From G-quartets to G-ribbon gel by concentration and sonication control

Article abstract of DOI:10.1039/c3ob27204d

Two guanosine analogues have been designed and synthesized by connecting one (1) or three adamantane branches (2). The compound containing a single adamantane branch formed G-quartets in acetonitrile solution, and was then transformed into a G-ribbon gel at concentrations higher than the critical gelation concentration. In contrast, the compound with three adamantane branches precipitated after a heating-cooling process. By means of circular dichroism and UV/visible spectra, NMR, SEM, and structural studies, the mechanism of the formation of the G-quartets and G-ribbon gel, as well as the difference in the self-assembly modes of the two compounds, have been fully elucidated. Compound 1 firstly self-assembled into G-quartets in solutions in the concentration range 5.0 × 10^-4 to 1.0 × 10^-2 M, and these G-quartets were transformed into a G-ribbon on further increasing the concentration. Gelation occurred when the G-ribbon self-assembled into a hexagonal columnar structure with the help of intermolecular hydrogen-bonding and hydrophobic interactions. This gel was sensitive to sonication and underwent a morphology change from a columnar structure to a flower-like structure composed of flakes. In contrast, due to steric hindrance, compound 2 only assembled into a spherical structure based on hydrophobic interactions.

Full text of DOI:10.1039/c3ob27204d

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From G-quartets to G-ribbon gel by concentration and
sonication control†
Cite this: Org. Biomol. Chem., 2013, 11,
1525
Luyan Meng, Keyin Liu, Shuli Mo, Yueyuan Mao and Tao Yi*
Two guanosine analogues have been designed and synthesized by connecting one (1) or three adaman-
tane branches (2). The compound containing a single adamantane branch formed G-quartets in aceto-
nitrile solution, and was then transformed into a G-ribbon gel at concentrations higher than the critical
gelation concentration. In contrast, the compound with three adamantane branches precipitated after a
heating–cooling process. By means of circular dichroism and UV/visible spectra, NMR, SEM, and structural
studies, the mechanism of the formation of the G-quartets and G-ribbon gel, as well as the difference in
the self-assembly modes of the two compounds, have been fully elucidated. Compound 1 firstly self-
assembled into G-quartets in solutions in the concentration range 5.0 × 10−4 to 1.0 × 10−2 M, and these
G-quartets were transformed into a G-ribbon on further increasing the concentration. Gelation occurred
when the G-ribbon self-assembled into a hexagonal columnar structure with the help of intermolecular
hydrogen-bonding and hydrophobic interactions. This gel was sensitive to sonication and underwent a
morphology change from a columnar structure to a flower-like structure composed of flakes. In contrast,
due to steric hindrance, compound 2 only assembled into a spherical structure based on hydrophobic
interactions.
Received 13th November 2012,
Accepted 10th January 2013
DOI: 10.1039/c3ob27204d
www.rsc.org/obc
template cation, except in a few special cases. Sessler et al.
attached a dimethylaniline moiety to the C8 position of guano-
Introduction
Supramolecular self-assembly through non-covalent inter- sine and obtained a G-quartet in the absence of alkali metal
actions, including hydrogen-bonding, π–π stacking, hydrophi- cations, which showed how synthetic chemistry could be used
lic and hydrophobic interactions, electrostatic interactions, to produce unnatural nucleobases for the non-covalent syn-
and van der Waals forces, offers potential applications in thesis of stable supramolecular assemblies.10 Besenbacher
biology and nanotechnology.1 DNA nucleobases carry the key et al. showed that guanine was able to form an “empty”
information of inheritance by utilizing a variety of cooperative G-quartet network on a gold surface, a process that was kineti-
and non-covalent interactions.1 Among nucleobases, guanine cally rather than thermodynamically controlled.11 Highly
is of particular interest. Due to the abundant proton donor ordered supramolecular motifs formed by guanosine deriva-
and acceptor sites, which are fundamental in self-recognition tives may be reversibly interconverted, as exemplified by the
and self-assembly processes, guanosine can self-assemble into transformation between G-quartet-based architectures and
dimeric,2 G-quartet,3 ribbon-like,4 and helical structures5 hydrogen-bonded G-ribbons with the help of an exterior syner-
under certain circumstances, which makes it a promising gistic effect of potassium ions, [2.2.2]cryptand, and trifluoro-
moiety for designing functional materials.6 In particular, methanesulfonic acid.12 Meanwhile, DNA G-quadruplexes have
G-quartets have attracted considerable attention over the past been proposed to play an important role in the maintenance
two decades because of their additional biological signifi- of telomere length as they cannot be extended by telomerase.
