
amino (9.79 and 10.12 ppm) proton signals are different in the
absence and presence of a potassium ion, respectively, indicating
that the two aggregates are different: one is an ‘empty’ quartet
and another is a cation assisted octamer.
Fig. 2 1H NMR (600 MHz) spectra of 1 at various temperatures in
Circular dichroism (CD) spectroscopy provides insight into the
chirality of this assembly in solution.13 As shown in Fig. 4, the
CD spectrum of 1 in CHCl3 in the absence of KPF6 at 25 1C
showed a random coil conformation due to the formation of the
oligomeric species. In sharp contrast, in the presence of 0.125 eq.
of KPF6, a significant change was shown in the CD spectrum: a
positive band at 262 nm and a very strong positive band at
334 nm were observed, being accompanied by a negative band
centered at 288 nm and a very strong negative band at 228 nm.
Although no detailed information on the electronic transitions is
available so far for this Au(I) complex, this difference indicates a
change in the conformation and/or secondary structure of 1 after
addition of KPF6. This finding might closely resemble those
reported for other unmodified lipophilic guanosines.3a,b
the absence of KPF6 in CDCl3.
The Au(I) complex 1 is quite soluble in CH2Cl2, CHCl3, THF
1
and others. The H NMR spectrum of 1 measured in CDCl3
showed the equivalent amino protons (NH2) as a broad singlet at
6.40 ppm while the amide proton (NH) resonates at 12.39 ppm at
25 1C (Fig. 2). These chemical shifts indicate that 1 self-associates
in a mixture of oligomeric species in the absence of an alkaline
cation. Warming the temperature to 45 1C, both the amino and
amide protons upfield shifted and became slightly sharper. In
contrast, lowering the temperature to À45 1C, the amino protons
are split into two signals. The downfield-shifted signal at
9.79 ppm is assigned to the hydrogen-bonded proton and
another signal around 4.50 ppm belongs to the ‘free’ amino
proton. At the same time, the amide proton signal is downfield
shifted to 12.75 ppm. These findings suggest that an empty
G-quartet is formed even without the assistance of a cation
template at the lower temperature.7,10 Interestingly, the water
signal downfield shifted from 1.59 ppm to 2.15 ppm during
lowering the temperature. This shift suggests that water molecules
are involved in stabilizing the empty G-quartet.4e
In the electronic spectrum of 1 in CHCl3 shown in Fig. 5, a
high-energy band at 250–300 nm and a more intense low-energy
absorption band at 320–340 nm were observed. Upon addition
of KPF6, a drop in intensity was observed with the absorption
band at about 330 nm, together with the concomitant growth of
a new low-energy shoulder in the region of approximately
380–430 nm. These observations indicate that the self-associate
of the guanosine 1 and aurophilicity is likely to bring the gold
atoms into close proximity, in which Au(I)–Au(I) interaction is
allowed to be present.
By adding 0.125 eq. of KPF6 to a CDCl3 solution of 1, the
1H NMR spectrum changed. As shown in Fig. 3, the amino
proton signal disappeared and the amide proton signal became
sharper at 25 1C. Lowering the temperature to À30 1C, the
Since Au(I) compounds are known to show rich lumines-
cence properties, the luminescence response of 1 towards
KPF6 was investigated. In the absence of the potassium ion,
1 exhibited an intense emission band around 400–600 nm
(Fig. 6a, lex = 380 nm). Upon addition of KPF6 to a CHCl3
solution of 1, a new low-energy emission band at ca. 510 nm
appeared (Fig. 6b, lex = 440 nm). The excitation spectra of 1
Fig. 3 1H NMR (400 MHz) spectra of 1 at various temperatures in
Fig. 4 CD spectra of 1 in the absence (red line) and presence (blue
the presence of KPF6 (0.125 eq.) in CDCl3.
line) of KPF6 (0.125 eq.) in CHCl3.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 4682–4684 4683

View Article Online
self-assembly serves as a reliable scaffold for the arrangement of
the Au(I) isonitrile moieties. Studies on the application of the
G-octamer induced metal ion aggregates including functional
materials and catalysts are now in progress.
The author X. M. expresses special thanks for the Global COE
(center of excellence) Program ‘‘Global Education and Research
Center for Bio-Environmental Chemistry’’ of Osaka University.
This work was supported by Grant-in-Aids for Science Research
on Innovative Areas (No. 22108516 and 21111512) from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan. Thanks are also due to the Analytical Center, Graduate
School of Engineering, Osaka University.
Fig. 5 UV-vis spectra of 1 in the absence (red line) and presence
(blue line) of KPF6 (0.125 eq.) in CHCl3.
Notes and references
1 (a) Bioorganometallics: Biomolecules, labeling, Medicine, ed.
G. Jaouen, Wiley-VCH, Weinheim, 2006 and references therein;
(b) R. H. Fish and G. Jaouen, Organometallics, 2003, 22, 2166;
(c) K. Severin, R. Bergs and W. Beck, Angew. Chem., Int. Ed.,
1998, 37, 1634.
2 (a) A. Nomoto, T. Moriuchi, S. Yamazaki, A. Ogawa and
T. Hirao, Chem. Commun., 1998, 1963; (b) T. Moriuchi,
A. Nomoto, K. Yoshida, A. Ogawa and T. Hirao, J. Am. Chem.
Soc., 2001, 123, 68; (c) T. Moriuchi, T. Nagai and T. Hirao, Org.
Lett., 2005, 7, 5265; (d) T. Moriuchi and T. Hirao, Acc. Chem.
Res., 2010, 43, 1040.
