Luminescent double-decker type guanine octets with trivalent lanthanide cations: In situ self-assembling and stability evaluation in homogeneous organic media

Article abstract of DOI:10.1007/s10847-011-9937-2

Silylated guanine formed luminescent double-decker type octet complexes with trivalent lanthanide cations (lanthanide cation : guanine = 1:8) in organic media, while a deaminated guanine derivative gave only 1:2 complexes. The octet formation was evidence

Full text of DOI:10.1007/s10847-011-9937-2

J Incl Phenom Macrocycl Chem (2011) 71:523–527
DOI 10.1007/s10847-011-9937-2
ORIGINAL ARTICLE
Luminescent double-decker type guanine octets with trivalent
lanthanide cations: in situ self-assembling and stability evaluation
in homogeneous organic media
•
Satoshi Shinoda Takashi Noguchi
•
•
Mari Ikeda Yoichi Habata Hiroshi Tsukube
•
Received: 1 February 2011 / Accepted: 15 February 2011 / Published online: 15 March 2011
Ó Springer Science+Business Media B.V. 2011
Abstract Silylated guanine formed luminescent double-
decker type octet complexes with trivalent lanthanide cat-
ions (lanthanide cation : guanine = 1:8) in organic media,
while a deaminated guanine derivative gave only 1:2
complexes. The octet formation was evidenced by char-
acteristic UV/Vis absorption changes, CSI-TOF MS and 1H
NMR spectra. The octet with Tb3? showed intense green
luminescence with a long lifetime by photo-excitation of
the guanine chromophore. The trivalent lanthanie cations
stabilized the octets more effectively than common mono-
and divalent metal cations.
Introduction
Guanosine and its derivatives are a family of the most pro-
grammed molecules to form structure-defined self-assemblies
[1, 2]. In particular, metal-templated quartets, octets and
higher assemblies have received wide attention, not only
because they are found in nature [3, 4] but because they can
provide useful scaffolds to develop self-assembled nano-
materials [5]. Some alkali and alkaline earth metal cations
worked as effective templates in self-assembly processes of
50-GMP and its analogs. Pinnavaia et al. [6] determined the
crystal structure of a Na?- or K?-templated quartet (1:4
complexation), in which one metal cation was coordinated by
four carbonyl oxygen atoms from the guanines. The double-
decker type of guanine octets (1:8 complexes) is another self-
assembly mode. These complexes are usually prepared by
solid (metal salt)–liquid (guanine solution) extraction, and
their self-assembling profiles have been characterized after
extraction procedures [7, 8]. Lanthanide cations have large
coordination numbers and high coordination flexibility, and
Wu et al. [9] reported that some of them promoted octet
formation in a similar fashion to the alkali and alkaline earth
cations. Since some nucleotides are known to enhance the
emission of Eu3? and Tb3? cations in solution [10, 11], proper
combinations of lanthanide cations and designed guanines
can provide a new series of functional assembled materials.
We report here that silylated guanine 1 forms double-
decker type octet complexes upon mixing with lanthanide
triflates in homogeneous CHCl3 solution. Compound 1 and
its deaminated derivative 2 were prepared in order to
examine in situ formation of the metal-templated octets
(Fig. 1). Previously reported guanines mostly had complex
lipophilic substituents and chiral sugar moieties to be
examined in the extraction processes, which often provided
additional interacting sites and diastereomerism in the metal
Keywords Lanthanides ꢀ G-Quadruplexes ꢀ
Self-assembly ꢀ Luminescence ꢀ
Cold-Spray Ionization Mass Spectrometry
Electronic supplementary material The online version of this
article (doi:10.1007/s10847-011-9937-2) contains supplementary
material, which is available to authorized users.