cance.7 For example, DNA sequences with guanine-repeated Mashimo et al. demonstrated that hairpin and triplex struc-
G-quadruplex secondary structures have been considered as tures were energetically feasible intermediates along the telo-
novel targets for anticancer drug therapy.8,9
meric DNA G-quadruplex folding pathway.13 Therefore,
Guanosine analogues usually self-assemble into hydrogen- exploration of the conversion between G-quartets and other
bonded dimers or ribbons in the absence of an appropriate self-assembled forms of guanosine is very useful for obtaining
a better understanding of the formation and stability of DNA
G-quadruplexes.
Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433,
Amphiphilic compounds have received much attention over
China. E-mail: yitao@fudan.edu.cn; Fax: +86-21-55664621; Tel: +86-21-55664330
the past few decades because of their particular structural
†Electronic supplementary information (ESI) available: Details of supplementary
spectra and images. See DOI: 10.1039/c3ob27204d
characteristics and importance in biological research.14 Some
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Synthesis of 1 and 2
The synthesis of 1 and 2 is shown in Scheme 1. 2′,3′-Isopropyli-
dene guanosine and (N-adamantylcarbamoyl) propionic acid
were synthesized according to the previous reports17 and
characterized by 1H NMR (see details in ESI†).
Synthesis of 1. A mixture of 2′,3′-isopropylidene guanosine
(0.32 g, 1.0 mmol), (N-adamantylcarbamoyl)propionic acid
(0.30 g, 1.2 mmol), DMAP (0.12 g, 1.0 mmol) and EDC HCl
(0.80 g, 4.0 mmol) in dry DMF (10 mL) was stirred overnight at
room temperature under an argon atmosphere. The solvent
was removed using a rotary evaporator equipped with high
vacuum. The residue was dissolved in CH2Cl2 (100 mL),
washed with water (50 × 3 mL). The organic layer was collected
and evaporated to dryness; the crude product was purified by
column chromatography [SiO2, CH2Cl2–MeOH (10 : 1, v/v)] to
give a white solid (yield 90%). Mp: 251–253 °C. 1H NMR
(400 MHz, DMSO-d6, 298 K): δ 10.69 (s, 1H), 7.87 (s, 1H), 7.32
(s, 1H), 6.53 (s, 2H), 6.00 (d, J = 2.1 Hz, 1H), 5.22 (dd, J = 6.3,
2.1 Hz, 1H), 5.15–5.07 (m, 1H), 4.30–4.19 (m, 2H), 4.19–4.09
(m, 1H), 2.42 (d, J = 7.2 Hz, 2H), 2.30 (d, J = 7.0 Hz, 2H), 1.97
(s, 3H), 1.88 (s, 6H), 1.59 (s, 6H), 1.51 (s, 3H), 1.32 (s, 3H).
13C NMR (100 MHz, DMSO-d6, 298 K): δ 172.37, 170.01,
156.90, 153.80, 150.61, 136.26, 116.97, 113.37, 88.42, 84.48,
83.86, 81.09, 64.03, 50.67, 41.04, 36.10, 30.59, 29.09, 28.90,
27.04, 25.31. HRMS (ESI, m/z): calcd for C27H36N6O7 [M + H]+,
557.2724; found: 557.2730.
