Photoinduced Electron Transfer Mediated by π-Stacked Thymine

Feb 24, 2011 - Youssef Helwa , Neeshma Dave , Romain Froidevaux , Azadeh Samadi ... Wenying Shi , Xiaolan Ji , Shitong Zhang , Min Wei , David G. Evan...
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Photoinduced Electron Transfer Mediated by π-Stacked Thymine-Hg2þ-Thymine Base Pairs Liangqia Guo, Na Yin, and Guonan Chen* Ministry of Education Key Laboratory of Analysis and Detection for Food Safety, Fujian Provincial Key Laboratory of Analysis and Detection Technology for Food Safety, and Department of Chemistry, Fuzhou University, Fuzhou, Fujian, 350002, China

bS Supporting Information ABSTRACT: Fluorescence spectra of thymine-containing oligonucleotides labeled with donor/ acceptor in the presence of Hg2þ ions were investigated. Formation of T-Hg2þ-T base pairs gives rise to a hairpin structure and makes both termini of the oligonucleotide close to each other. For fluorescein or tetramethylrhodamine single-labeled oligonucleotide, fluorescence quenching was observed on addition of Hg2þ ions. For fluorescein and tetramethylrhodamine double-labeled oligonucleotide, the apparent FRET efficiencies decrease unexpectedly in the presence of Hg2þ ions. The unusual fluorescence quenching in the presence of Hg2þ ions was ascribed to formation of T-Hg2þ-T base pairs, which can accept and mediate the electron transfer and provide an additional de-excitation process for the excited state of fluorophores via photoinduced electron transfer.

1. INTRODUCTION M-DNA (where M stands for divalent metal ions) is a complex of DNA and divalent metal ions such as Zn2þ, Ni2þ, and Co2þ at pH of 8-9.1-5 The structure of M-DNA consists of GC and AT base pairs, in which the imino protons of G and T are replaced by metal ions without disrupting the helical nature of the duplex DNA. M-DNA has been reported to exhibit metallic-like conduction by direct measurement of conductivity.6 Electrochemical studies of self-assembled M-DNA monolayer on a gold surface demonstrated the electron transfer rates are comparable to those for a bare gold surface.7-11 Fluorescence methods have also been employed to study the electron transfer process in M-DNA by the fluorescence quenching of donor/acceptor pairs in M-DNA.2,3,12-14 M-DNA has been observed to undergo electron transfer over long distance along the helix and dramatically increase the fluorescence resonance energy transfer (FRET) efficiency. Hg2þ ions are known to interact with DNA polymers. In 1952, Katz found that addition of HgCl2 caused a remarkable reversible decrease in the viscosity of calf-thymus DNA.15 Davidson’ group discovered that Hg2þ rather than HgCl2 was bound to the base moieties, and 1.8-2.0 protons were released with the binding of each equivalent of Hg2þ.16 Marzilli’s group examined intra- and interstrand cross-linking by Hg2þ-mediated TT base pairs by UV, CD, and 1H NMR spectroscopy.17 In 2006, Ono and coworkers demonstrated that addition of Hg2þ to duplex DNA oligomer which contains thymine-thymine (T-T) mismatch results in deprotonation and formation of thymine-Hg2þthymine (T-Hg2þ-T) base pairs.18,19 DNA melting and circular dichroism (CD) studies revealed that formation of the THg2þ-T base pairs increases the duplex stability when compared with the corresponding oligomer that contains A-T base pairs or the mismatched T-T base pairs, without affecting the double-helical structure. In this regard, formation of stable DNA π-stacked containing mercury-mediated thymine-thymine pairs r 2011 American Chemical Society

suggests a new approach to the development of M-DNA. In the past few years this thymine-Hg2þ-thymine coordination chemistry has successfully been applied in the design of oligonucleotide-based Hg2þ fluorescent,20,21 colorimetric,22-25 electrochemical,26 and resonance scattering27,28 sensors. Nevertheless, little attention has been focused on the effect of T-Hg2þ-T base pairs on DNA-mediated charge transfer efficiency.29-31 In this report, we reported the fluorescence spectra of donor/ acceptor pairs separated by thymine-containing oligonucleotide which can form mercury-mediated base pairs (T-Hg2þ-T) to give rise to a hairpin structure in the presence of Hg2þ ions. It was unexpectedly observed that the acceptor fluorescence acquired from the donor energy by FRET could be quenched upon formation of T-Hg2þ-T base pairs as well as donor fluorescence. This result indicated that formation of T-Hg2þ-T base pairs can accept and mediate the electron transfer through the πstacked DNA helix. To the best of our knowledge, this is the first experimental example to explore the effect of T-Hg2þ-T base pairs on DNA-mediated charge transfer by the fluorescence technique.

