Mg2+-mediated self-assembly of an amphiphilic pyrene derivative with single-stranded DNA

Article abstract of DOI:10.1016/j.tetlet.2008.02.024

An amphiphilic ethynyl-pyrene derivative modified with phosphonic acid groups (PyTOH) was synthesized and characterized. Single-stranded DNA (ssDNA), which carries negative charges, was found to induce the self-assembly of PyTOH through electrostatic inte

Full text of DOI:10.1016/j.tetlet.2008.02.024

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 2656–2660
Mg2+-mediated self-assembly of an amphiphilic pyrene
derivative with single-stranded DNA
Jinchong Xiao, Junbo Li, Cuihong Li, Changshui Huang,
Yuliang Li *, Shuang Cui, Shu Wang *, Huibiao Liu
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Center for Molecular Sciences,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China
Received 9 November 2007; revised 3 February 2008; accepted 6 February 2008
Available online 9 February 2008
Abstract
An amphiphilic ethynyl-pyrene derivative modified with phosphonic acid groups (PyTOH) was synthesized and characterized. Single-
stranded DNA (ssDNA), which carries negative charges, was found to induce the self-assembly of PyTOH through electrostatic inter-
actions with Mg2+ as co-complexation ion in aqueous solution. UV–vis, fluorescence, and circular dichroism (CD) spectra revealed that
left-handed helical architecture of PyTOH/Mg2+/DNA was formed. Scanning electron microscopy (SEM) and atomic force microscopy
(AFM) showed that nanoscale structures could be fabricated by self-assembly.
Ó 2008 Published by Elsevier Ltd.
Keywords: Self-assembly; Chiral; ssDNA; Aggregation
Design and synthesis of supramolecular assemblies that
mimic the molecular organization in biological systems is
of widespread interest, especially in the fields of information
storage, light energy conversion, and optical devices.1 In
general, the supramolecular systems can be constructed
through weak interactions such as hydrogen-bonding, p–p
stacking, electrostatic interaction, van der Waals forces,
and solvophobic interactions. DNA-templated fabrication
is a significant approach for constructing the supramole-
cular assemblies, and many advances have been achieved
in this field.2 The molecular recognition features present
in DNA have been incorporated into amphiphiles to afford
DNA analogs.3 They have also served as templates for the
deposition of conductive materials and silver metallization.4
Pyrene and its derivatives exhibit interesting electro-
chemical and photophysical properties, which have
attracted considerable research interests in their aggrega-
tion-induced emissions and nanometer-scale molecular
architectures.5 However, they are rarely exploited for fabri-
cating chiral supramolecular architectures with DNA
through metal-coordination interactions. Herein, we
describe the synthesis of an amphiphilic ethynyl-pyrene
(PyTOH) containing phosphonic acid groups and the eval-
uation of its self-assembly behavior with ssDNA in the
presence of Mg2+ ions in aqueous solutions (Scheme 1).
The importance of Mg2+ ions has been reported in cata-
lyzing the self-assembly and stabilizing the supramolecular
structures.6
*
Corresponding authors. Tel.: +86 10 62588934; fax: +86 10 82616576
(Y.L.).
E-mail addresses: steven_zhou1981@hotmail.com, ylli@iccas.ac.cn
Scheme 1. Chemical structure of PyTOH and single-stranded DNA.
(Y. Li).
0040-4039/$ - see front matter Ó 2008 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2008.02.024