Scheme 1 Chemical structure and synthetic route of 1 and 2.
amphiphilic molecules can form vesicles, tubes, or even gel
networks.15 Low-molecular-weight organogels (LMOGs) are
especially interesting due to their potential applications in
materials, sensors, drug delivery, and so on.16 Amphiphilic
guanosine derivatives obtained by modification of glycosyl
with a hydrophobic group not only possess the intrinsic prop-
erties of guanine itself, but also an additional hydrophobic
interaction, which may be exploited to manipulate molecular
self-assembly behavior by adjusting the micro-environment,
such as concentration, solvent, etc. In this work, we have
designed and synthesized two amphiphilic guanosine deriva-
tives by modifying the glycosyl group of guanosine with an
adamantane moiety (1 and 2 in Scheme 1). Surprisingly, we
found that 1 could form G-quadruplexes at low concentrations
in acetonitrile solution without templating ions, and these
G-quadruplexes were transformed into a gel network on
increasing the concentration. The driving force and the
mechanism of this structural transformation by molecular
self-assembly have been studied in depth.
Synthesis of 2. 2 was prepared with the same method for 1
using guanosine (0.09 g, 0.32 mmol) instead of 2′,3′-isopropyli-
1
dene guanosine (0.40 g, 1.0 mmol). Mp: 198–200 °C. H NMR
(400 MHz, DMSO-d6, 298 K) δ 10.73 (s, 1H), 7.92 (s, 1H), 7.34
(d, J = 26.8 Hz, 3H), 6.55 (s, 2H), 5.98 (d, J = 5.6 Hz, 1H),
5.84–5.65 (m, 1H), 5.49 (s, 1H), 4.31 (dd, J = 42.8, 9.8 Hz, 3H),
2.54 (d, J = 6.3 Hz, 2H), 2.40–2.20 (m, 6H), 2.08–1.74 (m, 27H),
1.59 (s, 18H). 13C NMR (100 MHz, DMSO-d6, 298 K) δ 172.28,
171.65, 171.52, 169.88, 169.77, 169.60, 156.81, 153.98, 151.21,
135.75, 130.72, 128.82, 116.94, 84.76, 79.72, 72.27, 70.35,
63.33, 50.71, 50.67, 41.05, 36.14, 30.54, 30.42, 28.94, 28.78.
HRMS (ESI, m/z): calcd for C52H70N8O11 [M + H]+, 983.5242;
found: 983.5223.
Experiment
General
Gelation test for organic fluids
The gelator and solvent were put in a septum-capped test tube
and heated (>80 °C) until the solid was dissolved. The sample
vial was then cooled to 25 °C (room temperature). Qualitatively,
gelation was considered successful if no sample flow was
observed upon inversion of the container at room temperature
(the inverse flow method).
All of the starting materials were obtained from commercial
suppliers and used as received. Moisture sensitive reactions
were performed under an atmosphere of dry argon. Amanta-
dine hydrochloride (99%) was provided by Alfa Aesar, and
other chemicals were supplied from Sinopharm Chemical
Reagent Co., Ltd (Shanghai). Column chromatography was
carried out on silica gel (200–300 mesh). The 1H NMR
(400 MHz) and 13C NMR (100 MHz) spectra were recorded on a
Techniques
Mercury plus-Varian instrument. Proton chemical shifts are FTIR spectra were recorded by using an IRPRESTIGE-21 spec-
reported in parts per million downfield from tetramethylsilane trometer (Shimadzu). SEM images were obtained using an
(TMS). HR-MS was recorded on an LTQ-Orbitrap mass spec- FE-SEM S-4800 (Hitachi) instrument. Samples were prepared
trometer (Thermo Fisher, San Jose, CA). Melting points were by spinning the samples on glass slices and coating with Au.
determined on a hot-plate melting point apparatus XT4-100A The samples were prepared by coating the diluted wet gels on
without correction.
a copper grid at room temperature and freeze drying (EYELA,
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FDU-1200) for 24 h. Powder X-ray diffractions were generated template in chlorinated organic solvents.4 Protic solvents, such
by using Philips PW3830 sealed-tube X-ray generator as ethanol and methanol, are not conducive to molecular self-
a
(Cu target, λ = 0.1542 nm) with a power of 40 kV and 50 mA. assembly of lipophilic guanosine derivatives because of their
CD (circular dichroism) spectra and UV-Vis absorption spectra competing hydrogen bonding. As we expected, 1 and 2 were
were recorded on a MOS-450 spectropolarimeter and a UV-vis found to dissolve freely in many polar solvents, including
2550 spectroscope (Shimadzu), respectively.
chloroform, ethanol, methanol, tetrahydrofuran, dimethylform-
amide, dimethylsulfoxide and dioxane, whereas they proved
to be insoluble in hexane and precipitated from ethyl acetate.
