Quartet formation of a guanine derivative with an isopropyl group: Crystal structures of naked G-quartets and thermodynamics of G-quartet formation

Article abstract of DOI:10.1039/c2ob26877a

The formation of guanine quartets with 9-isopropylguanine (ⁱPG) is discussed in organic solvents. Crystal structures of the ⁱPG quartets were determined by X-ray crystallography with template cations (Na ⁺ and Ca²⁺) and the structure without a template cation was also obtained by virtue of the stabilization by intermolecular hydrogen bonding with water molecules of crystallization. The difference in the quartet formation of ⁱPG in the presence and absence of a template cation was clearly demonstrated by ¹H NMR measurements in CDCl ₃-CH₃OH mixed solvents. The quartet formation is mainly governed by the enthalpy gain due to the electrostatic interaction between the O6 oxygen in ⁱPG and the template cations in the presence of the cations rather than the intermolecular hydrogen bonding, while desolvation of ⁱPG is the dominant factor for the formation in the absence of cations. In the presence of Na⁺ and Ca²⁺, ΔH and ΔS values in the formation of ⁱPG-4-Na⁺ and ⁱPG-4-Ca²⁺ complexes were determined to be ΔH = -8.4 kcal mol^-1 and ΔS = +50 cal mol^-1 K^-1 for Na⁺ and ΔH = -12.9 kcal mol^-1 and ΔS = +34 cal mol^-1 K^-1 for Ca²⁺ on the basis of van't Hoff plots attained from the results of temperature-dependent UV-Vis spectroscopic measurements. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2ob26877a

Organic &
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Quartet formation of a guanine derivative with
an isopropyl group: crystal structures of “naked”
Cite this: Org. Biomol. Chem., 2013, 11,
758
G-quartets and thermodynamics of G-quartet formation†
Yuji Inui,a,b Motoo Shiro,c Shunichi Fukuzumi*a,d and Takahiko Kojima*b
The formation of guanine quartets with 9-isopropylguanine (iPG) is discussed in organic solvents. Crystal
i
structures of the PG quartets were determined by X-ray crystallography with template cations (Na+ and
Ca2+) and the structure without a template cation was also obtained by virtue of the stabilization by
intermolecular hydrogen bonding with water molecules of crystallization. The difference in the quartet
formation of iPG in the presence and absence of a template cation was clearly demonstrated by 1H NMR
measurements in CDCl3–CH3OH mixed solvents. The quartet formation is mainly governed by the
i
enthalpy gain due to the electrostatic interaction between the O6 oxygen in PG and the template
cations in the presence of the cations rather than the intermolecular hydrogen bonding, while desolva-
tion of iPG is the dominant factor for the formation in the absence of cations. In the presence of Na+ and
Ca2+, ΔH and ΔS values in the formation of iPG-4–Na+ and iPG-4–Ca2+ complexes were determined to be
ΔH = −8.4 kcal mol−1 and ΔS = +50 cal mol−1 K−1 for Na+ and ΔH = −12.9 kcal mol−1 and ΔS = +34 cal
mol−1 K−1 for Ca2+ on the basis of van’t Hoff plots attained from the results of temperature-dependent
UV-Vis spectroscopic measurements.
Received 24th September 2012,
Accepted 27th November 2012
DOI: 10.1039/c2ob26877a
www.rsc.org/obc
Introduction
Since Gellert and coworkers suggested the guanine tetrad
structure was linked by intermolecular hydrogen bonding in
1962,1 the characteristic guanine-quartet (hereafter referred to
as G-4) structure (Fig. 1) has attracted a great deal of attention
of researchers in many different areas.2–8 Especially, integrated
G-4 structures which exist at the edge of the DNA chain, called
guanine quadruplexes (hereafter denoted as G-quadruplex),
have been reported in large numbers.9–15 So far, many kinds of
G-quadruplexes included in higher-order structures have been
crystallised with various template cations.16,17 In parallel,
studies on the interaction of molecules to show a certain selec-
tivity for the part of the G-quadruplex have been
Fig. 1 Schematic description of G-quartet (G-4).
