Luminescent Helical Nanofiber Self-Assembled from a Cholesterol

Sep 20, 2016 - Compared to pure organic amphiphiles, metalloamphiphiles display distinctive features, including luminescence, magnetism and catalytic ...
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Luminescent Helical Nanofiber Self-Assembled from a CholesterolBased Metalloamphiphile and Its Application in DNA Conformation Recognition Hairui Lei, Jing Liu,* Junlin Yan, Jingmiao Quan, and Yu Fang Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an 710062, People’s Republic of China S Supporting Information *

ABSTRACT: Compared to pure organic amphiphiles, metalloamphiphiles display distinctive features, including luminescence, magnetism and catalytic properties. However, the selforganization of metalloamphiphiles is commonly driven by solvophobic effects. Alkyl chains and oligomeric ethylene glycol moieties are thus the most frequently used aggregation units to drive the self-assembly of metalloamphiphiles. We expect novel metallo-supramolecular structures with exciting functions to be created if additional noncovalent interaction modes are incorporated. In this work, a new type of metalloamphiphile, consisting of a Tb(III) complex head and a cholesteryl unit (TbL3+(I)), was designed and synthesized. TbL3+(I) spontaneously self-assembles into helical nanofibers (d = 6 nm) in water. This synthetic multivalent nanoscale binding array displays powerful capability for the recognition of DNA conformations through a turn-on luminescence sensing mechanism. ssDNA-kit1 triggered a 26-fold increase in the luminescence intensity of TbL3+(I). Its corresponding G-quadruplex structure (Gquadruplex-kit1), however, induced a 6.6-fold enhancement under the same conditions. Consequently, TbL3+(I) nanofibers can monitor DNA folding. In contrast, neither ssDNA-kit1 nor G-quadruplex-kit1 markedly promoted the luminescence of molecularly dispersed TbL3+(II), illustrating that the multivalent electrostatic interactions between the phosphate groups on the backbone of DNA and TbL3+(I) self-assembled into nanofibers could greatly improve the efficiency of the energy transfer between the guanine units and the organized TbL3+(I). The TbL3+(I) nanofibers could bind and distinguish not only the kit1ssDNA/G-quadruplex but also the conformations of other G-rich DNA, such as spb1, htelo, and intermolec-htelo. The selfassembly of luminescent metalloamphiphiles thus provides a general and convenient strategy for the efficient recognition and conversion of molecular information.

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INTRODUCTION As a subfield of supramolecular chemistry, the self-assembly of metalloamphiphiles has received considerable attention for its contribution to the fundamental knowledge and understanding of vital biological processes, such as oxygen transport, gene activation, and catalysis.1 Compared to their corresponding counterparts (pure organic amphiphiles), metalloamphiphiles display distinctive features, including luminescence,2−4 magnetism,5,6 and catalytic properties,6−8 that are derived from the metal elements and coordination structures. However, this field is still in its infancy. The self-organization of metalloamphiphiles is mainly driven by the solvophobic effects or the combination of solvophobic effects and other noncovalent interactions.9,10 Alkyl chains and oligomeric ethylene glycol moieties are thus the most frequently used aggregation units to drive the self-assembly of metalloamphiphiles. In contrast, pure organic amphiphiles with all types of aggregation units have been widely employed to construct various self-assemblies with different functional characteristics and potential applications through all types of noncovalent interactions, including π−π interactions, van der Waals forces, hydrogen bonds, solvopho© XXXX American Chemical Society

bic effects, host−guest interactions, and electrostatic interactions.11−16 Therefore, we expect that novel metallo-supramolecular structures with exciting functions will be produced if additional noncovalent interaction modes are incorporated. The cholesteryl unit is one of the most frequently used building blocks in creating molecular gels17−20 and pure organic self-assemblies in solution21,22 because of its versatile aggregation ability. Cholesterol consists of a rigid planar framework with multiple chiral centers, which facilitates the directional stacking of the cholesteryl group through van der Waals interactions.23 The chiral self-assembly can hierarchically produce a rich variety of advanced nanostructures, such as fibers, ribbons, helices, and tubes.24 The stacking of the cholesteryl groups is postulated to produce a one-dimensional helical array with the pendant functional groups organized in a helical fashion at the periphery of the columnar core.23 Although several cholesteryl-containing amphiphilic metal Received: August 27, 2016 Revised: September 19, 2016