However, 1 formed a transparent gel and an opaque gel after a
heating–cooling process with critical gelation concentrations
(CGCs) of 5 mg mL−1 and 25 mg mL−1 in dichloromethane
and acetonitrile, respectively. Moreover, ultrasound had an
influence on the gel formation of 1. Sonication reduced its
CGC to 20 mg mL−1 in acetonitrile, and promoted the gelling
of 1 in chloroform, with a CGC of 25 mg mL−1. It is obvious
that the balance between hydrophobicity and hydrophilicity, as
well as the steric effect react on the gelation properties of these
compounds. The three adamantane moieties in compound 2
increase the hydrophobic property and the steric effect, thus
reduce its gelation capability. Compound 2 was found to be
soluble in hot acetonitrile (above 100 °C), but gradually preci-
pitated after cooling to room temperature. Hence, the self-
assembly behavior of the two compounds in acetonitrile was
studied in more detail.
Calculation of the equilibrium constant K for the G-quartet
The equilibrium for the formation of G-quartet structure was
shown by eqn (1) and the equilibrium constant, K, was given
by eqn (2):18
4½Gꢀ $ ½G4ꢀ
ð1Þ
ð2Þ
4
4
K ¼ ½G4ꢀ=½Gꢀ ¼ ½G4ꢀ=ðc0 ꢁ 4½G4ꢀÞ
where c0 represents the total concentration of initial 1. As we
know, the CD ellipticity (θ) meets eqn (3):
θ ¼ 2:303ðAL ꢁ ARÞ=4 ¼ 0:576Δεcl
ð3Þ
where Δε, c and l represent the difference values of molar
extinction coefficients for left and right circularly polarized
light, the molar concentration of an optically active molecule
and the path length, respectively. Thus eqn (3) can be simpli-
fied into (4). The equilibrium constant K for the G-quartet can
be obtained by eqn (5):
At the present time, the outstanding sensitivity of circular
dichroism (CD) makes it the main tool for studying biological
macromolecular chirality and perturbations by external
factors.20 In order to gain information on the structure and
mechanism of gel formation, a large range of concentrations
of 1 in CH3CN from solution (1.0 × 10−4 M) to stable gel state
(0.1 M) were investigated by CD spectroscopy, and the spectra
are shown in Fig. 1. For guanosine derivatives, if the tetramers
of the G-quartet-based assemblies were rotated with respect to
one another, a double-signed CD signal would be expected in
the region 230–330 nm, characteristic of the π–π* transitions
of the guanosine chromophore. It is evident from Fig. 1a that
½G4ꢀ ¼ kθ
ð4Þ
ð5Þ
K ¼ kθ=ðc0 ꢁ 4kθÞ
where k represents the coefficient.
Results and discussion
The solubility and gelation properties of 1 and 2 are summar-
ized in Table 1. To our knowledge, lipophilic guanosine deriva-
tives are often present in the form of monomeric species in
aprotic solvents, especially in dimethylsulfoxide,19 but are
likely to form G-ribbon structures in the absence of a cationic
the CD signal of 1 at a concentration lower than 2.5 × 10−4
M
was very weak. When the concentration was increased to
5.0 × 10−4 M, two opposite signed bands at 263 and 293 nm
appeared, the intensity of which increased with increasing
concentration up to 5.0 × 10−2 M (Fig. 1a and 1b). According to
a previous report, the presence of opposite signed bands at ca.
290 and 260 nm is diagnostic of heteropolar stacking.21 The
CD spectra of 1 strongly suggest a G-quartet conformation with
heteropolar stacking having D4 symmetry. The equilibrium
constant of G-quartet formation was calculated from the CD
spectral change in the concentration range 5.0 × 10−4 to
3.0 × 10−3 M (see details in the experiment part). The influence
of concentration on the CD ellipticity at 263 nm is illustrated
in the inset in Fig. 1a. The equilibrium constant K for
G-quartet formation was estimated to be 6.3626 × 109 M−3
from a linear fitting with a coefficient of k = 4.7455 × 10−6
Table 1 Gelation properties of 1 and 2a
Solvent
1
2
CH3CN
CH2Cl2
CHCl3
MeOH
EtOH
EtOAc
THF
DMSO
DMF
Hexane
G (25)b, G (20)c
P
S
S
S
S
P
S
S
S
I
G (5)b
Sb, G (25)c
S
S
P
S
S
S
I
a G = gel; P = precipitation; S = solution; I = insoluble. The critical (R2 = 0.9956).