conducted.18–27 The main purpose of those studies has been
the acquirement of an anticancer effect. The effect is based on
inhibition of telomerase binding to the G-quadruplex moiety
of DNA by protecting the moiety with the molecules. G-quadru-
plexes have also been utilized to develop functional biomater-
ials: the cavity of G-quadruplex can include guest ions, which
can be applied to ion-conducting materials and so on.28,29
The study on solution behaviour of guanine and its deriva-
tives in organic solvents should be important for elucidating
the dynamics of the G-4 structure formation by intermolecular
hydrogen bonding in detail. However, the formation dynamics
of a discrete G-4 structure is yet to be well investigated. The
aDepartment of Material and Life Science, Graduate School of Engineering, Osaka
University and ALCA (JST), 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan.
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp; Fax: (+) 81-6-6879-7370
bDepartment of Chemistry, Graduate School of Pure and Applied Sciences, University
of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8571, Japan.
E-mail: kojima@chem.tsukuba.ac.jp; Fax: (+) 81-29-853-4323
cX-ray Laboratory, Rigaku Corporation, 3-9-12 Matsubara, Akishima, Tokyo
196-8666, Japan
dDepartment of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
†Electronic supplementary information (ESI) available. CCDC 898242, 898243,
898244, 898245. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2ob26877a
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Fig. 2 The structure of 9-isopropylguanine (iPG).
lack of the thermodynamic data on the formation of a discrete
and single-layer G-4 structure has hampered the discussion on
the details of the G-4 formation, including the driving force of
the formation. In fact, G-quadruplex structures involve too
many complicated factors including noncovalent intermolecu-
lar interactions30 provided by many functional groups such as
sugar and phosphate moieties, which cover the part of G-4
cores.31,32 Therefore, interactions of external molecules with a
G-quadruplex have been observed as interactions with the per-
ipheral groups, not direct interactions with the G-4 units. In
addition, the conformational regulation of the peripheral moi-
eties has been considered to be difficult for the formation of
supramolecular assemblies of guanine derivatives linked only
by intermolecular hydrogen bonding in solutions.33–35 Thus,
G-4 formation with the use of a guanine derivative without
complicated peripheral groups is indispensable to clarify the
intrinsic thermodynamics of the complementary hydrogen
bonding and interaction of G with a template cation in a dis-
crete and single-layer G-4 structure.
In this work, 9-isopropylguanine (iPG)36 having the isopro-
pyl group at the 9-N position was used to form the G-4 struc-
ture (Fig. 2). The introduction of the small isopropyl group
allowed us not only to improve the solubility into organic
solvents but also to accommodate the crystallization of
“naked” G-4 supramolecules, without any steric protection of
the G-4 core by such deoxyribose-phosphate moieties in
guanosine derivatives reported so far. The strategy presented
herein makes it possible for the first time to discuss the
intrinsic characteristics of the G-4 structure and the detailed
thermodynamics of the G-4 formation to elucidate the role of
the template cations. Based on the observations, we would
clarify the switching of the driving force of the G-4 formation
from desolvation of the guanine molecule in the absence of
template metal ions to electrostatic attractive interactions
i
Fig. 3 Crystal structures of quartets of PG with and without template cations:
i
(a) ORTEP drawing of the PG-4 structure without template cations (70% prob-
ability thermal ellipsoids); (c) that with Na+ derived from NaBPh4 (30% prob-
ability thermal ellipsoids). BPh4− was omitted for clarity; (e) that with Ca2+ (70%
probability thermal ellipsoids). Partial packing structures for (a), (c), and (e) are
depicted in (b), (d) and (f), respectively.
(iPG-4) structure in the crystal with the aid of two water mole-
cules (iPG-4–H2O; Fig. 3(a) and Fig. S1 in ESI†). As observed in
the G-quadruplex, the G-4 assembly was formed by the comp-
lementary intermolecular hydrogen bonding between the O6
and N1H atoms and that between the 2-amino group and the
7-nitrogen atom. Intermolecular hydrogen bonding was
observed among the water molecule of crystallization and the
i
i
between PG and metal ions in the presence of the template
oxygen atoms at the 6-position in PG showing the interatomic
cations.
distance (O1⋯O3(W) and O2⋯O3(W), 2.85 and 3.03 Å, respecti-
vely). This hydrogen bonding could assist the G-4 formation
to compensate for the lack of stabilization by template cations.