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exhibits distinctive emission properties, including a long luminescence lifetime, large Stokes shift, narrow emission lines and resistance to photobleaching.32 However, its intrinsic luminescence is weak due to the f−f transitions of Tb(III) ion are Laporte forbidden. When an appropriate chromophore (antenna) is placed near Tb(III) ion, antenna captures light to the singlet excited state, undergoes intersystem crossing to the triplet state and transfers its energy to the excited 5D4 state of Tb(III) ion. The transition of electrons from the excited 5D4 state to the 7FJ state produces luminescence of Tb(III) ion.32 According to this mechanism, various Ln(III)-based luminescent probes have been developed. There are two general strategies to design lanthanide complex based luminescent probes for anions.32 One is modulating the number of the inner coordinated water molecules of the Ln(III) ion. The analyte molecules replace bound water and sensitize the luminescence of the Ln(III) ion. However, selective detection of guest molecules is difficult with this method because coexisting molecules with the same anion units can display similar effects as the guest molecules. Another approach is regulating the distance between the Ln(III) ion and the antenna group to modulate the energy transfer efficiency. On the basis of this strategy, turn-on luminescent sensing systems can be designed through the binding of chromophoric guest molecules to the Ln(III) complex with weak luminescence. TbL3+(I) contains no aromatic units acting as sensitizers. Its intrinsic luminescence is therefore minimized due to the forbidden f−f transitions.33,34 Such a metalloamphiphile can identify biomolecules with antenna groups through the energy transfer from the excited aromatic units to the Tb(III) ion. The metalloamphiphile spontaneously self-assembles into helical nanofibers (d ≈ 6 nm) driven by the characteristic aggregation of the cholesteryl unit in aqueous solution (as displayed in Figure 1). This synthetic

complexes have been anchored in vesicle matrices to associate with substrate molecules,25−29 studies on the self-assembly of cholesteryl-based metalloamphiphiles are far from sufficient to design desirable syntheses for structure−function studies.30,31 Moreover, there is a need to explore the assembly process to realize the full potential of metallo-supramolecular materials. Therefore, in this work, a new type of metalloamphiphile consisting of a 3+ charged terbium(III) complex head and a cholesteryl tail was designed and synthesized (TbL3+(I), Figure 1). The Tb(III) complex was selected because the Tb(III) ion

Figure 1. Self-assembly process of TbL3+(I) in H2O and binding to ssDNA and G-quadruplex DNA.