With a further increase in concentration from 1.0 × 10−2 to
gelation concentrations of the gelators are given in the parentheses
(mg mL−1). b Heated to dissolve, then cooled to room temperature.
c Heated to dissolve, then treated with ultrasound for 30 s, ultrasound
power: 0.16 W cm−2, 40 kHz.
5.0 × 10−2 M (gel formation), the band located at 263 nm
gradually shifted to longer wavelengths and diminished in
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Fig. 2 (a) CD spectral changes of the gel formed by 1 in acetonitrile (0.1 M)
upon addition of 0, 1/3, 1, 3, 9, 12.3 to 19 equivalent volumes of acetonitrile;
(b) temperature variable CD spectra of G-quartets (2.5 × 10−3 M, cell length
0.5 mm) and gel (inset, 0.1 M, cell length 0.1 mm) of 1 in acetonitrile, tempera-
ture from 25 to 60 °C with an interval of 5 °C.
The corresponding UV/visible absorption spectra of 1 were
also examined and showed two absorption bands at 254 and
325 nm at lower concentrations before the G-quartets were
formed (5.0 × 10−4 M) (Fig. 1e), which could be ascribed to
π–π* and n–π* transitions (C6vO) of the guanine chromo-
phore, respectively. When the concentration was higher than
5.0 × 10−4 M, G-quartets were formed with C6vO being
involved in the intermolecular hydrogen bonds, so that the
n–π* transition of the guanine chromophore was weakened. As
a result, the absorption band centered at 325 nm disappeared
(Fig. 1f). In the gel formed at a concentration of 5.0 × 10−2 M,
a red-shifted shoulder band at 284 nm was observed, indicat-
ing a π–π interaction between the chromophores. This conjec-
ture was consistent with the experimental CD data.
Fig. 1 CD spectra of 1 in CH3CN under different concentrations: (a) 2.5 ×
10−4–3.0 × 10−3 M (inset is the Job’s plot for CD ellipticity at 263 nm in the con-
centration range of 5.0 × 10−4–3.0 × 10−3 M; cell length, 1.0 mm); (b) 5.0 ×
10−3–0.1 M (cell length, 0.1 mm); CD spectra of 1 in CH3CN with concentrations
of (c) 3.6 × 10−2 M and (d) 1.0 × 10−2 M before (T) and after sonication (S); UV-
visible absorption spectra of 1 in CH3CN solution with different concentrations
of (e) 5.0 × 10−5–5.0 × 10−4 M (cell length, 0.5 mm), and (f ) 1.0 × 10−3–3.0 ×
10−3 M and gel in CH3CN (5.0 × 10−2 M) (cell length, 0.1 mm; absorbance of gel
was divided by 2 for clear comparison).
To check the reversibility of the self-assembly process, the
transformation from the gel network to G-quartets upon
dilution of the self-assembled structure with acetonitrile was
investigated by CD spectroscopy (Fig. 2a). When the gel was
diluted from 0.1 M to 7.5 × 10−2 M in acetonitrile, two opposite
signed bands at 269 and 295 nm appeared in place of the orig-
inal negative band located at 297 nm. Upon further dilution of
the gel to 2.5 × 10−2 M, the opposite signed bands were gradu-
ally blue-shifted to 263 and 293 nm, at which they remained
upon further dilution with acetonitrile. Thus, the acetonitrile
gel of 1 completely reverted to G-quartets upon dilution.