In the packing structure, the iPG-4 planes overlap partially due
to CH/π and π–π interactions (Fig. 3(b)). The integration of the
iPG-4 planes by stacking as seen in the G-quadruplex could not
be observed, because the two hydrogen atoms of the water
Results and discussion
1. Crystal structures of G-quartets
In all cases, crystallization of G-4 assemblies was conducted in
methanol solutions with vapour diffusion of diethyl ether.
Crystal structures of the G-4 assemblies made of iPG are
shown in Fig. 3. Even in the absence of a template cation, it
was clearly demonstrated that iPG could form the quartet
i
molecule of crystallization can direct only to one PG-4 plane
and the water molecule cannot link two iPG-4 planes. Although
a
G-4 structure without template cations has been
reported,37,38 no report has appeared on “naked” G-4
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Table 1 Summary of the interatomic distances related to template cations,
i
those of N1H⋯O6 as intermolecular hydrogen bonding in PG-4 structures, and
interplane distances between two G-4 assemblies
iPG-4–H2O
iPG-4–Na+
iPG-4–Ca2+
Water⋯O (Å)
Cation⋯O (Å)
O3⋯O1
2.852(1)
Na1⋯O1
Na1⋯O2
Na1⋯O3
Na1⋯O4
2.257(4)
2.300(3)
2.320(4)
2.357(3)
Ca1⋯O1
Ca1⋯O2
2.3212(9)
3.3232(9)
O3⋯O2
3.030(1)
NH⋯O (Å)
N8⋯O1
N3⋯O2
2.849(2)
2.782(1)
N18⋯O1
N3⋯O2
N8⋯O3
N13⋯O4
2.876(5)
2.878(6)
2.872(4)
2.906(5)
N8⋯O1
N3⋯O2
2.941(2)
2.943(1)
G-4 plane⋯G-4 plane (Å)
3.597
3.626
3.652
Selected bonds
Na1⋯O4′
2.729(4)
2.609(7)
Ca1⋯Cl1
2.6980(4)
Na1⋯O5
Fig. 4 Temperature dependent 1H NMR studies of iPG with NaBPh4. Con-
ditions: (a) iPG (19 mM) and NaBPh4 (5.4 mM) in CDCl3 and CH3OH (600 μl
(30 : 1 v/v)); (b) iPG (3.6 mM) in CDCl3 and CH3OH (600 μl (5 : 1 v/v)).
structures without steric protection by large substituents
attached to the guanine scaffold.
In the presence of sodium cation derived from Na[B(C6H5)4]
i
(NaBPh4), the crystal structure of PG clearly demonstrated the
formation of the G-4 dimer. The Na+ ion resided nearly in the
2. Discussion of G-quartet formation in organic solvents
i
mean plane of the PG-4 assembly. Two of the G-4 assemblies
i
formed a dimer by the electrostatic interaction between Na+
and O6 of guanine in the other iPG-4 plane, showing the
interatomic distance of Na⋯O4′ to be 2.729(4) Å (Fig. 3(c, d)
and Fig. S2 in ESI†). One diethyl ether molecule was found to
bind to the Na+ ion with the interatomic distance of 2.609(7) Å
to cap the dimeric structure, preventing further stacking with
Observations of the PG-4 structure formation in organic sol-
vents were carried out using ESI-MS, UV-Vis and 1H NMR
i
measurements. Since PG is capable of forming a variety of
supramolecular structures by virtue of intermolecular hydro-
gen bonding, including a ribbon structure as shown in Fig. S4
in ESI.† Thus, the regulation of the supramolecular structure
should be carried out under strictly optimized conditions in
organic solvents to form the G-4 structure selectively.
i
the PG-4 assemblies. In the dimeric unit in the crystal, π–π
interactions are operating with the closest interatomic distance
In general, the following issues have been considered: (1)
the proton of N1H fixed into a G-4 assembly by intermolecular
hydrogen bonding does not undergo exchange with deuterium
of solvents, and (2) noncovalent intermolecular interactions
including hydrogen bonding and electrostatic interaction in
G-4 become stronger at low temperature, and then the G-4
structure is stabilized by the decrease of enthalpy beyond the
decrease of entropy.40 In the presence of Na+, the proton
signal of N1H of iPG forming the iPG-4 assembly can be
detected around 12 ppm in the mixed solvent of CH3OH and
CDCl3. The signal became clearer on cooling down to 253 K
(Fig. 4(a)). On the other hand, in the absence of Na+, the iPG-4
structure could not be formed even on cooling down to 253 K.