J. Xiao et al. / Tetrahedron Letters 49 (2008) 2656–2660
2657
Scheme 2. Synthetic routine of PyTOH.
The amphiphilic PyTOH consists of two phosphonic
acid groups, which are linked at both ends of the aromatic
ethynyl-pyrene core. The synthesis of PyTOH is shown in
Scheme 2.7 Phosphonate (2) was afforded from 4-iodo-
benzyl bromide (1) by a Michaelis–Arbuzov reaction,
followed by treatment with compounds (3) under basic
condition to yield the corresponding precursor PyTOEt
(4). The Sonogashira reaction was used to build triple
bonds. Cleavage of the ethyl protecting groups afforded
the target PyTOH (5). PyTOH was characterized by
NMR spectroscopies and ESI mass spectrometry. These
spectra indicated that PyTOH existed as mono-ammonium
salt with triethylamine.
PyTOH in water exhibits the absorption maxima at
410 nm with a shoulder peak at 455 nm (for the ethynyl-
pyrene core) and the emission maxima at 430 nm upon
excitation with 400 nm in dilute solution. The fluorescence
quantum yield (U) of PyTOH in water was measured to be
0.05 using rhodamine 6G as standard. The addition of
Mg2+ to the solution of PyTOH in water resulted in an
obvious decrease in the absorbance of the peak at 410 nm
and maintained peak position of absorption maximum.
The emission intensity was also observed to decrease upon
the addition of Mg2+ (Fig. 1). The stoichiometry of
PyTOH/Mg2+ complex was determined by the method of
continuous variations (Job’s plot). The result obtained
indicates the formation of a 1:2 PyTOH/Mg2+ complex.
To investigate the possible expression of chirality within
the aggregates and thus gain more information on the
structure of the assemblies, absorption, fluorescence, and
circular dichroism (CD) experiments were carried out to
study the self-assembly behavior of PyTOH/Mg2+/DNA
in aqueous solution. The CD spectrum of ssDNA itself in
aqueous solution shows a positive Cotton effect (from
303 nm to 260 nm) followed by a negative Cotton effect
at short wavelengths (from 260 nm to 230 nm) (Fig. 2C).
The concentration of DNA/Mg2+ ([DNA] = 1.0 Â 10À4 M,
1:2) was kept constant, and the change of the CD spectra of
the complex PyTOH/Mg2+/DNA was followed as a func-
tion of increasing concentration of PyTOH in 0.1 M Tris/
HCl buffer solution (Fig. 2A). At the concentration of
PyTOH above 3.0 Â 10À5 M, the CD spectra of the com-
plex showed obvious negative cotton signal from 460 nm
to 380 nm, which corresponded to the p–p* absorption of
PyTOH. This result indicates that the chirality of PyTOH
is induced by the DNA upon complexation leading to a
left-handed helical stacking of the PyTOH.8 In the control
experiments, solutions of PyTOH, PyTOH/Mg2+, and
PyTOH/DNA were CD-inactive above 350 nm wave-
length, which indicated that Mg2+ played an important
Fig. 1. (A) UV–vis absorption, and (B) fluorescence (excitation at 400 nm) titration of Mg2+ to PyTOH in 0.1 Tris/HCl buffer solution.
[PyTOH] = 1.0 Â 10À5 M, [Mg2+] = (a) 0; (b) 5.0 Â 10À6 M; (c) 1.0 Â 10À5 M; (d) 2.0 Â 10À5 M.