Furthermore, to gain insight into the stability of the G-quartets
and the gel, the temperature dependences of the CD spectra of
the G-quartets (2.5 × 10−3 M) and the G-ribbon (gel state,
0.1 M) of 1 in acetonitrile were measured (Fig. 2b). The pos-
itions of the CD bands at 263 and 293 nm characteristic of
G-quartets remained unchanged with increasing temperature,
but they showed a sharp decrease in intensity and eventually
disappeared at 60 °C. This indicated that the G-quartet struc-
ture was thermodynamically unstable, dissociating to the
monomer at higher temperature. In contrast, the CD spectra of
the G-ribbon structure in the gel state showed no obvious
changes in either location or intensity when the temperature
was increased from 25 to 60 °C, which indicated that the
G-ribbon structure was comparatively stable within the studied
temperature range (Fig. 2b, inset).
intensity, while the opposite band located at 293 nm intensi-
fied with no wavelength change. Beyond 5.0 × 10−2 M, the
single band located at 293 nm was gradually shifted to 297 nm
and slightly diminished in intensity during the stable gel for-
mation. According to previously reported results,22 a negative
band of moderate intensity at around 300 nm supported
the presence of stacked monomers or stacked dimers. In
summary, 1 first self-assembled into a G-quartet at low concen-
tration. The G-quartet dissociated with increasing concen-
tration, and the molecules subsequently aggregated in the
form of stacked monomers or stacked dimers upon gel
formation.
The CD spectra of the sample (3.6 × 10−2 M, 20 mg mL−1
)
before and after sonication were compared. The positions of
the CD bands of the gel (S-gel) after sonication were slightly
red shifted from 294 to 295 nm, but the intensity obviously
decreased (Fig. 1c). This phenomenon is similar to the effect
of increasing concentration. The decreased intensity of the
S-gel compared to the sol with the same concentration can be
ascribed to the increased accumulation after sonication. This
may be the reason for the decrease of CGC by sonication.
However, for the sample with the concentration lower than
CGC of S-gel, sonication has almost no influence on the CD
spectra (Fig. 1d).
Moreover, the CD spectra of 1 in other solvents, such as
tetrahydrofuran, chloroform, dimethylsulfoxide (DMSO), and
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Fig. 4 1H NMR spectra of 1 with different concentrations (a) 5.0 × 10−3 M, (b)
1.0 × 10−2 M, (c) 2.5 × 10−2 M and (d) 5.0 × 10−2 M in CD3CN.
Fig. 3 CD spectra of 1 in (a) tetrahydrofuran and (c) chloroform in the concen-
tration range of 2.5 × 10−4–1.0 × 10−2 M; temperature variation CD spectra of 1
in (b) tetrahydrofuran and (d) chloroform (2.5 × 10−3 M) (cell length, 0.5 mm).
anti-conformer decreased and all of the signals became broad.
methanol, at different concentrations were also measured. The When the concentration was lower than 1.0 × 10−2 M, the
CD spectra of 1 in tetrahydrofuran were similar to those in
typical signal of H-C8 was split into two bands, located at
acetonitrile, with two opposite signed bands appearing at 263
δ ≈ 7.7 and 7.4 ppm. The signal of H-C8 in the syn conformer
and 293 nm, the intensity of which decreased with increasing was observed at δ ≈ 7.4 ppm, which is typical of structures
formed by assembled G-quartets.23 The signal of the –NH2
temperature (Fig. 3a and 3b). In chloroform (Fig. 3c and 3d),
although two opposite bands were seen at 266 and 294 nm,
protons originally located at δ = 6.5 ppm in DMSO was split
neither the positions nor the intensities of these two bands into two broad bands at δ = 9.3 ppm (HA-NH2) and 6.0 ppm
(HB-NH2) in the concentration range 5.0 × 10−3 to 1.0 × 10−2 M
showed any obvious changes with increasing temperature,
indicating that the G-quartet self-assembly of 1 in chloroform
in CD3CN. This splitting of the resonance of the –NH2 protons
was more stable than that formed in acetonitrile. The was also indicative of G-quartet formation, with HA-NH2 being
involved in H-bonding.24 Other protons showed almost no
G-quartet formed in chloroform could not be transformed into
a gel by increasing the concentration. However, solutions of 1
change in chemical shifts but broadened with the increased
in chloroform at concentrations higher than 25 mg mL−1 concentration. Therefore, we could conclude that G-quartets
(4.5 × 10−2 M) could be gelled with the aid of sonication. No
signal was detected in the CD spectrum of 1 in DMSO, indicat-
were formed before the concentration reached 1.0 × 10−2 M.