2. EXPERIMENTAL SECTION 2.1. DNA Sequences. Oligonucleotides with high-performance liquid chromatography purification were purchased from Shanghai Sangon Biological Engineering Technology & Services Co., Ltd. of China. 5(6)-Carboxyfluorescein (FAM) and tetramethylrhodamine (TAMRA) were chosen as the donor and acceptor, respectively, since they are the most common pair used for FRET study. Oligonucleotide solutions were dissolved and diluted by 10 mM 3-(N-morpholino) propanesulfonic acid buffer Received: September 1, 2010 Revised: January 20, 2011 Published: February 24, 2011 4837

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Table 1. Fluorophore-Labeled Oligonucleotides Used in This Study ssDNA1

50 -FAM-TTG TTC TTC CCC CCT TGT TCT T-Tamra-30

ssDNA2

50 -FAM-TTG TTC TTC CCC CCT TGT TCT T-30

ssDNA3

50 -TTG TTC TTC CCC CCT TGT TCT T-Tamra-30

ssDNA4

50 -Cy3-TTG TTC TTC CCC CCT TGT TCT T-Cy5-30

(MOPS) with 100 mM NaNO3 pH 7.10. The base sequence and fluorophore label position of the oligonucleotides are shown in Table 1. 2.2. Fluorescence and Absorption Spectroscopy. Fluorescence spectra of ssDNA1 and ssDNA2 were recorded by a FS920 fluorimeter (Edinburgh, U.K.) at room temperature. Thirty microliters of Hg(NO3)2 aqueous solution of different concentration was mixed with 30 μL of 1.0 μM oligonucleotide solution; the mixtures were diluted with 10 mM MOPS buffer (pH 7.10) to a total volume of 300 μL and incubated for 24 h before conducting fluorescence measurements. Fluorescence spectra were corrected automatically by the fluorimeter. Fluorescence of fluorescein and rhodamine B in the presence of Hg(NO3)2 was obtained by a Cary Eclipse fluorospectrophotometer (Varian, USA). The absorption spectra of ssDNA2 and SSDNA3 in the presence of Hg(NO3)2 were recorded by a Lambda 750 UV-vis spectrophotometer (PE, USA).

3. RESULTS AND DISCUSSION 3.1. Effect of Hg(NO3)2 on the Fluorescence of DoubleLabeled and Single-Labeled Oligonucleotide. ssDNA1,

ssDNA2, and ssDNA3 have the same oligonucleotide sequence which can be divided into two parts, namely, the thymine-rich sequence and the linker sequence poly(dC). ssDNA2 and ssDNA3 were modified only with FAM at the 50 -termini or TAMRA at the 30 -termini, respectively. ssDNA1 was modified with FAM and TAMRA at the 50 - and 30 -termini as the donor and acceptor, respectively. On addition of Hg2þ ions to ssDNA1, THg2þ-T base pairs will be formed between thymine-rich sequences. Photometric titration experiments (see Figure S1 in the Supporting Information) showed the optical density around 260-280 nm decreased as the Hg2þ ion concentration increased, which suggested formation of a double-helical structure with the help of T-Hg2þ-T base pairs.18 The cooperativeness of T-Hg2þ-T base pairs helps ssDNA1 to give rise to a hairpin. The distance of both termini of the oligonucleotides changes dramatically upon formation of a hairpin structure, and its formation is strongly dependent on the concentration of Hg2þ ions. Ono and co-workers adopted a structure like ssDNA1 to design a Hg2þ sensor.20 A dark quencher 4-(dimethylaminoazo)benzene-4-carboxylic acid, which does not fluoresce, was used as the acceptor in the sensing system. Thus, the fluorescence quenching of donor could only be observed. Fluorescence spectra of ssDNA1 at the donor excitation wavelength (480 nm) on addition of Hg(NO3)2 are shown in Figure 1. As expected, the fluorescence at 520 nm of FAM decreased and that at 580 nm of TAMRA gradually increased due to the energy transfer from the donor to the acceptor via the FRET process upon addition of Hg2þ ions. However, the fluorescence at 580 nm of TAMRA unexpectedly decreased in the presence of a high concentration of Hg(NO3)2 (curve G in Figure 1). This phenomenon was different from the Kþ-stabilized guanine quartet.32 The fluorescence intensity corresponding to FAM decreased, whereas that of 6-TAMRA straight increased when

