Preparation and properties of DNA–lipid complexes carrying pyrene and anthracene moieties

European Polymer Journal 47 (2011) 370–377

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Preparation and properties of DNA–lipid complexes carrying pyrene and anthracene moieties Tong Zhang a, Jinqing Qu a,⇑, Naoya Ogata b, Toshio Masuda c a

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China Ogata Research Laboratory Limited, Kashiwa-dai Minami 1-3-1, Chitose 066-0009, Japan c Faculty of Engineering, Department of Environmental and Biological Chemistry, Fukui University of Technology, 3-6-1 Gakuen, Fukui 910-8505, Japan b

a r t i c l e

i n f o

Article history: Received 11 August 2010 Received in revised form 10 December 2010 Accepted 19 December 2010 Available online 30 December 2010 Keywords: DNA complexes Lipid Pyrene Anthracene Fluorescence

a b s t r a c t Novel DNA–lipid complexes carrying pyrene and anthracene were prepared by substituting sodium counter cations with cationic amphiphilic lipids, namely lipid(PY) and lipid(Anth), in which the actual mole ratios of phosphate to lipid were 1:1.11 and 1:1.03, respectively. DNA–lipid(PY) and DNA–lipid(Anth) complexes were soluble in common organic solvents including CHCl3, CH2Cl2, methanol and ethanol, while insoluble in THF, toluene, and aqueous solutions. CD spectroscopy revealed that DNA–lipid(PY) and DNA–lipid(Anth) complexes took a predominantly double helical structure in methanol and that the helical structure was fairly stable against heating. The solution of DNA–lipid(PY) complex emitted fluorescence in 27.8% quantum yield, which were higher than that of the corresponding lipid(PY) (16.8%), while the fluorescence quantum yields of the solution of DNA–lipid(Anth) (45.4%) was lower than that of the lipid(Anth) (53.0%). The onset temperatures of weight loss of DNA–lipid(PY) and DNA–lipid(Anth) were both 220 °C according to TGA in air. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction DNA is an anionic polyelectrolyte, which can be quantitatively precipitated with cationic surfactants in water to form DNA/cationic–lipid complexes. DNA complexes have been studied intensively over recent years due to their potential in gene delivery and transfection methodologies. DNA complexes also remind us of the coil-to-globule transition in synthetic polymers, exhibiting intriguing liquid crystalline and polyelectrolyte behavior [1–3]. Much work has been dedicated to revealing supramolecular structures and morphologies in DNA/cationic–lipid complexes, which is primarily stimulated by nonviral gene delivery [4–9]. Specifically, cryo-TEM [10], freeze-fracture electron microscopy [11], synchrotron X-ray scattering [12], optical and fluorescence microscopies [13], and small-angle X-ray scattering (SAXS) [14] have given a fairly good picture of

⇑ Corresponding author. Tel.: +86 20 87110247. E-mail address: [email protected] (J. Qu). 0014-3057/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2010.12.010

the structure of these complexes as a function of the lipid content and charge ratio between the cationic lipid and DNA. However, few efforts have been made about the development of DNA–cationic lipid complexes carrying functional groups in the lipid moieties as organic advanced materials for electronic optical applications and DNA molecule probes used in various biological studies. On the other hand, pyrene and its derivatives have attracted special attention because of their interesting photophysical properties such as high fluorescence quantum yield, well characterized long-lived excited state, the sensitivity of its excitation spectra to microenvironment changes, high ability for excimer formation, and the sensitivity of its fluorescence to quenching. These characteristics of pyrene and its derivatives are suitable for the application as a sensor to microscopically probe an environment around the molecules under study [15,16]. Polymers containing pyrene moieties in the main chain or side chain have been widely studied because of their unique properties, which allow them to be applied to various photoelectronic materials including photoconductive,