The broadening and the downfield shift of the signal of
ing that the molecule remained predominantly in a mono- HA-NH2 with increasing concentration signified dissociation of
meric form in this solvent (Fig. S1, ESI†). CD spectra recorded
in methanol were similar to that in DMSO, except for some
the G-quartets and the formation of a G-ribbon structure.
When the concentration reached 5.0 × 10−2 M, the NH proton
slight fluctuation at high concentrations, which may have been signal of the amantadine group at δ = 5.62 ppm split into two
due to hydrogen-bonding competition between 1 and metha-
nol (Fig. S2, ESI†).
peaks at δ = 5.76 and 5.57 ppm, which indicated that the
amantadine amide group participated in the intermolecular
The corresponding 1H NMR spectra also proved the for- H-bond self-assembly. In other words, two kinds of intermole-
mation of G-quartets. The 1H NMR spectral region between
cular interactions were involved in the aggregation of 1 in the
δ = 4 and 13 ppm proved to be useful for characterizing the
self-assembly of guanosine derivatives, especially the associ-
ation of G-quartets. The 1H NMR spectrum of 1 in DMSO-d6
showed sharp and well-defined signals that were indicative of
the predominant presence of monomeric species (Fig. S3,
ESI†). In CD3CN (Fig. 4), the singlet signal of the imino proton
(H-N1) of 1 was downfield shifted from δ = 10.69 ppm and
split into three peaks in the region δ = 12.1–12.3 ppm. This
indicated G-quartet stacking involving both anti (located at
lower field) and syn conformers.23 The chemical shifts of these
signals remained unchanged when the concentration of
the solution was increased from 5.0 × 10−3 to 5.0 × 10−2 M,
but the relative intensity of the signals attributable to the
gel state, the first being the assembly of two molecules of 1
into a G-ribbon building block through multiple intermolecu-
lar hydrogen bonds between guanine moieties, and the second
being the further aggregation of the G-ribbons through inter-
molecular hydrogen bonding of amantadine amide groups
with the cooperation of hydrophobic interactions.
The morphology and structure of the gel formed by 1 were
investigated by scanning electron microscopy (SEM) and X-ray
diffraction analysis (XRD). SEM images of the xerogel of 1
obtained after a heating–cooling process (T-xerogel) showed a
hexagonal columnar structure with the width of the column in
the range 4–9 μm (Fig. 5a and 5b). Meanwhile, SEM images of
the S-xerogel of 1 obtained by sonication revealed a twisted
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Fig. 7 IR spectra of xerogel of (a) T-gel, (b) S-gel of 1 and (c) precipitation of 2.
In contrast, with three adamantane groups connected to
the glycosyl group of guanosine, 2 precipitated from aceto-
nitrile after a heating–cooling process. SEM images of the pre-
cipitate revealed globular structures of differing diameters in
the range 100–300 nm (Fig. 5e and 5f). Transmission electron
microscopy (TEM) revealed that these globular structures were
solid (Fig. S4, ESI†). The XRD profile of the precipitate of 2
showed a peak at 23.4 Å (Fig. S5, ESI†), close to the length of
the molecule of 2 (Fig. S6, ESI†). Evidently, the significant
steric effect caused by the three adamantane branches greatly
weakened the intermolecular hydrogen-bonding interaction
between the guanine moieties of 2. Therefore, 2 shows better
solubility in most polar solvents.
Fig. 5 SEM images of T-xerogel (a and b) and S-xerogel (c and d) of 1 in
CH3CN; SEM images of precipitation of 2 in CH3CN (e and f ); scale bar: (a)
50 μm, (b) 10 μm, (c) 50 μm, (d) 10 μm, (e) 2 μm, (f ) 300 nm.