Instead, the chemical shift of the proton of N1H of iPG
of 3.422(8) Å.
i
We also examined the formation of the PG-4 structures in
the presence of a calcium ion derived from CaCl2. In this case,
i
a PG-4 structure was formed and the Ca2+ ion also resided in
i
the mean plane of the PG-4 structure. Two chloride counter
anions interacted with the calcium ion with the Ca2+⋯Cl− dis-
tance of 2.6980(4) Å both above and below the mean plane of
i
the PG-4 bound to Ca2+ (iPG-4–Ca2+) (Fig. 3(e) and Fig. S3
shown in ESI†). The packing structure is similar to that of the
i
water-assisted PG-4 assembly: As well as in the case of the
water-assisted PG-4, two of the PG-4–Ca2+ assemblies stack
via CH/π and π–π interactions to form a dimeric motif in the
crystal (Fig. 3(f)).
i
i
Each distance between the cation and O6 of guanine in the
i
i
without forming the PG-4 structure (δ = 10.5 ppm) could be
corresponding PG-4 plane and that between N1H and O6 for
the hydrogen bonding to form the iPG-4 structure were slightly
observed clearly at low temperature. In addition, on cooling,
the signal of N1H showed a downfield shift due to intermole-
cular hydrogen bonding probably between the guanine deriva-
tive and methanol (Fig. 4(b)). This result indicates that, in the
absence of electrostatic interaction with template cations, the
i
different among three kinds of PG-4 structures. However, the
distances fall in the normal range of those reported in G-quad-
ruplexes, i.e., 2.3 Å and 2.9 Å, respectively,39 as summarized in
Table 1. Thus, the G-4 formation can be assisted by electro-
static interactions of O6 in guanine with a cationic entity or
water molecules having polarized O–H bonds, regardless of
the steric protection by bulky peripheral substituents.
i
solvation of the PG molecule by methanol exerts a stronger
effect in the methanol solution of iPG rather than intermolecu-
i
lar hydrogen bonding to form the PG-4 structure. This result
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i
Fig. 5 ESI-MS spectra of self-assembled species of PG with (a) NaBPh4 and (b)
i
CaCl2; in methanol at room temperature under N2. Conditions: (a) PG (15 mM),
NaBPh4 (6.8 mM); (b) iPG (18 mM), CaCl2 (8.4 mM). Peaks: (a) 1: iPG–Na+ (m/z =
216.0), 2: iPG-2–Na+ (m/z = 409.1), 3: iPG–NaBPh4–Na+ (m/z = 558.1); (b)
i
i
i
1: PG-2–Ca2+ (m/z = 213.0), 2: PG-3–Ca2+ (m/z = 309.6), 3: PG-5–Ca2+ (m/z =
502.7), 4: iPG-8–Ca2+ (m/z = 792.3).
suggests that the gain of enthalpy by Na+⋯O interaction can
compensate for the instability caused by desolvation.
i
ESI-MS measurements of PG in the presence of template
i
cations allowed us to detect the formation of the PG-4 struc-
Fig. 6 UV-Vis spectral change by adding (a) NaBPh4, (b) titration curve at
295 nm and (c) Job’s plot of the PG–Na+ complex: Conditions: (a) PG (207 μM)
in CH3OH and CHCl3 (1 : 74 v/v), the additional NaBPh4 (15.5 mM) in CH3OH
and CHCl3 (1 : 9 v/v); (b) the total mole concentration was adjusted to 41.3 μM
in CH3OH and CHCl3 (1 : 374 v/v), iPG (0.62 mM) in CH3OH and CHCl3 (1 : 24 v/v),
NaBPh4 (0.62 mM) in CH3OH and CHCl3 (1 : 24 v/v); all the measurements
were carried out at room temperature.
i
i
ture clearly even in the methanol solution. Upon addition of
NaBPh4 to provide Na+ as a template cation, a peak cluster was
observed at m/z = 795.2 (calcd, 795.4), assignable to that
derived from PG-4–Na+ (Fig. 5(a) and Fig. S5 in ESI†). In the
i
presence of Ca2+, a peak cluster assigned to [iPG-4–Ca2+]2+ was
observed at m/z = 406.1 (calcd, 406.2; see Fig. S6 in ESI†), also
confirming its formation.