Scheme 1. Synthesis of Ligand(I) and Complex [TbL(I)]Cl3

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NCH2 ring), 2.54−2.74 (4H, NCH2 ring), 2.25 (4H, CH2CO and NCH2CH2CH2CO), 1.45 (27H, C(CH3)3), 0.68−2.03 (45H, CH2CH2CO and cholesteryl unit). 13C NMR δC (600 MHz, CDCl3): 172.59 (1C, CO), 156.16 (1C, NCO), 155.73 (1C, NCO), 155.40 (1C, NCO), 139.58 (1C, CCH), 122.74 (1C, CCH), 79.38 (3C, C(CH3)3), 74.03 (1C, CH−O), 56.76 (1C, CH), 56.17 (1C, CH), 54.89 (1C, CH2), 53.59 (2C, CH2), 50.12 (1C, CH), 47.98 (6C, CH2), 42.34 (1C, CH), 39.79 (1C, CH2), 39.53 (1C, CH2), 38.19 (1C, CH2), 37.00 (1C, CH2), 36.57 (1C, CH), 36.20 (1C, CH2), 35.80 (1C, CH), 32.51 (1C, CH), 32.03 (1C, CH2), 31.79 (1C, CH2), 31.70 (1C, CH), 28.71 (1C, CH2), 28.54 (2C, CH3), 28.51 (1C, CH3), 28.24 (3C, CH3), 28.23 (2C, CH3), 28.02 (1C, CH3), 27.84 (1C, CH2), 24.29 (1C, CH2), 23.84 (1C, CH2), 22.82 (2C, CH3), 22.57 (1C, CH2), 21.08 (1C, CH2), 19.37 (1C, CH3), 18.73 (1C, CH3), 11.87 (1C, CH3). Elemental analysis (%): calculated for C54H94N4O8 (926.7072): C 69.94, H 10.22, N 6.04, found: C 70.27, H 10.13, N 5.66. Synthesis of Compound 4. Compound 3 (3.13 g, 3.4 mmol) was dissolved in 1,4-dioxane (30 mL) and stirred at 0 °C. Trifluoroacetic acid (7.75 mL, 101.2 mmol) was added slowly to the stirred solution. The mixture was stirred for 30 min and allowed to react at room temperature for 16 h. Then, the organic solvent was evaporated under reduced pressure to produce a condensed solution (10 mL). Abundant white powder precipitated when concentrated hydrochloric acid (20 mL) was added to the condensed solution. After filtration, the filter residue was washed with an acetone/methanol mixture (120 mL, 5:1, v/v) under reflux for 1 h to give the hydrochloride salt of compound 4 as a white solid (1.92 g, yield = 78%). The hydrochloride salt of compound 4 (0.73 g, 1 mmol) and triethylamine (4.18 mL, 30 mmol) were dispersed in anhydrous CHCl3 (50 mL), and the mixture was refluxed for 5 h. Then, the final mixture was washed with water (50 mL × 3), 0.01 mol/L NaOH (50 mL × 3) and saturated NaCl solution (50 mL × 3). The organic phase was dried with anhydrous Na2SO4 and evaporated to dryness. The residue was recrystallized from petroleum ether to give 4 as a white powder (0.56 g, yield = 89%). M.p.: 118.7−119.3 °C. IR (KBr, vmax/ cm−1): 3296 (N−H), 2939, 2797 (C−H), 1730 (CO), 1459 (C C), 1377 (C−O), 1268 (C−N). MS (m/z, ESI+): calculated for C39H70N4O2, 627.5577 ([M+H+]); found, 627.5574. 1H NMR δH (600 MHz, CDCl3, Me4Si): 5.36 (1H, CCH), 4.63 (1H, O−CH), 2.78 (4H, CH2CO and CCH2C−O), 2.56−2.64 (12H, NCH2 ring), 2.48 (2H, NCH2CH2CH2CO), 2.32 (4H, NCH2 ring), 0.67−1.98 (46H, NH, CH2CH2CO and cholesteryl unit). 13C NMR δC (600 MHz, CDCl3, Me4Si): 172.87 (1C, CO), 139.73 (1C, CCH), 122.58 (1C, CCH), 73.85 (1C, CH−O), 56.71 (1C, CH), 56.16 (2C, CH2), 53.87 (1C, CH), 51.53 (1C, CH2), 50.05 (1C, CH), 47.16 (4C, CH2), 46.24 (2C, CH2), 42.33 (1C, CH), 39.75 (1C, CH2), 39.53 (1C, CH2), 38.16 (C, CH2), 37.01 (1C, CH2), 36.61 (1C, CH), 36.20 (1C, CH2), 35.90 (1C, CH), 32.47 (1C, CH), 32.11 (1C, CH2), 31.88 (1C, CH2), 28.38 (1C, CH), 27.96 (1C, CH2), 27.81 (1C, CH2), 24.29 (1C, CH2), 23.84 (2C, CH3), 22.82 (1C, CH2), 22.57 (1C, CH2), 21.04 (1C, CH2), 19.38 (1C, CH3), 18.73 (1C, CH3), 11.86 (1C, CH3). Elemental analysis (%): calculated for C39H70N4O2 (626.5499): C 74.71, H 11.25, N 8.94, found: C 74.70, H 10.93, N 8.64. Synthesis of Ligand(I). Compound 4 (0.37 g, 0.5 mmol) was dissolved in absolute ethanol (50 mL) and stirred under a N2 atmosphere. 2-Bromoacetamide (0.25 g, 1.8 mmol), triethylamine (0.49 mL, 7 mmol) and KI (0.10 g, 0.6 mmol) were added to the above solution. The mixture was refluxed for 72 h. Then, the solvent was removed under reduced pressure. The residue was purified by a silica gel column successively eluted with chloroform/methanol mixtures with ratios of 5:1 and 1:5 to give a pale yellow solid. The obtained solid was purified further by recrystallization from acetone (120 mL) three times to give the product as a white solid (0.29 g, yield =71%). M.p.: 261.0−261.8 °C. IR (KBr, vmax/cm−1): 3312, 3161 (N− H), 2946, 2831 (C−H), 1678 (CO), 1459 (CC), 1336 (C−O), 1187 (C−N). MS (m/z, ESI+): calculated for C45H79N7O5: 798.6221 ([M + H]+), 820.6040 ([M + Na]+); found: 798.6235, 820.6055. 1H NMR δH (600 MHz, DMSO): 7.18−7.75 (6H, CONH2), 5.35 (1H,

multivalent nanoscale binding array displays powerful DNA binding and recognition (purine nucleobase as the antenna).