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J. Xiao et al. / Tetrahedron Letters 49 (2008) 2656–2660
Fig. 2. (A) CD spectra, (B) UV–vis absorption titration of PyTOH to DNA/Mg2+ in 0.1 Tris/HCl buffer solution. [DNA] = 1.0 Â 10À4 M, [PyTOH] =
(a) 0; (b) 3.0 Â 10À5 M; (c) 6.0 Â 10À5 M; (d) 8.0 Â 10À5 M; (e) 1.0 Â 10À4 M. (C) CD spectrum of ssDNA in buffer solution (c = 1.0 Â 10À5 M).
role in catalyzing the self-assembly and stabilizing the
supramolecular structures. The self-assembly behavior of
PyTOH/Mg2+/DNA is strongly dependent on the concen-
tration of PyTOH used.
We also measured the effect of DNA concentration on
the CD signals of PyTOH/Mg2+/DNA in buffer solution
at 15 °C. Although the solution of PyTOH/ Mg2+ is CD-
silent, upon the addition of DNA, the CD spectra with
the negative cotton effect at 455, 410 nm was observed
(Fig. 3). Noted that the CD signal was not changed with
varying DNA concentration. The results indicate that the
DNA drives the helical structure of PyTOH and contrib-
utes to the stability of the complex.
plex PyTOH/Mg2+/DNA in aqueous solution. When the
temperature was increased from 5 to 60 °C, the absorbance
intensities of the complex PyTOH/Mg2+/DNA at 410 and
455 nm obviously decreased, but those at 340 nm and 480–
680 nm increased (Fig. 4B). While the solution of PyTOH
in water was cooled down from 60 to 5 °C, the emission
intensities were enhanced by probably 25-fold accompany-
ing by a red-shift of 10 nm (Fig. 4C). These results indi-
cated that the rotation of the alkyne-aryl moiety was
effectively limited leading to the enhanced emission effi-
ciency when the temperature was decreased.9 The presence
of two well-defined isosbestic points at 367 and 478 nm,
respectively, showed the clean conversion of twisted struc-
ture to coplanar type. Although it was well known that CD
spectra induced by the formation of chiral aggregates were
sensitive to temperature,10 the intensity of CD spectra of
the complex PyTOH/Mg2+/DNA in buffer solution did
not change much within the temperature range from 5 to
60 °C. This behavior probably resulted from the strong
binding force between PyTOH and DNA through Mg2+
ions co-coordinating interactions.
To further take an insight into the self-assembly process
among the three building blocks, temperature-dependent
optical measurements were carried out for the 1:2:1 com-
The morphologies of the PyTOH/Mg2+/DNA complex
were studied by scanning electron microscopy (SEM) and
atomic force microscopy (AFM). Samples were prepared
by casting PyTOH/Mg2+/DNA from water solution onto
silicon slices or mica. Figure 5a showed that the complex
in water ([PyTOH] = 5.0 Â 10À5 M, [DNA] = 1.0 Â
10À4 M, [Mg2+] = 5.0 Â 10À5 M) could form the aggre-
gated nanostructures consisting of nanoparticles with
diameters in the range of 30–150 nm. The AFM image of
the complex also shows the formation of spherical assem-
Fig. 3. CD spectra of PyTOH/Mg2+/DNA as a function of varying DNA
concentration in 0.1 Tris/HCl buffer solution. [PyTOH] = 1.0 Â 10À4 M,
[DNA] = (a) 0; (b) 4.0 Â 10À5 M; (c) 6.0 Â 10À5 M; (d) 1.0 Â 10À4 M.
Fig. 4. (A) Temperature dependence of the CD spectra, (B) UV–vis absorption of self-assembly of PyTOH/Mg2+/DNA ([PyTOH] = 1.0 Â 10À4 M, mol.
ratio: 1:2:1), (C) FL spectra ([PyTOH] = 1.0 Â 10À5 M) in 0.1 Tris/HCl buffer solution; (a) 5 °C; (b) 15 °C; (c) 50 °C; (d) 60 °C.