T. Zhang et al. / European Polymer Journal 47 (2011) 370–377

electroluminescent, and photorefractive materials [17,18]. Meanwhile, anthracene-based polymers consisting of layered p-electron and oriented p-electron systems have potential applications in optoelectronic devices and single-molecular devices such as single molecular wires [19,20]. Thus far, most of the studies on pyrene- and anthracene-containing polymer materials have been carried out using synthetic polymers. However, there is a growing interest in natural polymers and biomacromolecules for practical applications as functional materials especially from the viewpoints of bio- and nanotechnologies and sustainable materials science. Among various biomacromolecules, DNA is one of the most abundant substances in the biosphere and quite interesting as a candidate of source material for these applications. Previously, we synthesized DNA–cationic lipid complexes carrying 2,2,6,6-tetramethyl-1-piperidine-1-oxy (TEMPO), applied them as positive electrode materials of organic radical battery (ORB), and found that the complexes displayed two-stage discharge process [21]. The total capacity of one TEMPO-containing DNA–cationic lipid complex reached 192% of the theoretical value for one electron redox reaction, suggesting two-electron redox reactions between the cation and the anion. We have also studied the DNA–lipid complexes carrying carbazole, triphenylamine [22,23], and azobenzene [24] moieties and found that their solution displayed electrochemical properties. Although no research has been performed about DNA complexes carrying pyrene and anthracene, incorporation of pyrene and anthracene moieties into DNA will possibly lead to the development of novel functional materials based on synergistic actions of pyrene and anthracene and DNA main chain. Such polymeric materials may form helical pyrene and anthracene strands based on the helical DNA main chain, which may endow efficient photoelec-

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tronic properties for potential applications such as nonlinear optics, field-effect transistors, photovoltaics, and so on. Especially, DNA–lipid complexes carrying pyrene and anthracene should be useful as photoluminescence probes in various biological gene therapies [25]. In the present paper, we would like to report for the first time the preparation and properties of pyrene- and anthracene-carrying DNA–lipids, namely Dna–lipid(PY) and DNA–lipid(Anth) (Scheme 1), aiming at the future development of advanced polymeric functional materials. 2. Experimental part 2.1. Materials Sodium salts of DNA from salmon testes (>95%) were donated from Japan Chemical Feeding Company, and used without farther purification. According to the data of Japan Chemical Feeding Company, the weight-average molecular weight of the DNA sample is 6.6  106 (ca. 30 000 bp) (tested by electrophoresis). N,N0 -Dicyclohexylcarbodiimide (DCC, Aldrich), 4-dimethylaminopyridine (DMAP; Wako), 11-bromoundecanoic acid (Aldrich), 4-(pyren-2-yl)butanoic acid (Aldrich), 11-bromoundecan-1-ol (Aldrich); (anthracen-10-yl)methanol (Aldrich) were purchased and used without further purification. 2.2. Measurements 1 H (400 MHz) and 13C (100 MHz) NMR spectra were recorded on a JEOL EX-400 spectrometer using tetramethylsilane as an internal standard. IR, UV–vis, and fluorescence spectra were measured on JASCO FT/IR-4100, V-550, and FP750 spectrophotometers, respectively. Circular dichroism (CD) spectra were recorded on a JASCO

Scheme 1. The structure of DNA–lipid complexes.

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J-820 spectropolarimeter. Melting points (m.p.) were measured on a Yanaco micro melting point apparatus. Elemental analysis was carried out on an Elementar Vario EL-III instrument. Thermogravimetric analysis (TGA) was carried out on a Shimadzu TGA-50 thermal analyzer in air. The content of Na ion was determined by inductively coupled plasma (ICP) emission spectrometry using a Shimadzu ICP-1000 IV spectrometer; DNA–lipid samples were dissolved in 2 N HCl. Polarized optical microscope images were observed with a Nikon ECLIPSE LV100POL. 2.3. Synthesis of lipids [15] Scheme 2 illustrates the synthetic procedures of 4-pyren2-yl-butanoyloxy-undecyl-pyridinium bromide [lipid(PY)] and 11-anthracen-9-ylmethoxy-11-oxoundecyl-pyridinium bromide [lipid(Anth)] via 11-bromoundecyl 4-(pyren-2-yl)butanoate }) and (anthracen-9-yl)methyl 11-bromoundecanoate (2), respectively. Lipid(Py) 11-bromoundecan-1-ol 4-pyren-2-yl-butanoic acid. Compound 1 was prepared as follows: 11-Bromoundecan-1-ol (1.26 g, 5.0 mmol) was added to a solution of EDCHCl (1.0 g, 5.2 mmol) and DMAP (60 mg, 0.50 mmol) in CH2Cl2 (45 mL) at room temperature. The 4-pyren-2yl-butanoic acid (1.64 g, 5.7 mmol) was added to the solution and the resulting mixture was stirred at room temperature overnight. The reaction mixture was washed with water (50 mL) three times, and the organic layer was dried over anhydrous MgSO4. It was filtered, and the filtrate was concentrated on a rotary evaporator. The residual mass was purified by silica gel column chromatography eluted with n-hexane/ethyl acetate = 19/1 (volume ratio) to give 1 as a white solid. Yield 1.44 g (55%). 1H NMR