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EXPERIMENTAL SECTION

Materials. All chemicals were purchased and used as received without further purification. All organic solvents were of analytical grade and purified according to literature procedures. Water was purified by the Direct-Q water purification system (18.2 mΩ, Millipore Co.). All oligonucleotides were purchased from Sangon Biological Engineering Technology & Service Co., Ltd. (Shanghai, China). The sequence of oligonucleotides is displayed as follows: kit1/5′-AGGGAGGGCGCTGGGAGGAGGG-3′, intermolechtelo/5′-(TTAGGG)3-3′, spb1/5′-GGCGAGGAGGGGCGTGGCCGGC-3′, htelo/5′-GG(TTAGGG)4TTAG-3′. Preparation of the G-Quadruplex. To prepare the G-quadruplex structure, an ssDNA sequence (kit1, spb1, htelo, or intermolec-htelo) was dissolved in KCl-containing (100 mM) Tris-HCl buffer solution (10 mM, pH = 7.4). The resulting solution (CssDNA = 1.0 μM) was heated at 95 °C for 10 min, followed by rapid cooling to 4 °C and storage at this temperature overnight.35 The formation of the Gquadruplex structure was confirmed by circular dichroism (CD) measurements, and the results are displayed in Figure S11 (Supporting Information, SI). To monitor the ssDNA folding, specimens were prepared by dissolving ssDNA (1.0 μM) in Tris-HCl buffer solution (pH = 7.4, 10 mM) containing various concentrations of KCl, ranging from 0 to 100 mM. The samples were heated to 95 °C for 10 min, followed by rapid cooling to 4 °C and storage at this temperature overnight. Characterization. Luminescence measurements were conducted on a time-correlated single-photon-counting fluorescence spectrometer, Edinburgh Instruments Ltd. FLS 920. For the luminescence lifetime analysis, the specimen in D2O was prepared by completely drying up the aqueous dispersion and successively adding the same volume of D2O to the residue. CD spectra were obtained using a Chirascan plus spectropolarimeter, Applied Photophysics Ltd. The ζ potential measurement was performed on a Malvern Zetasizer NanoZS90. Transmission electron microscopy (TEM) images were obtained using a JEOL JEM-2010 instrument (acceleration voltage, 120 kV). For TEM observation, the sample solution was drop-coated on a carbon-meshed copper grid. After 1 min, the excess solution was removed. The grid was dried in vacuum. The resultant grid was observed after negative staining. AFM measurement was conducted on a SOLVER P47 PRO system. The sample was prepared by drop the solution of TbL3+(I) on a mica substrate. After 10 min, the excess solution was removed with filtration paper and the resultant mica was dried in vacuum. ESI-MS spectra were collected using a Bruker maxis UHR-TOF mass spectrometer. FTIR spectra were obtained using a Fourier Transform Infrared Spectrometer, Bruker Vertex 70v. 1H NMR and 13C NMR measurements were conducted on Bruker AV 600 NMR spectrometer. Synthesis of Ligand(I) and Complex [TbL(I)]Cl3. The synthesis procedure of ligand(I) and complex [TbL(I)]Cl3 is described in Scheme 1. Synthesis of Compound 3. Compound 136 (1.96 g, 4.0 mmol) was dissolved in anhydrous acetonitrile (70 mL) and stirred under a N2 atmosphere. Compound 237 (1.89 g, 4.0 mmol), NaHCO3 (0.84 g, 10 mmol), and KI (0.80 g, 4.8 mmol) were successively added to the above solution. The mixture was then refluxed for 96 h. The organic solvent was removed under reduced pressure. The residue was dissolved in CHCl3 (50 mL) and washed with water (30 mL × 3). The organic phase was dried with anhydrous Na2SO4 and evaporated to dryness. The obtained oil was purified by a silica gel column eluted with ethyl acetate/petroleum ether (1:2, v/v) to give 3 as a white powder (1.95 g, yield = 53%). M.p.: 43.3−44.5 °C. IR (KBr, vmax/ cm−1): 2942 (C−H), 1692 (CO), 1412(CC), 1358 (C−O), 1244 (C−N). MS (m/z, ESI+): calculated for C54H94N4O8, 927.7150 ([M + H]+); found, 927.7166. 1H NMR δH (600 MHz, CDCl3, Me4Si): 5.37 (1H, CCH), 4.61 (1H, O−CH), 3.16−3.74 (12H, C