In the range under 0.5 mole fraction in the Job’s plots, allo-
steric effects could not be observed clearly on the sequential
i
Further scrutiny was given to the formation of PG-4 in the
presence of the template cations in CH3OH–CHCl3 mixed sol-
ution by UV-Vis absorption spectroscopy. The absorption
spectra of PG showed red shifts in the course of addition of
i
process relating to the formation of PG-4–cation complexes in
both the cases of Na+ and Ca2+ (see Fig. 6(b) and see the inset
of Fig S7(a) in ESI†). This indicates that the intermolecular
hydrogen bonding to form the G-4 structure is much weaker
than the electrostatic interaction between the O6 oxygen of
guanine and the template cation. Therefore, in the presence of
a template cation, the main driving force of the G-4 formation
is the interaction of the guanine O6 oxygen with the template
cation.
i
NaBPh4 or CaCl2 (Fig. 6(a) and see Fig. S7(a) in ESI†). The shift
was assigned to form G-4–cation complexes: a Job’s plot for
iPG and NaBPh4 gave the maximum at 0.2 mole fraction, indi-
cating the 4 : 1 complexation of iPG with sodium ion (Fig. 6(c)).
In the presence of an excess amount of Na+, however, the pres-
ence of a 1 : 1 complex of iPG with Na+ was observed by ESI-MS
spectrometry (see Fig. S8 in ESI†). In contrast, the Job’s plot
for iPG and CaCl2 afforded two maximums at 0.2 and 0.5, indi-
cating that iPG-4–Ca2+ is formed at lower concentration of
Ca2+, however, a 1 : 1 complex (iPG–Ca2+) became dominant at
higher concentrations of Ca2+ (see Fig. S7(b) in ESI†). It should
be noted that the Ca2+ ion binds to guanine more tightly than
the Na+ ion due to the higher cationic charge, even though the
Cl− anion may interact with the Ca2+ ion in solution to reduce
the Lewis acidity.
3. Thermodynamics of the formation of G-quartet analysed
by van’t Hoff plots
As mentioned above, the electrostatic interaction among the
template cation and the O6 oxygen of guanine is much stron-
ger than the intermolecular hydrogen bonding to form the G-4
structure. In addition, the contribution of equilibrium of G-4
formation by the metal-free guanine molecules should be
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Scheme 1
negligible. Furthermore, as observed in variable-temperature
i
i
NMR measurements (Fig. 4), PG molecules can form a PG-4
structure in CH3OH–CDCl3 at temperatures above 253 K,
however, below 253 K, the formation is negligible. Therefore,
in order to simplify the equilibrium of G-4 formation in the
presence of a template cation, spectroscopic titration to deter-
i
mine the equilibrium constants of the PG-4 formation was
conducted at temperatures below 253 K. Under such con-
ditions, the equilibrium can be elucidated as shown in
Scheme 1.
i
Fig. 7 (a) Absorbance change of PG at 295 nm upon addition of NaBPh4 at
various temperatures in the course of the formation of PG-4–Na+; (b) a van’t
i
i
Hoff plot for the equilibrium constants. Conditions: PG (215 μM) in CH3OH and
CHCl3 (1 : 74 v/v), the additional NaBPh4 (15.5 mM) in CH3OH and CHCl3 (1 : 9
Spectroscopic titration to determine the equilibrium con-
i
stant of the PG-4–Na+ formation was carried out at 253 K or
v/v).
lower and under 0.5 of the equivalence of NaBPh4 relative to
iPG, i.e., under 0.333 of the mole fraction of NaBPh4 for iPG. At
temperatures below 253 K, the plot of UV-Vis absorbance
changes at 295 nm by adding NaBPh4 exhibited no sigmoidal
change, suggesting the absence of the allosteric effect (Fig. 7).