(400 MHz, CDCl3): d = 1.21–1.81 (m, 18H, 9CH2(CH2)9CH2, 2.14–2.20 (m, 2H, –OCOCH2CH2CH2-PY), 2.41–2.45 (m, 2H, CH2PY), 3.32–3.37 (m, 2H, –CH2Br), 4.05–4.08 (m, 2H, –CH2COO), 7.80–8.28 (m, 9H, PY). 13C NMR (100 MHz, CDCl3): d = 24.4, 25.8, 28.6, 28.7, 28.8, 28.9, 29.0, 31.8, 33.8, 41.6, 50.2, 61.8, 108.5, 119.1, 120.2, 122.8, 125.7, 128.3, 140.2, 144.8, 144.9, 173.4 (–CO2–). Anal. Calcd for C31H37BrO2: C, 71.39; H, 7.15. Found: C, 71.46; H, 7.04. Lipid(PY) was synthesized by dissolving 1 (1.25 g, 2.4 mmol) in 50 mL of pyridine and stirring at reflux temperature for 2 days. After cooling to room temperature, the reaction mixture was concentrated on a rotary evaporator, and then poured into a large amount of diethyl ether to precipitate lipid(PY). The white crystals of lipid(PY) were dried in vacuum. Yield 1.1 g (78%, 1.8 mmol); mp 121–123 °C; IR (KBr): 3425, 3112, 3032, 2923, 2850, 1724, 1623, 1477, 1369, 1245, 1172, 848, 779, 686 cm–1. 1H NMR (400 MHz, CDCl3): d = 1.19–1.89 (m, 18H, CH2(CH2)9CH2), 2.15 (m, 2H, –OCOCH2), 2.43–2.45 (m, 2H, CH2PY),4.05–4.07 (m, 2H, CH2COO), 4.86 (m, 2H, –CH2–pyridine), 7.27 (m, 2H, pyridine+), 7.70–8.37 (m, 9H, Py), 9.35(m, 3H, pyridine+). 13C NMR (100 MHz, CDCl3): d = 25.7, 26.8, 28.5, 28.9, 29.0, 29.2, 31.8, 32.7, 33.9, 61.9, 64.5, 123.3, 124.8, 125.8, 126.6, 127.3, 127.4, 128.2, 131.3, 144.9, 145.2, 173.5. Anal. Calcd for C36H42BrNO2: H, 7.05; C, 71.99; N, 2.33. Found: C, 72.16; H, 6.94; N, 2.10. Lipid(Anth) was synthesized in a manner similar to that of Lipid(PY). Compound 2: Yield 88%. Pale yellow solid. 1H NMR (400 MHz, CDCl3): d = 1.18–1.84 (m, 18H, CH2(CH2)9CH2, 2.29–2.33 (m, 2H, CH2COO), 3.35–3.39 (m, 2H, Br-CH2), 6.13 (s, 2H, CH2-Anth), 7.45–8.48 (m, 9H, Anth). 13C NMR (100 MHz, CDCl3): d = 24.9, 28.0, 28.6, 29.0, 29.1, 29.3, 32.8, 34.0, 34.3, 58.6, 123.9, 125.0, 126.5, 129.1, 131.0,

Scheme 2. Synthesis process of lipid(PY) and lipid(Anth).