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Figure 2. TEM image (A) and AFM image (B) of TbL3+(I) nanofibers. Temperature-dependent CD spectra (C) of TbL3+(I) in 10 mM Tris-HCl buffer (pH = 7.4). The concentration of TbL3+(I) was 150 μM.

aggregation mode of TbL3+(I) in water, CD spectra of the TbL3+(I) solution were measured. As shown in Figure 2C, a negative CD signal appeared at 205 nm. The CD intensity decreased as the temperature increased and almost vanished at high temperature (T = 90 °C), which can be ascribed to the gradual deaggregation of the helical stacking architectures. The above conclusion was further confirmed by the CD spectrum measurement conducted for the molecularly dispersed TbL(I)3+ in methanol (Figure S3A, SI). The CD spectrum of the methanol solution of TbL(I)3+ almost did not display obvious CD signals, which indicates that the observed CD signal in Figure 2C is attributed to the chiral self-assemblies rather than a single TbL(I)3+ molecule. The CD spectra of the hydrochloride salt of compound 4 (Scheme 1) at different temperature were also measured. As shown in Figure S3B (SI), a CD signal at the same position (λ = 205 nm) was also observed, and it almost vanished at high temperature. Therefore, it is reasonable that the covalently bound cholesteryl moieties aggregated in a specific, chiral direction, which forced Tb(III) complex head to interact in an asymmetric manner. Considering the molecular length of TbL3+(I) (ca. 2.9 nm, Figure S4, the molecular simulation details are described in the SI) and the chiral aggregation characteristics of the cholesteryl unit,23 TbL3+(I) molecules should pack helically, as schematically shown in Figure 1. The photophysical properties of TbL3+(I) were studied in Tris-HCl buffer (10 mM, pH = 7.4). TbL3+(I) contains no aromatic group as an antenna; its luminescence is consequently minimized due to the forbidden f-f transitions32 (Figure S5, SI). Upon addition of kit1, a single-stranded G-rich DNA (ssDNA) sequence in an oncogenic promoter, the luminescence of TbL3+(I) was drastically triggered (Figure S5). In the emission spectra, luminescence peaks were observed at 495 (5D4 → 7F6), 545 (5D4 → 7F5), 586 (5D4 → 7F4) and 621 nm (5D4 → 7F3), with a maximum intensity at 545 nm, which are the photophysical characteristics of the Tb(III) ion.38 However, kit1 produced no luminescence under the same conditions (Figure S5), indicating that the observed luminescence should be sensitized by the binding of ssDNA. The guanine unit on the ssDNA backbone harvests light to the singlet excited state, undergoes intersystem crossing to the triplet state and transfers its energy to the excited 5D4 state of Tb(III) ion. The transition of electrons from the excited 5D4 state to the 7FJ state produces luminescence of Tb(III) ion.39,40 This was proved by the excitation spectrum monitored at 545 nm, which displayed a peak assigned to the guanine group at 270 nm. Successive addition of ssDNA-kit1 induced a regular increase in