The analysis of the spectral change at each temperature was
made to determine an apparent equilibrium constant based
on the following equation.
Table 2 Equilibrium constants at each temperature and the enthalpy and
entropy of the formation of iPG-4–Na+ and iPG-4–Ca2+
iPG-4–Na+
T (K)
K (M−4 a
)
ΔHb (kcal mol−1
)
ΔSb (cal mol−1 K−1
)
253
243
233
223
1.4 × 1018
2.5 × 1018
4.7 × 1018
1.6 × 1019
−8.4
+50
Â
Ã
iPG-4ꢀNaþ
K ¼ K1K2K3K4 ¼
ð1Þ
4
½iPGꢁ ½Naþꢁ
iPG-4–Ca2+
In the case of CaCl2, the treatments were the same as
described above for NaBPh4, using [Ca2+] in eqn (1) in place of
[Na+].
T (K)
K (M−4 a
)
ΔHb (kcal mol−1
−12.9
)
ΔSb (cal mol−1 K−1
)
308
303
298
4.9 × 1016
6.3 × 1016
1.0 × 1017
+34
i
Equilibrium constants (K, M−4) of the formation of PG-4–
Na+ and iPG-4–Ca2+ at various temperatures are summarized in
Table 2. The enthalpy and entropy changes (ΔH and ΔS,
respectively) in the formation of iPG-4–Na+ were determined to
be −8.4 kcal mol−1 and +50 cal mol−1 K−1, respectively, on the
basis of a van’t Hoff plot (ln K vs. 1/T) as depicted in Fig. 7(b).
a Each equilibrium constant was obtained by temperature-dependent
UV-Vis measurements.41 b Each value was estimated by the slope and
the intercept of the van’t Hoff plot; ln K = −ΔH/RT + ΔS/R.
i
Similarly, the ΔH and ΔS values of the PG-4–Ca2+ formation
i
were also determined to be −12.9 kcal mol−1 and +34 cal the main driving force to form PG-4, the enthalpy change
mol−1 K−1, respectively (see Fig. S9 in ESI†). The results indi- should be derived from the attractive electrostatic interaction
i
cate that the more positive Ca2+ ion binds more tightly to the between PG and the template-cation. In addition, although
iPG-4 assembly to stabilize the quartet structure than the Na+ the formation of iPG-4–Na+ and iPG-4–Ca2+ should be
ion does.
unfavourable in terms of entropy changes, the entropy loss can
i
The enthalpy change can be elucidated as the summation be compensated for by the desolvation of methanol from PG
of the stabilization by the coulombic interaction between PG as well as Na+. The degree of contributions to the entropy
i
i
and the template cation (Na+ or Ca2+), intermolecular change from the desolvation of PG and the template cations,
i
hydrogen bonding between each PG and the desolvation of however, remains unclear due to the lack of the thermodyn-
i
i
methanol from PG. As noted at the end of the former section, amic parameters for the PG-4 formation in the absence of a
since the intermolecular hydrogen bonding cannot become template cation.
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SHIMAZU UV2450. ESI-MS measurements were recorded on an
Applied Biosystems QStar Pulsar i (ESI-TOF; positive mode)
Conclusions
In this study, we determined crystal structures of “naked” G-4 spectrometer.
i
assemblies without bulky substituents using PG as a building
i
block. In the crystal structure of PG-4–H2O, the quartet struc-
ture is stabilized by intermolecular hydrogen bonding among 1H NMR measurements
the O6 oxygen of iPG in the quartet and the water molecules of
A mixed solvent of CDCl3–CH3OH was used for measurements
and chemical shifts were determined relative to an internal
standard (TMS). Variable-temperature NMR measurements
were made on a sample containing 9-isopropylguanine and
each template cation after incubating for over 3 minutes at a
certain temperature.
crystallization. In the structures of PG-4–Na+ and PG-4–Ca2+,
the template cations such as Na+ and Ca2+ reside at the centre
of the quartets. It should be noted that the iPG-4 structure can
be obtained without the presence of the template cations and
bulky peripheral substituents to protect the hydrogen-bonded
supramolecular G-tetramer.