The difference in the intermolecular hydrogen-bonding pat-
terns seen in 1 and 2 was corroborated by infrared (IR) spec-
troscopy. The IR spectra of the gels of 1 obtained with or
without sonication were almost the same, showing two peaks
at ν = 1688 and 1638 cm−1 corresponding to a hydrogen-
bonded CvO vibration in the gel state.25 This implies that
C6vO plays a central role in the intermolecular hydrogen
bonding of guanosine derivatives. Compared with the spec-
trum of 1, sharp peaks appeared at ν = 1745 and 1649 cm−1 in
the infrared spectrum of 2, evidencing that C6vO and
branched ester groups did not participate in the intermole-
cular hydrogen-bonding interaction. This result was consistent
with the morphological and structural studies (Fig. 7).
On the basis of the data presented above, the self-assembly
processes of 1 and 2 can be clearly understood. Both the CD
and NMR spectra support the formation of G-quartets consist-
ing of two stacked G4 units with D4 symmetry through mul-
tiple complementary N1–H⋯O6 and N2–H⋯O7 hydrogen
bonds between the guanosine groups of 1 in acetonitrile in the
concentration range 5.0 × 10−4 to 1.0 × 10−2 M (Fig. S7a, ESI†).
It is remarkable that a derivative such as 1 can form G4-quar-
tets in the absence of templating ions. We checked all of the
experimental details and confirmed that no contaminant ions
were present during the synthetic process. We noticed the for-
mation of intermolecular interactions between amantadine
groups, which may stabilize the stacking of G-quartets in
dilute acetonitrile solution and also promote the formation
Fig. 6 Powder X-ray diffraction patterns of xerogel of 1 from CH3CN at room
temperature.
flower-like structure made up of numerous sheets, with the
average width of each sheet being about 2 μm (Fig. 5c and 5d).
However, the xerogels of 1 formed with or without sonication
displayed similar X-ray diffraction profiles (Fig. 6). This indi-
cated that the two kinds of gels were constructed from the
same substructure. Small-angle X-ray scattering measurements
showed a series of peaks corresponding to d-spacings of 28.5,
16.8, and 14.0 Å, with a ratio of 1 : 1/√3 : 1/√4, indicating the
formation of a Colh structure with a structure parameter of
a = 32.9 Å, consistent with the result from the SEM images.
Molecular modeling suggested that two molecules of 1 were
assembled through intermolecular hydrogen bonds between
guanine moieties at a spacing of approximately 32.7 Å (Fig. S7,
ESI†), which is very close to the value of the parameter
a (32.9 Å). Thus, we speculated that 1 self-assembled into a
G-ribbon structure via dimer units and further aggregated into
a hexagonal columnar structure in the gel state.
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G-quartets and other self-assembled forms of guanosine has
provided insight into the formation and stability of DNA
G-quadruplexes. Moreover, it is evident that structural differ-
ences have a significant impact on the self-assembly mode in
the construction of functional materials based on guanosine.
This work was supported by the China National Funds for
Distinguished Young Scientists (21125104), National Natural
Science Foundation of China (91022021), National Basic
Research Program of China (2009CB930400, 2013CB733700),
Program for Innovative Research Team in University (IRT1117),
Program of Shanghai Subject Chief Scientist (12XD1405900),
and Shanghai Leading Academic Discipline Project (B108).
Fig. 8 The proposed structures of self-assembled G-quartets and G-ribbons by
molecules of 1.
and growth of G-ribbons with increasing concentration until
the CGC is reached (Fig. 8). There are two kinds of G-ribbon
structures, with the pairs of hydrogen bonds being N1–H⋯N7
and N2–H⋯O6 (type A) or N2–H⋯N7 and N1–H⋯O6 (type B),
respectively (Fig. S7b and c, ESI†). Red-shifts of the CD and
UV/visible absorption bands with increasing concentration
indicated an increased dipole polarization in the aggregated
state, which strongly supported the formation of a G-ribbon of
type A. NMR spectra also indicated a decrease in the amount
of the anti-rotamer of 1 in the formation of the G-ribbon. The
most important finding was the tunable self-assembly of 1 by
concentration and sonication, that is, the G-ribbon structure
in the gel state could revert to G-quartets by dilution of the gel.
In contrast, it was difficult to form ordered aggregates of 2 in a
solvent due to its steric effect. This study has furthered our
understanding of the formation, stability, and transformation
of G-quartets.
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