CCH), 4.45(1H, O−CH), 2.94−3.05(6H, NCH2CO), 2.19− 2.51(22H, NCH2CH2CH2CO, CCH2C−O and NCH2 ring), 0.67− 1.95(43H, CH2CH2CO and cholesteryl unit). 13C NMR δc (600 MHz, CD3OD, Me4Si): 176.43 (1C, OCO), 176.01 (1C, NH2C O), 174.64 (2C, NH2CO), 141.08 (1C, CCH), 123.68 (1C, C CH), 75.48 (1C, C−O), 58.16 (3C, CH2), 57.87 (1C, CH), 57.63 (1C, CH), 57.31 (2C, CH2), 54.63 (1C, CH2), 51.66 (6C, CH2), 43.54 ((1C, CH)), 41.16(1C, CH), 40.72(1C, CH2), 39.26(1C, CH2), 38.29(1C, CH2), 37.81(1C, CH2), 37.42(1C, CH), 37.14(1C, CH2), 33.36(1C, CH), 33.24 (1C, CH), 33.05 (1C, CH2), 29.33 (1C, CH2), 29.17 (1C, CH), 28.90 (1C, CH2), 25.33 (1C, CH2), 24.98 (1C, CH2), 23.21 (1C, CH2), 22.97 (2C, CH3), 22.18 (1C, CH2), 21.87 (1C, CH2), 19.81 (1C, CH3), 19.30 (1C, CH3), 12.36 (1C, CH3). Synthesis of [TbL(I)]Cl3. Ligand(I) (199.40 mg, 0.25 mmol) was dissolved in water (30 mL) and stirred at room temperature. An aqueous solution of terbium(III) chloride (93.35 mg, 0.25 mmol, 40 mL) was added dropwise to the above solution. The reaction mixture was stirred at 50 °C for 24 h. Then, the solvent was removed under reduced pressure. The residue was purified by dissolving it in methanol (2 mL) and precipitating from diethyl ether (20 mL) three times. The obtained solid was purified further by recrystallization in CH2Cl2 (20 mL) two times to give the complex as a white powder (230 mg, yield = 79%). IR (KBr, vmax/cm−1): 3375 (N−H), 2939, 2872 (C−H), 1663, 1594 (CO), 1466 (CC), 1316 (C−O), 1092 (C−N). MS (m/z, ESI+): calculated for C45H79Cl3N7O5Tb: 1026.4784 ([M-Cl]+), 495.7545 ([M-2Cl]2+); found: 1026.4770, 495.7535. Elemental analysis (%): calculated for C45H79Cl3N7O5Tb (1061.4478): C 50.82, H 7.49, N 9.22, found: C 51.25, H 7.78, N 9.09. Preparation of Assemblies. To prepare the self-assemblies of TbL3+(I) in water, a solution of [TbL(I)]Cl3 (150 μM) in Tris-HCl buffer solution (10 mM, pH = 7.4) was heated in a hot water bath at 60 °C for 3 h, followed by slow cooling to room temperature and storage at this temperature overnight.

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RESULTS AND DISCUSSION To investigate the self-assembly behavior of TbL3+(I), its critical aggregation concentration (CAC) in Tris-HCl buffer (10 mM, pH = 7.4) was determined by measuring the surface tension (γ) as a function of the TbL3+(I) concentration. As shown in Figure S2 (Supporting Information, SI), TbL3+(I) displayed a linear decrease in γ below 56 μM and remained almost unchanged beyond this concentration, indicating that the CAC of TbL3+(I) is approximately 56 μM. The selfassembled structures of TbL3+(I) in water were observed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). As shown in Figure 2A, TbL3+(I) molecules self-assembled into nanofibers with an average diameter of ∼6 nm and an average length of several micrometers. The formation of TbL3+(I) nanofibers was further confirmed by AFM (Figure 2B). To further reveal the D

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Figure 3. Luminescence spectra of TbL3+(I) in the absence and presence of ssDNA-kit1 (A); I/I0 evolution of TbL3+(I) with increasing concentrations of ssDNA-kit1, dsDNA-kit1, and G-quadruplex-kit1. Inset: photographs of the aqueous solutions of (a) TbL3+(I) + ssDNA-kit1, (b) TbL3+(I) + dsDNA-kit1, (c) TbL3+(I) + G-quadruplex-kit1, and (d) TbL3+(I) under a 254 nm ultraviolet lamp (B). The concentration of TbL3+(I) was 150 μM in 10 mM Tris-HCl buffer (pH = 7.4), λex = 260 nm.