3 (a) J. T. Davis and G. P. Spada, Chem. Soc. Rev., 2007, 36, 296;
(b) J. T. Davis, Angew. Chem., Int. Ed., 2004, 43, 668; (c) S. Lena,
S. Masiero, S. Pieraccini and G. P. Spada, Mini-Rev. Org. Chem.,
´
2008, 5, 262; (d) C. Aime, R. Nishiyabu, R. Gondo, K. Kaneko
and N. Kimizuka, Chem. Commun., 2008, 6534.
Fig. 6 Emission spectra of 1 in the absence (red line) and presence
(blue line) of KPF6 (0.125 eq.) in CHCl3 ((a) lex = 380 nm;
(b) lex = 440 nm).
4 (a) S. Masiero, G. Gottarelli and S. Pieraccini, Chem. Commun.,
2000, 1995; (b) S. Martic, X. Liu, S. Wang and G. Wu, Chem.–Eur.
´
J., 2008, 14, 1196; (c) S. Pieraccini, S. Bonacchi, S. Lena,
S. Masiero, M. Montalti, N. Zaccheroni and G. P. Spada, Org.
Biomol. Chem., 2010, 8, 774; (d) G. P. Spada, S. Lena, S. Masiero,
S. Pieraccini, M. Surin and S. Samorı, Adv. Mater., 2008, 20, 2433;
(e) D. Gonzalez-Rodrıguez, P. G. A. Janssen, R. Martın-Rapu´ n,
´ ´ ´
I. De Cat, S. De Feyter, A. P. H. J. Schenning and E. W. Meijer,
J. Am. Chem. Soc., 2010, 123, 4710.
5 (a) H. Schmidbaur, Chem. Soc. Rev., 1995, 24, 391; (b) P. Pyykko,
¨
Chem. Rev., 1997, 97, 597; (c) M. J. Katz, K. Sakai and
D. B. Leznoff, Chem. Soc. Rev., 2008, 37, 1884;
(d) H. Schmidbaur and A. Schier, Chem. Soc. Rev., 2008, 37, 1931.
6 (a) V. W.-W. Yam, C.-K. Li and C.-L. Chan, Angew. Chem., Int.
Ed., 1998, 37, 2857; (b) X. He, E. C.-C. Cheng, N. Zhu and
V. W.-W. Yam, Chem. Commun., 2009, 4016; (c) H. Ito,
T. Saito, N. Oshima, N. Kitamura, S. Ishizaka, Y. Hinatsu,
M. Wakeshima, M. Kato, K. Tsuge and M. Sawamura, J. Am.
Chem. Soc., 2008, 130, 10044; (d) X. He and V. W.-W. Yam, Inorg.
Chem., 2010, 49, 2273; (e) T. Moriuchi, K. Yoshi, C. Katano and
T. Hirao, Chem. Lett., 2010, 841; (f) T. Moriuchi, K. Yoshi,
C. Katano and T. Hirao, Tetrahedron Lett., 2010, 51, 4030.
7 J. L. Sessler, M. Sathiosatham, C. T. Brown, T. A. Rhodes and
G. Wiederrecht, J. Am. Chem. Soc., 2001, 123, 3655.
8 F. Maya and J. M. Tour, Tetrahedron, 2004, 60, 81.
9 C. P. McArdle, S. Van, M. C. Jennings and R. J. Puddephatt,
J. Am. Chem. Soc., 2002, 124, 3959.
10 M. Nikan and J. C. Sherman, Angew. Chem., Int. Ed., 2008, 47,
4900.
11 S. Lena, P. Neviani, S. Masiero, S. Pieraccini and G. P. Spada,
Angew. Chem., Int. Ed., 2010, 49, 3657.
12 In addition, analysis of the sample by CSI-TOF MS yielded the
Fig. 7 A possible diagram showing the formation of Au(I)–Au(I)
interaction upon addition of KPF6.
in the presence and in the absence of KPF6 revealed that the
low-energy and the high-energy bands are derived from
different excited state origins (Fig. S1, ESIw). The excitation
spectrum showed a weak low-energy shoulder at 440 nm in the
presence of KPF6 (Fig. S1, ESIw), which coincides with the
new absorption band at about 400–450 nm in the UV-vis
spectrum resulting from the Au(I)–Au(I) interaction. Upon
addition of KPF6, the sandwich-like octamer was formed,
thereby bringing the two quartets containing Au(I) centers
into close proximity (Fig. 7). On the basis of related literature,
the distance between the two quartets might be about 3.3 A,3
which is suitable for the overlap of the orbital on each Au(I)
atom. Therefore, the low energy emission band at 510 nm is
considered to be attributed to the Au(I)–Au(I) interaction.6
In conclusion, a bioorganometallic Au(I) complex possessing
the guanosine moiety was designed and synthesized. The designed
guanosine-based Au(I) isonitrile complex was demonstrated to
form the quartet and octamer in the absence and presence of a
potassium ion, respectively, exhibiting a switchable emission
based on Au(I)–Au(I) interaction, wherein the G-octamer via
spectra with
a peak m/z = 4122 that matched the mass
[18+2Na]2+ in CHCl3, which may be due to cation exchange
during sample preparation and analysis. M. S. Kaucher and
J. T. Davis, Tetrahedron Lett., 2006, 47, 6381.
13 (a) D. M. Gray, J. D. Wen, C. W. Gray, R. Repges, C. Repges,
G. Raabe and J. Fleishhauer, Chirality, 2008, 20, 431;
(b) G. Gottarelli, S. Masiero and G. P. Spada, Enantiomer, 1998,
3, 429; (c) G. Gottarelli, S. Lena, S. Masiero, S. Pieraccini and
G. P. Spada, Chirality, 2008, 20, 471.
c
4684 Chem. Commun., 2011, 47, 4682–4684
This journal is The Royal Society of Chemistry 2011