J. Xiao et al. / Tetrahedron Letters 49 (2008) 2656–2660
2659
Fig. 5. SEM image of PyTOH/Mg2+/DNA (mol. ratio, 1:2:1) (a), AFM image (b), and dynamic laser light scattering (c) of PyTOH/Mg2+/DNA (from
CONTIN analysis of the autocorrelation function) at a scattering angle of 90°. Samples were prepared from solution and transferred to fresh silica slice or
mica by drop-casting.
blies (Fig. 5b). The size of the aggregates of PyTOH/Mg2+
/
129, 6078; (i) Gan, H. Y.; Li, Y. L.; Liu, H. B.; Wang, S.; Li, C. H.;
Yuan, M. J.; Liu, X. F.; Wang, C. R.; Jiang, L.; Zhu, D. B.
Biomacromolecules 2007, 8, 1713.
DNA in aqueous solution was measured by dynamic light
scattering (DLS) (Fig. 5c). The CONTIN analysis of the
autocorrelation function shows a broad peak correspond-
ing to an average hydrodynamic radius (Rh) of approxi-
mately 210 nm at 298 K.
On the basis of the optical and CD properties described
above, it can be concluded that the self-assembly of
PyTOH/Mg2+/DNA formed left-handed helical stack
structures. The aggregation was initiated by the electro-
static interactions of PyTOH and DNA with Mg2+ as
co-complexation ion in aqueous solution. The mutation
rotation in the same direction avoided steric hindrance of
PyTOH and the aggregation could self-sort into helical
nanostructures.
In summary, we have synthesized an amphiphilic pyrene
derivative and demonstrated the self-assembly of PyTOH/
Mg2+/DNA in aqueous solution. Induced CD spectra were
observed in water indicating the formation of helical supra-
molecular structures. These observations should facilitate
the future design of related self-assembled materials with
the biological and electrooptical properties.
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This work was supported by the National Natural
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References and notes
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6.76 mmol) was stirred for 4 h at 140–145 °C. The reaction mixture
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J. Xiao et al. / Tetrahedron Letters 49 (2008) 2656–2660
(2.34 g, 97%). 1H NMR (300 MHz, CDCl3, 298 K, TMS): d = 7.63 (d,
water and extracted with chloroform (20 mL Â 3). The organic layer
was separated, dried with anhydrous Na2SO4 and concentrated under
reduced pressure. The residue was suspended in hexane and sonicated
for 5 min, and then the supernatant was centrifuged. This process was
repeated three times. The solution was poured into methanol with
sonication, and then was poured into diethyl ether and the precipitate
was collected. This process was also repeated three times. The
precipitate was collected and dried in vacuum to afford a yellow solid
5 (32 mg, 65%). >200 °C Dec; 1H NMR (300 MHz, DMSO-d6, 398 K,
TMS): d = 8.44 (d, J = 8.7 Hz, 2H), 8.14–8.06 (m, 6 H), 7.58 (d,
J = 7.5 Hz, 2H), 7.36 (d, J = 7.5 Hz, 2H), 6.47 (br, 3H), 2.97 (d,
J = 21.0 Hz, 4 H), 2.84 (q, 6H), 1.10 (t, J = 6.9 Hz, 9H). 31P NMR
(160 MHz, DMSO-d6, 298 K, TMS): 19.8. ESI MS: Calcd. for
J = 7.8 Hz, 2H), 7.04 (dd, 1J = 7.8 Hz, 2J = 2.1 Hz, 2H), 4.07–3.97
(m, 4H), 3.07 (d, J = 21 Hz, 1H), 1.28–1.23 (t, J = 7.2 Hz, 6H). MS
(EI): 354 [M+]. Compound PyTOEt (4): A mixture of 1,6-di-
ethynylpyrene (3, 80 mg, 0.32 mmol), compound
2 (248 mg,
0.70 mmol), CuI (3 mg, 0.016 mmol), PPh3 (4 mg, 0.016 mmol), and
Pd(PPh3)2Cl2 (12 mg, 0.016 mmol) in anhydrous Et3N (50 mL) was
stirred for 12 h at 90 °C under nitrogen. After the mixture was cooled
down to room temperature, the mixture was filtrated and concen-
trated under reduced pressure. The residue was purified by column
chromatography (silica gel) with dichloromethane/ethyl acetate as
eluent to give 4 (105 mg, 47%). Mp: 163–165 °C; 1H NMR (400 MHz,
CDCl3, 298 K, TMS): d = 8.67 (d, J = 8.8 Hz, 2H), 8.23–8.15 (m,
6H), 7.67 (d, J = 8.0 Hz, 2H), 7.38 (d, J = 8.0 Hz, 2H), 4.10–4.03 (m,
8H), 3.22 (d, J = 22.0 Hz, 4H), 1.28 (t, J = 7.2 Hz, 12H). 13C NMR
(150 MHz, CDCl3, 298 K, TMS): 132.4, 132.3, 132.1, 131.9, 131.3,
130.1, 130.0, 128.3, 126.4, 125.3, 124.3, 122.1, 118.5, 95.4, 88.8, 62.4,
34.5, 33.5, 16.5. 31P NMR (160 MHz, CDCl3, 298 K, TMS): 25.9.
MALDI-TOF MS: Calcd. for C42H40O6P2: [M]+ = 702.2,
[M+Na]+ = 725.2. Found: 702.6 [M+], 725.6 [M+Na]+. HR-MS
calcd for C42H40O6P2 702.2300; found, 702.2305. Compound PyTOH
(5): PyTOEt (4, 50 mg, 0.071 mmol) and triethylamine (800 lL,
5.74 mmol) were dissolved in 50 mL of chloroform, and then
bromotrimethylsilane (546 lL, 2.13 mmol) was added to the solution.
The mixture was stirred overnight under nitrogen at 65 °C. The
reaction mixture was cooled down to room temperature, washed with
C
40H39NO6P2 [M] = 589.10, 691.2. Found: 589.50, 689.1. HR-MS
calcd for C34H23O6P2, 589.1000; found, 589.0996.
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