6.47 and 10.65 ppm were assigned to the NH2 and NH protons on the guanine base. The addition of K+ caused the two peaks to shift to 6.62 and 10.79 ppm, suggesting the occurrence of guanine aggregation through intermolecular hydrogen bonding. The aggregation of guanine units could favor photoelectron transfer (PET) between them and thus deactivate the energy transfer from the guanine group (antenna) to TbL3+(I).43,44 This conclusion was confirmed by the luminescent titration experiment of the double-stranded DNA, dsDNA-kit1. As shown in Figure 3B, the sensitization efficiency of dsDNA-kit1 is between the two previous compounds; a 13-fold increase was obtained when the concentration of dsDNA-kit1 exceeded 0.6 μM. The formation of intermolecular hydrogen bonds between the complementary base pairs also deactivated the energy transfer process. The binding of ssDNA-kit1 to TbL3+(I) nanofibers was verified by the variation in the ζ potential (Figure S7, SI). The ζ potential measured for the TbL3+(I) helical nanofibers (ζ = +49.6 mV) showed a marked decrease with increasing concentration of ssDNA-kit1, presented a gradual decline above a ssDNA-kit1 concentration of ∼0.8 μM, and then reached the lowest ζ value of 43.8 mV. This threshold was almost in agreement with that observed in the luminescence titration curve (Figure 3B). The TbL3+(I) nanofibers could sense and distinguish the kit1-ssDNA/G-quadruplex and the conformations of other ssDNA. As shown in Figures S8A, S9A, and S10A (SI), ssDNAspb1, ssDNA-htelo, and ssDNA-intermolec-htelo significantly triggered the luminescence of TbL3+(I). Approximately 27.8-, 25.8-, and 31.8-fold increases (I/I0 = ITbL(I)+ssDNA/ITbL(I)) were observed for ssDNA-spb1, ssDNA-htelo, and ssDNA-intermolec-htelo, respectively. However, the sensitization efficiency (I/ I0 = ITbL(I)+G‑quadruplex/ITbL(I)) of their corresponding Gquadruplex conformations was much lower, with approximately 5.4-, 10.5-, and 14.9-fold enhancements (Figures S8B, S9B, and S10B, SI). Therefore, the nanofibers self-assembled from amphiphilic TbL3+(I) provide a fascinating platform for sensing and distinguishing the conformations of DNA sequences. In addition to recognizing the conformations of ssDNA, the TbL3+(I) nanofibers could monitor the ssDNA-to-G-quadruplex conversion, which is an important process in living organism. K+ was chosen to facilitate this conversion process, as reported elsewhere.45−47 The luminescence emission spectra of the TbL3+(I)/ssDNA-kit1 mixture in the presence of K+ with various concentrations were recorded and are shown in Figure 5. As demonstrated above, ssDNA-kit1 remarkably triggered

luminescence intensity, as shown in Figure 3A. The luminescence intensity of TbL3+(I) was enhanced 26-fold when the concentration of ssDNA-kit1 exceeded 0.9 μM (Figure 3B). In contrast, G-quadruplex-kit1 (see Experimental Section for the preparation details) induced a 6.6-fold enhancement under the same conditions (Figure 3B). The apparent binding constants (Ka) of TbL3+(I) with ssDNA-kit1 and G-quadruplex-kit1 were estimated as 3.57 × 107 and 9.01 × 106 M−1, respectively,41 revealing strong interactions between the DNA sequence and TbL3+(I). TbL3+(I) is a 3+ charged compound, and DNA contains abundant negative charges on its backbone. The multivalent electrostatic interactions between TbL3+(I) assembled as nanofibers and DNA chains could lead to a short distance and efficient energy transfer between the guanine unit and the Tb(III) complex (this will be confirmed later by luminescence lifetime measurements). However, neither ssDNA-kit1 nor its corresponding G-quadruplex structure remarkably promoted the luminescence of the molecularly dispersed complex, TbL3+(II)3,42 (Figure 4),

Figure 4. I/I0 evolution of TbL3+(I) and TbL3+(II) as a function of the concentration of ssDNA-kit1 (square) and G-quadruplex-kit1 (circle). The concentration of TbL3+(I) and TbL3+(II) for all experiments was 150 μM in 10 mM Tris-HCl buffer (pH = 7.4), λex = 260 nm.

illustrating that the interaction between the single TbL3+(II) molecule and DNA chains is not sufficiently strong to trigger efficient energy transfer from the DNA chains to TbL3+(II). This supports the above preliminary conclusions. To understand the different sensitization efficiencies of the two conformations, a 1H NMR titration experiment was conducted, and the results are displayed in Figure S6 (SI). The signals at E

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induced a 6.6-fold enhancement under the same conditions. Consequently, the nanofibers could monitor the ssDNA-to-Gquadruplex conversion. In contrast, neither ssDNA-kit1 nor its corresponding G-quadruplex markedly promoted the luminescence of molecularly dispersed TbL3+(II), illustrating that the multivalent electrostatic interactions between the phosphate groups on the backbone of DNA and TbL3+(I) self-assembled into nanofibers could greatly improve the efficiency of the energy transfer between the guanine units and the organized TbL3+(I). The TbL3+(I) nanofibers could bind and distinguish not only the kit1-ssDNA/G-quadruplex but also the conformations of other G-rich DNA, such as spb1, htelo and intermolec-htelo. Although vesicles of Na+ complexes containing a cholesteryl unit have been developed to sense amino acid anions, premodification of the target molecules made online detection impossible because the amphiphilic Na+ complex is not luminescent.31 Therefore, the self-assembly of cholesterolbased luminescent metalloamphiphiles provides a general and convenient strategy for the efficient recognition and conversion of molecular information. Considering the comprehensive characteristics of supramolecular chemistry, self-assembly of metalloamphiphiles will find more interesting multidisciplinary applications, including separation, catalysis and biotherapy.