S. Shinoda (&) ꢀ T. Noguchi ꢀ H. Tsukube
Department of Chemistry, Graduate School of Science, Osaka
City University, Sugimoto, Sumiyoshi-ku, Osaka 558-8585,
Japan
e-mail: shinodas@sci.osaka-cu.ac.jp
S. Shinoda ꢀ H. Tsukube
JST, CREST, Osaka City University, Sugimoto, Sumiyoshi-ku,
Osaka 558-8585, Japan
M. Ikeda ꢀ Y. Habata
Department of Chemistry, Faculty of Science, Toho University,
2-2-1 Miyama, Funabashi, Chiba 274-8510, Japan
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Fig. 1 Guanine derivatives and
double-decker type guanine
octet
complexation. We demonstrate here that simply silylated
guanine 1 forms more stable octets with trivalent lanthanide
cations than those with common alkali and alkaline earth
cations. Thus, the present results provide various possibili-
ties that new synthetic strategies toward self-assembled
materials can be developed for coordination, material, bio-
chemical and related applications.
protons of guanine 1 exhibited separate signals at 8.9 and
5.4 ppm. One of them, HA, shifted dramatically upon for-
mation of an intermolecular hydrogen bond and the other,
HB, shifted only slightly. Since the amide proton also
appeared at two distinct positions, it was concluded that the
resulting octet was composed of two diastereomers. The
La3? octet exhibited NOE cross peaks between N8 and the
amide protons, indicating that four guanines self-assembled
in the metal complex (Fig. S3 in Supplementary material).
The cold spray ionization (CSI) TOF–MS method, well
recognized as an effective method of non-covalent com-
plexes [12], was applied in detection of the guanine–metal
cation assemblies. When 1/8 equivalent of metal triflate
(K?, Ca2?, Ba2?, Y3?, La3? or Tb3?) was added to the
solution of guanine 1 (CHCl3:CH3CN = 4:1), intense mass
signals from the double-decker type octets were observed
(Fig. 3 and Fig. S4 in Supplementary material): m/z = 3748
for [18–K]?; 1874 for [18–Ca]2?; 1923 for [18–Ba]2?; 1266
Results and discussion
We prepared guanine derivatives 1 and 2 by silylation of
acyclovir and 4-[(2-hydroxyethoxy)methyl]-1,9-dihydroxy-
6H-purine-6-one in 64 and 86% yields, respectively. UV
spectral changes of guanine 1 upon stepwise addition of
La3?, Tb3? and Ca2? cations are shown in Fig. 2. When
La(OTf)3 was added to a CHCl3 solution of guanine 1, a new
peak appeared around 310 nm. Its intensity increased at
La3?/guanine 1 \ 1/8, and then decreased at La3?/guanine
1 [ 1/8. The plots clearly showed that La3? and Tb3?
cations predominantly formed the octets with guanine 1 at
low ratios of metal cation/guanine, while Ca2? cation gave
the octet under wider conditions. Since guanine 2 formed 1:2
complexes, the N2–amino-hydrogen atoms of guanine 1
formed strong intermolecular hydrogen bonds. Guanine 1
exhibited characteristic fluorescence spectral changes upon
addition of metal triflates. These spectral changes indicated
that La3? cation formed both 1:8 and 1:2 complexes but
Ca2? cation gave only the 1:8 complex (see Fig. S1 in
Supplementary material). 1H NMR spectra also demon-
strated that guanine 1 gave a 1:8 complex in the presence of
1/8 equivalent of La3? cation (see Fig. S2 in Supplementary
material). As observed with the Ca2? complex, two amino
for [18–Y]3?; 1283 for [18–La]3?; and 1289 for [18–Tb]3?
.
The octet with Tb3? cation exhibited intense green
luminescence signals upon excitation of the guanine chro-
mophore via efficient energy transfer. Its luminescence
lifetime was estimated to be 1.6 ms, indicating that the Tb3?
ion was effectively protected from solvation in the octet (Fig.
S5 in Supplementary material). The luminescence was
monitored not only by spectrometry but alsoby the naked eye
(Fig. 4). Competitive luminescence experiments were car-
ried out with combinations of equimolar Tb3? and other
metal cations to estimate the relative stability of the octets.
Addition of other metal ions gave only small enhancements
of the absorbance at the excitation wavelength (285 nm).
When Ca(OTf)2 was added to the CHCl3 solution of guanine
1 and Tb(OTf)3 (Ca2?:Tb3?:guanine 1 = 1:1:8), the
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Fig. 2 Absorbance changes
of guanine 1 upon stepwise
addition of La(OTf)3, Tb(OTf)3
and Ca(OTf)2 in CHCl3. [1] =
1.0 9 10-4
M
luminescence intensity of the resulting solution slightly
decreased compared to the guanine 18–Tb3? complex (91%).
Although Ca2? cation formed an octet, its stability was
confirmed to be much lower than that of the Tb3? complex.