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131.3, 174.0. Calcd for C26H31BrO2: C, 68.57; H, 6.86. Found: C, 68.71; H, 7.02. Lipid(Anth): Yield 57%, pale grey solid, m.p. 91.0–93.0 °C. IR (KBr): 3409, 3054, 2927, 2854, 1708 (vC@O), 1589, 1489, 1315, 1272, 1173, 1107, 760, 694, 524 cm–1. 1H NMR (400 MHz,CDCl3): d = 1.10–1.98(m, 18H, CH2(CH2)9CH2), 2.20–2.35 (m, 2H, CH2COO), 4.29–4.32 (m, 2H, pyridine– CH2–), 6.08 (s, 2H, CH2-Anth), 7.42–7.46 (m, 2H, pyridine+), 7.42–8.46 (m, 9H, Anth), 5.52–8.56 (m, 1H, pyridine+), 8.94– 9.96 (m, 2H, pyridine+). 13C NMR (100 MHz, CDCl3): d = 24.4, 27.0, 28.6, 29.9, 30.1, 32.3, 35.0, 59.5, 63.0, 125.0, 126.2, 127.6, 127.7, 129.4, 130.1, 132.2, 132.8, 145.8, 146.8, 175.4. Calcd. for C31H36BrNO2: C, 69.66; H, 6.79; N, 2.62. Found: C, 69.51; H, 6.52; N, 2.34. 2.4. Preparation of DNA–lipid complexes [12,13] A small amount of lipid (2.0 mmol) in 10 ml HF was added slowly into double-distilled 100 mL H2O to form a uniform solution. An aqueous solution (200 mL) of DNA–Na from salmon testes (0.50 g, 0.68 mmol bp1) was added dropwise into the aqueous lipid solution (the feed mole ratio of phosphate to lipid was 1.50). The formed DNA–lipid complex immediately precipitated out from the aqueous solution. After mixing for 24 h, the precipitate was collected by filtration, washed with H2O to remove free DNA, and then dried in a vacuum oven at 50 °C for 24 h. The white DNA–lipid complex was dissolved in chloroform and reprecipitated in THF two times. The obtained DNA– lipid complex was subjected to elemental analysis to determine the actual composition of phosphate anion and the cationic lipid in the DNA–lipid complex. DNA–lipid(PY): IR (KBr): 3424, 2927, 2854, 1735, 1635, 1462, 1365, 1238, 1172, 1091, 964, 844, 682, 478 cm–1. Anal. Calcd for DNA–lipid(PY) complex with 1:1 ratio of phosphate anion to cationic lipid(PY): C, 61.36; H, 6.60; N, 10.10; P, 3.88. Found: C, 61.09; H, 6.70; N, 9.34; P, 3.36. DNA–lipid(Anth): IR (KBr): 3423, 2926, 2854, 1725, 1655, 1638, 1542, 1525, 1489, 1273, 1173, 963, 774, 734, 529 cm–1. Anal. Calcd for DNA–lipid(Anth) complex with 1:1 ratio of phosphate anion to cationic lipid(Anth): C, 63.77; H, 6.58; N, 9.35; P, 3.59. Found: C, 63.82; H, 6.55; N, 9.26; P, 3.61. 2.5. Calculation of the yields of DNA–lipid complexes We define the yield of DNA–lipid complex as the ratio of the actual weight of DNA–lipid complex to the theoretical weight of DNA–lipid complex based on DNA–Na. The yield of DNA–lipid complex was calculated based on the following equation.

yield ð%Þ ¼

W DNAlipid  100   M M W DNANa 1 þ lipidM NaBr

ð1Þ

base

where WDNA–lipid is the actual weight of DNA–lipid complex; WDNA–Na is the feed weight of DNA–Na; Mlipid and MNaBr are the molecular weights of lipid and NaBr, respectively, Mbase is the average molecular weight of base groups in the repeating unit of DNA–Na [the value is 347.91 calculated from the structures of base couple (according to the

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fragment sequence of Salmon’s DNA with an AT/GC ratio of approximately 56:44)] [26]. 2.6. Determination of the actual mole ratios of phosphate to lipid in the DNA–lipid complexes The actual mole ratio of phosphate to lipid in the DNA–lipid complexes was estimated from the amounts of phosphorus before and after complexation. Phosphorus was determined by elemental analysis. The actual percent phosphorus content in a DNA–lipid (Pa) was calculated based on the following equation:

Pa ¼

1þ

P0 M lipid MNaBr NMbase

ð2Þ

where P0 is the percent phosphorus content in DNA–Na (the value was 8.90% according to elemental analysis), Mlipid and MNaBr are the molecular weights of lipid and NaBr, respectively, Mbase is the average molecular weight of base groups in the repeating unit of DNA–Na [the value was calculated to be 347.91 from the structures of base couple (according to the fragment sequence of Salmon’s DNA with an AT/GC ratio of approximately 56:44)], [26] and N is the actual mole ratio of phosphate to lipid in the DNA–lipid complexes. When solved for N, the equation is