i
i
i
We also discussed the thermodynamics of the PG-4 for-
mation in CH3OH and CH3OH–CH(D)Cl3 solutions on the
basis of spectroscopic measurements. Concerning the PG-4
UV-Vis titration to determine thermodynamic parameters
i
UV-Vis spectroscopic titrations of iPG upon addition of
template cations were conducted in mixed solvents of CHCl3/
CH3OH. Sodium tetraphenylborate and calcium dichloride
were used as the sources of the template cations. In order to
determine the K value (expressed by eqn (1) in the text), absor-
bance at an appropriate wavelength (295 nm for iPG-4–Na+ and
formation, it is clarified that the electrostatic interactions
i
between the template cations and the O6 oxygen of PG con-
tribute more dominantly to the stabilization than the multiple
intermolecular hydrogen bonding among the guanine mole-
i
cules in the PG-4 structure. In the absence of the template
i
cations, the PG-4 formation is governed by entropy change
i
i
288 nm for PG-4–Ca2+) was monitored. Thermodynamic para-
due to the desolvation of methanol from PG. In sharp con-
meters were determined using the van’t Hoff plot (see the
footnote of Table 2).
trast, in the presence of template cations, the charge of the
cations, i.e., the Lewis acidity of the cations, is important to
i
stabilize the PG-4 assemblies by virtue of the electrostatic
attractive interaction between the template cations and the
electrostatically negative O6 oxygen of iPG. This is clearly
X-ray crystallography
i
i
reflected on the more negative ΔH values of the iPG-4–Ca2+ for- Single crystals of PG-4 assemblies made of PG with water
i
mation than that of the PG-4–Na+ formation. Thus, the di- molecules of crystallization and template cations were attained
cationic Ca2+ ion is more favourable to stabilize the iPG-4 by recrystallisation from methanol with vapour diffusion of
structure thermodynamically than the monocationic Na+ ion diethyl ether. The crystallization of the G-ribbon type was
(Fig. S10 in ESI†).
carried out in the mixed solvent of methanol and chloroform
The results obtained in this work provide a solid thermo- with vapour diffusion of diethyl ether. All measurements were
dynamic basis of the driving force of the G-quartet formation performed on a Bruker SMART APEX II ULTRA CCD diffract-
without any bulky substituents on the guanine scaffold. This ometer at 120 K or 90 K with graphite-monochromated Mo Kα
work will also contribute to the manipulation of G-quartet radiation (λ = 0.71073 Å). The structures were solved by direct
structures toward their further application in the construction methods and expanded using Fourier techniques. Non-hydro-
of supramolecular materials.
gen atoms were refined anisotropically including solvents
except disordered molecules (ether in the crystal structure of
the G-4 with sodium tetraphenylborate). Refinements were
carried out by full-matrix least squares techniques on F2 with
scattering factors42 and including anomalous dispersion
effects.43 All calculations were performed using the Yadokari-XG
crystallographic software package,44 and structure refinements
were made by using SHELXL 97.45 All the X-ray crystallographic
data are summarized in Table S1 in ESI.†
Experimental section
Materials and methods
CHCl3 (Wako Pure Chemical Industries) was purified by distil-
lation with CaH2. CH3OH (Wako Pure Chemical Industries)
was dried using Mg and I2. CDCl3 was purchased from Cam-
bridge Isotope Laboratories, Inc. Other chemicals and solvents
were purchased from Wako Pure Chemical Industries and
Tokyo Chemical Industries. These reagents were used as
received. 9-Isopropylguanine was synthesized according to the
literature method (see ref. 36 in the text).
Acknowledgements
This work was supported by Grants-in-Aids (20108010,
23111703, 24245011) from the Japan Society of Promotion of
Apparatus
1H NMR spectra were measured on a JNM-EX270 spectrometer Science and Technologies, and by KOSEF/MEST through the
(JEOL). UV-Vis absorption spectra were recorded on
a
WCU project (R31-2008-000-10010-0) of Korea.
This journal is © The Royal Society of Chemistry 2013
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4
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764 | Org. Biomol. Chem., 2013, 11, 758–764
This journal is © The Royal Society of Chemistry 2013