In contrast, addition of La3? and Y3? cations significantly
decreased the Tb3? luminescence intensities (33% for La3?
and 47% for Y3?), indicating that their 1:8 complexes had
comparable stability to that of the Tb3? complex. Thus, our
luminescence studies confirmed the following stability order
of the octets in a semi-quantitative sense: 18–La3? [ 18–
Tb3? and 18–Y3?[18–Ca2?[18–K? and 18–Ba2?. Since
the employed metal cations have similar ionic radii [13], this
order emphasizes the significant effect of highly charged
lanthanide centers on octet stability.
double-decker typeoctets. Sincetheselanthanidecationshave
multiple functionalities, double-decker type octets can have
wide applications.
Experimental section
Acyclovir and its deamino derivative were purchased from
Maybridge. Chloroform was a spectroscopic grade of
Wako Pure Chemical Industries, Ltd. and used after pass-
ing through alumina column.
Guanine 1: Acyclovir (111 mg), imidazole (91.5 mg),
and t-butyldiphenylsilyl chloride (244 mg) were dissolved
in dry DMF (2 ml) and stirred for 8 h at room temperature.
Water (5 ml) was added and all the solvent was removed
by evaporation. The product was separated by chroma-
tography (silica gel, CHCl3-MeOH, 10:1) and purified by
washing with hexane, and then water. After dying in vacuo,
1 was obtained as white solids (141 mg, 64%). m.p.
248–250 °C; 1H NMR (DMSO-d6, 400 MHz): d = 0.94 (s,
9H), 3.60 (t, 2H), 3.70 (t, 2H), 5.36 (s, 2H), 6.48 (s, 2H),
7.39–7.58 (s, 10H), 7.80 (s, 1H) and 10.6 (s, 1H); 13C NMR
(DMSO-d6, 100 MHz): d = 18.7, 26.5, 62.7, 69.8, 72.0,
116.5, 127.8, 129.8, 133.0, 135.0, 137.6, 151.4, 153.8 and
156.8; FAB-MS m/z 464.2 (M?) and 486.2 (M?Na?);
Conclusions
We have developed an in situ preparation method of double-
decker type octets in organic media, in which the trivalent
lanthanide cations effectively organized a coordination mode
and hydrogen bonding pattern to promote self-assembly of the
silylated guanine 1. Competitive luminescence experiments
with Tb3? and other metal cations revealed that trivalent
lanthanide cations such as La3?, Tb3? and Y3? formed stable
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Fig. 4 Pictures of green luminescence from Tb3?–guanine octet
photo-excited by UV illuminator (8 W, 302 nm) and spectral changes
upon addition of other metal cations in CHCl3. [1] = 1.0 9 10-4 M,
[Tb(OTf)3] = 1.3 9 10-5 M, [metal triflate] = 0 or 1.3 9 10-5 M, kex
= 285 nm, slit widths: 5.0 (ex) and 2.5 nm (em)
Anal. calcd for C23H27N5O3Si: C, 62.18; H, 6.30; N, 15.11,
found: C, 62.40; H, 6.33; N, 15.15.
Guanine 2 was obtained (133 mg, 86%) from deamino-
acyclovir (100 mg), imidazole (97 mg), and t-butyldi-
methylsilyl chloride (108 mg) in a similar way. m.p.
238–239 °C; 1H NMR (300 MHz, DMSO-d6): d = –0.04 (s,
6H), 0.79 (s, 9H), 3.55 (t, 2H), 3.62 (t, 2H), 5.54 (s, 2H), 8.07
(s, 1H), 8.20 (s, 1H) and 12.36 (s, 1H); 13C NMR (DMSO-d6,
100 MHz): d = –5.4, 17.9, 25.7, 61.8, 70.5, 72.7, 124.0,
140.6, 146.0, 148.5 and 156.6; FAB-MS m/z 324.2 (M?);
Anal. calcd for C14H24N4O3Si: C, 51.83; H, 7.46; N, 17.27,
found: C, 51.64; H, 7.46; N,17.23.
Since these rarely extracted lanthanide triflates, the
guanine octets were not synthesized under common
extraction conditions.
Acknowledgments This research was supported in part by a Grant-
in-Aid for Scientific Research (No. 20245014) from the Japan Society
for the Promotion of Science.
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