N¼

Pa  ðM lipid  M NaBr Þ ðP0  P a Þ  M base

ð3Þ

3. Results and Discussion 3.1. Synthesis of lipids Scheme 2 illustrates the synthetic routes for the pyrene- and anthracene-containing lipids, lipid(PY) and lipid(Anth). Lipid(PY) was synthesized by the reaction of 11-bromoundecan-1-ol with 4-pyren-2-yl-butanoic acid using EDCHCl as a condensation agent, and DMAP as a catalyst, followed by the reaction of the product with pyridine. Lipid(Anth) was prepared similarly by the condensation of the 11-bromoundecanoic acid with (anthracen-10-yl)methanol and the subsequent reaction with pyridine. The lipids were identified by 1H NMR, 13C NMR, and IR spectra besides elemental analysis. 3.2. Preparation of DNA–lipid complexes Table 1 summarizes the conditions and results of preparation of DNA–lipid(PY) and DNA–lipid(Anth) complexes. When the aqueous solution of DNA–Na was added into the aqueous lipid solutions, the DNA–lipid complexes immediately precipitated from the aqueous solution. After 24 h, the white flocculous polymers were easily isolated by filtration to afford DNA–lipids in good yields (90% and 92%). The ICP data revealed that the Na ion was almost completely replaced by the lipids. The formed DNA–lipids were completely soluble in CHCl3, CH2Cl2, methanol and ethanol, while insoluble in water, THF, toluene, diethyl ether, and n-hexane. The elemental analysis showed that the obtained DNA–lipid complexes possessed 1:1.11 and

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Table 1 Preparation of DNA–Lipid complexes. Lipid

Yield (%)a

Ratio of replaced Na+(%)b

Mole ratio of phosphate to lipid (N)c

Lipid(PY) Lipid(Anth)

90 92

98 97

1:1.11 1:1.03

a DNA–lipid complex is the insoluble part in water. The yield was determined according to Eq. (1). b Determined by ICP. c Calculated by P elemental analysis according to Eq. (3).

1:1.03 compositions of a phosphate anion to the cationic amphiphile.

DNA–Na, DNA adopts the B-form conformation [17]. The absorption peak at 1695 cm–1 in this spectrum is attributed to the hydrogen-bonded C@O stretching in the base pairs [thymine (T), guanine (G), and cytosine (C)]. The unsymmetrical shape in this absorption band is due to the C@N and C@C stretching in the aromatic bases of DNA around 1640 cm–1. In the spectrum of lipid(PY), the absorptions at 2930 and 2850 cm–1 are assignable to –CH2 stretching vibration in lipid(PY), and the absorption at 1728 cm–1 to the non-hydrogen-bonded ester C@O stretching vibration in lipid(PY). The absorption band observed at 1600 cm–1 in this spectrum is due to the stretching of C@C in aromatic PY rings. The spectrum of the DNA–lipid(PY) complex displays almost all the absorption bands of both lipid(PY) and DNA–Na, suggesting the

3.3. Properties of DNA–lipid complexes Fig. 1 shows the FTIR spectra of DNA–Na, lipid(PY), and DNA–lipid(PY) complex in the range of 4000–400 cm–1. Judging from the absorption band at 1234 cm–1 (asymmetric stretching vibration of PO2–) in the spectrum of

Fig. 1. IR spectra of DNA–Na, lipid(PY), lipid(Anth), DNA–lipid(PY), and DNA–lipid(Anth) (KBr pellet).

Fig. 2. CD and UV–vis spectra of lipid(PY), lipid(Anth), DNA–lipid(PY) and DNA–lipid(Anth) in methanol (c = 0.025 mg/mL) and of DNA–Na in water (c = 0.04 mg/mL) at 22 °C.

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presence of both lipid(PY) and double-strand DNA in the complex. The spectrum of the DNA–lipid(Anth) also exhibits absorptions characteristic of both lipid(Anth) and DNA–Na. The top part of Fig. 2 shows the CD spectra of DNA– lipid(PY) and DNA–lipid(Anth) measured in methanol, along with the one of DNA–Na in water for comparison. The pristine DNA in an aqueous solution exhibited a positive Cotton effect at 270 nm and a negative one at 245 nm, while lipid(PY) and lipid(Anth) were CD inactive as expected. On the other hand, both Dna–lipid(PY) and DNA–lipid(Anth) displayed a large plus CD signal at 290 nm and no negative Cotton effect in a range of 245–260 nm in methanol. Thus it is evident that DNA–lipid(PY) and Dna–lipid(Anth) adopt a double helical C-form conformation different from that of the virgin DNA. [6]. In the UV–vis spectra, DNA–lipid(PY) and lipid(PY) exhibited almost the same absorption peaks at 265, 280, 315, 330, and 343 nm attributable to PY. The UV–vis spectra of DNA–lipid(Anth) and lipid(Anth) showed