Figure 5. Luminescence spectra of the TbL3+(I)/ssDNA-kit1 mixture in the absence and presence of KCl. The concentration of KCl ranged from 0 to 100 mM. Inset: 1 − I/I0 evolution of TbL3+(I) with increasing concentration of KCl. The luminescence intensity at 545 nm was selected for the analysis. The concentration of TbL3+(I) and ssDNA were 150 and 1.0 μM, respectively, in 10 mM Tris-HCl buffer (pH = 7.4), λex = 260 nm.

the luminescence of TbL3+(I) (ITbL(I)+ssDNA‑kit1/ITbL(I) = 26), but its corresponding G-quadruplex induced a 6.6-fold enhancement in luminescence intensity. Therefore, it is reasonable that the TbL3+(I)/ssDNA-kit1 mixture showed a significant decrease in luminescence intensity upon the addition of K+ (Figure 5). The decrease in the luminescence intensity of the TbL3+(I)/ssDNA-kit1 mixture (1-I/I0 = 1-ITbL(I)+G‑quadruplex‑kit1/ ITbL(I)+ssDNA‑kit1) reached a maximum value of 0.80 when the concentration of K+ exceeded 25 mM (Figure 5, inset). The formation of G-quadruplex-kit1 was confirmed by CD spectra (Figure S11, SI). Moreover, TbL3+(I) could also monitor the conformational conversion of other G-rich ssDNA, as shown in Figure S12 (SI). To reveal the manner of interactions between TbL3+(I) and the ssDNA chains, the hydration number of the Tb(III) complex, q, was determined by measuring the excited state lifetimes (τ) of TbL3+(I) in different solutions (Figure S13, SI).48 The experimental results indicated that approximately one H2O molecule binds to TbL3+(I) in aqueous solution (Table S1, SI). Considering that the coordination number of Tb(III) is 8 or 9,49 the coordination sphere of TbL3+(I) is saturated by ligands and bound water, leaving no coordination site for the phosphate anion of DNA chains. The q value remained unchanged after adding ssDNA-kit1 or G-quadruplexkit1 (Table S1, SI). No ligand exchange occurred with the coordinated water molecule during the binding of ssDNA or Gquadruplex to TbL3+(I). Therefore, the simultaneous electrostatic interactions between the TbL3+(I) nanofiber and DNA chains play vital roles in DNA binding.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b03181. FTIR spectra of ligand(I) and [TbL(I)]Cl3, determination of CAC for [TbL(I)]Cl3, CD spectra of [TbL(I)]Cl3, molecular dynamics simulation for the length of TbL3+(I), photophysical characteristics of [TbL(I)]Cl3, 1H NMR spectra of guanosine and guanine tetrad, luminescence spectra of [TbL(I)]Cl3 and [TbL(I)]Cl3/DNA mixtures, CD spectra of various G-rich DNA, Zeta potential evolution of [TbL(I)]Cl3, luminescence lifetime measurements (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work was supported by the Natural Science Foundation of China (Grant 21273143, 21573141, 21503128), Foundation for the Author of National Excellent Doctoral Dissertation of China (Grant 201223), Program for New Century Excellent Talents in University (Grant NCET-130887), Fundamental Research Funds for the Central Universities (Grant GK201504006), and Program of Introducing Talents of Discipline to Universities (Grant B14041).

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CONCLUSIONS In summary, a new family of metalloamphiphiles consisting of a Tb(III) complex head and a cholesteryl group (TbL3+(I)) was designed and synthesized. This amphiphilic Tb(III) complex spontaneously self-assembles into helical nanofibers (d = 6 nm) in aqueous solution. The self-assembled nanofibers of TbL3+(I) proved to be a powerful synthetic multivalent nanoscale binding array to sense and distinguish the conformations of G-rich DNA through a turn-on luminescence sensing mechanism. ssDNA-kit1 triggered a 26-fold luminescence intensity increase of TbL3+(I). G-quadruplex-kit1; however,

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DOI: 10.1021/acs.langmuir.6b03181 Langmuir XXXX, XXX, XXX−XXX