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absorption bands at 260, 343, 363, 383 nm due to anthracene. The temperature dependence of the CD and UV–vis spectra of DNA–lipid(PY) and Dna–lipid(Anth) was examined (Fig. 3). When the measuring temperature was raised from –10 to 60 °C in methanol, the magnitude of Cotton effect decreased only slightly. This phenomenon resembles the aqueous DNA–Na solution which exhibited slight changes of Cotton effect in a temperature range of 10– 90 °C in aqueous solution (the figure is not shown). It can be said that the helical structure of the Dna–lipids is thermally very stable in the measured temperature range. Fig. 4 shows the fluorescence spectra of DNA–lipid(PY) and DNA–lipid(Anth) along with those of lipid(PY) and lipid(Anth). The solutions of lipid(PY) emitted luminescence at 380 and 400 nm with fluorescence quantum yields ð/Þ of 16.8% upon excitation at 350 nm, which should come from PY. DNA–lipid(PY) emitted fluorescence in a manner

Fig. 3. Temperature-variable CD and UV–vis spectra of DNA–lipid(PY) and DNA–lipid(Anth) measured in a range of –10 to 60 °C in MeOH (c = 0.025 mg/ mL).

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Fig. 5. TGA curves of DNA–Na, DNA–lipid(PY) and DNA–lipid(Anth) measured at a heating rate of 10 °C/min in air.

loss of DNA–lipid(PY), DN–lipid(Anth) and DNA–Na were above 220 °C under air. None of these samples completely lost weight even at 900 °C, which is attributable to remaining inorganic materials including sodium phosphate and phosphorus oxide. We subjected the DNA–lipid(PY) and DNA–lipid(Anth) complexes to DSC to find that the DSC diagrams of DNA and DNA–lipid complexes showed no glass transition temperature (Tg) but a melting temperature (Tm) at 81 °C, which agrees with the literature [19]. 4. Conclusions

Fig. 4. Fluorescence spectra of lipid(PY), lipid(Anth), DNA–lipid(PY) and DNA–lipid(Anth) measured in methanol at 22 °C. DNA–lipid(PY) was excited at 350 nm, U = 27.8%; lipid(PY) excited at 350 nm, U = 16.8%; DNA–lipid(Anth) excited at 343 nm, U = 45.4%; lipid(Anth) excited at 343 nm, U = 53.0%; The intensities have been normalized based on the concentration of the carbazole (c = 0.0025 mg/mL).

similar to lipid(PY), while the ð/Þ values of DNA–lipid(PY) were slightly larger than that of the corresponding lipid (PY) (27.8%). DNA–lipid(PY) further exhibited a large emission around 480 nm, which is attributable to an excimer emission [27]. This indicates that the pyrene moieties in DNA–lipid(PY) easily form an excimer. On the other hand, the solutions of lipid (Anth) and Dna–lipid(Anth) emitted luminescence at 380 and 420 nm assignable to the monomer emission of anthracene upon excitation at 343 nm, whose fluorescence quantum yields ð/Þ were 53.0% and 45.4%, respectively. Fig. 5 depicts the TGA traces of DNA–Na, DNA–lipid(PY) and DNA–lipid(Anth). The onset temperatures of weight

Novel DNA–lipid complexes carrying pyrene and anthracene were prepared by substituting sodium counter cations with cationic amphiphilic lipid(PY) and lipid(Anth); the actual mole ratios of phosphate to lipid were 1:1.11 and 1:1.03, respectively. The DNA–lipid(PY) and DNA– lipid(Anth) were soluble in common organic solvents including CHCl3, CH2Cl2, methanol and ethanol, but insoluble in THF, toluene and aqueous solutions. CD spectroscopic studies revealed that DNA–lipid(PY) and DNA–lipid(Anth) took predominantly double helical structure in CHCl3 and methanol, and the helical structure was very stable against heating. The solutions of DNA–lipid(PY) and DNA–lipid (Anth) emitted fluorescence in 27.8% and 45.4% quantum yields. The onset temperatures of weight loss of DNA–lipid (PY) and DNA–lipid(Anth) were both 220 °C under air. Acknowledgements We gratefully thank the ‘‘Program for New Century Excellent Talents in University’’ (NCET-08-0204), ‘‘National Natural Science Foundation of China’’ (20976060), and ‘‘the Fundamental Research Funds for the Central Universities, SCUT’’ (2009ZM0069) for financial support of this work.

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