Figure 1. Fluorescence spectra (λex = 480 nm) of ssDNA1 (0.1 μM) on addition of Hg(NO3)2. The concentration of Hg(NO3)2 (from top to bottom) is as follows: (A) 0, (B) 0.2, (C) 0.4, (D) 0.6, (E) 0.8, (F) 1.0, and (G) 10.0 μM.

the potassium sensing oligonucleotide shifted its structure from the random coil form toward the tetraplex form upon addition of Kþ ions. We also investigated the effect of Hg2þ on the fluorescence spectra of ssDNA4, which has the same oligonucleotide sequence with ssDNA1 but labeled with Cy3/Cy5 as donor/acceptor paris. Similar phenomena were observed on addition of Hg2þ to ssDNA4 (see Figure S2 in the Supporting Information). The fluorescence at 565 nm of Cy3 decreased as Hg2þ ion increased. The fluorescence at 665 nm of Cy5 gradually increased due to the energy transfer from the donor to the acceptor via the FRET process in the presence of a low concentration of Hg2þ. However, the fluorescence at 665 nm of Cy5 unexpectedly decreased in the presence of a high concentration of Hg2þ. Although the donor intensity decreases on addition of Hg2þ ions, the energy is not completely transferred to the acceptor, which suggests that an additional quenching pathway associated with the T-Hg2þ-T base pairs has to be taken into account to explain the observed fluorescence quenching of the acceptor. Besides, the fluorescence intensity of FAM (curve B in Figure 1) in the presence of 0.2 μM Hg(NO3)2, which is not enough to form a hairpin structure in oligonucleotide ssDNA1, is higher than that in the absence of Hg(NO3)2 (curve A in Figure 1). The observed fluorescence enhancement may correlate with the ionic strength effect on the dissociation equilibrium of FAM. The pKa increases from 6.43 for fluorescein in the free form to about 6.90 for fluorescein attached to an oligonucleotide.33 At neutral pH, FAM is present in either the mono- or the dianion form with a different quantum yield. An increase in the ionic strength should shift the equilibrium toward the dianionic form with a higher quantum yield.34 To further explore the effect of T-Hg2þ-T base pairs on fluorescence quenching of fluorophore-labeled oligonucleotide, fluorescence spectra of the donor or acceptor single-labeled oligonucleotide were also investigated on addition of Hg(NO3)2. The fluorescence intensity of FAM single-labeled ssDNA2 decreases gradually as the concentration of Hg(NO3)2 increases (Figure 2). Under the same condition, the fluorescence intensities at 520 nm in Figure 2 are higher than those corresponding in Figure 1, which corroborates that the donor transfers its energy 4838

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Figure 2. Fluorescence spectra (λex = 480 nm) of ssDNA2 (0.1 μM) on addition of Hg(NO3)2. (Insert) Fluorescence intensity as a function of Hg(NO3)2 concentration. The concentration of Hg(NO3)2 is as follows: (A) 0, (B) 0.2, (C) 0.4, (D) 0.6, (E) 0.8, (F) 1.0, and (G) 10.0 μM.

to the acceptor via FRET in ssDNA1 in the presence of Hg(NO3)2. Fluorescence at 580 nm of ssDNA1 excited at the acceptor excitation wavelength at 540 nm, where the donor could not fluoresce, was also quenched on addition of Hg(NO3)2 (Figure 3). Similar to the double-labeled oligonucleotide, fluorescence enhancement phenomena in the presence of a low concentration of Hg(NO3)2 due to the ionic strength effect can also be observed in Figures 2 and 3. Fluorescence quenching of FAM or TAMRA single-labeled oligonucleotide in the presence of Hg2þ ions further reveals that formation of T-Hg2þ-T base pairs provides a nonradiative de-excitation process for the excited state of fluorophore. 3.2. Effect of Hg(NO3)2 on the Absorption of SingleLabeled Oligonucleotide. The absorption spectra of FAM single-labeled ssDNA2 and TAMRA single-labeled ssDNA3 on addition of Hg(NO3)2 are shown in Figures S3 and S4, respectively, in the Supporting Information. In order to obtain a relative intensive absorption signal, the concentration of oligonucleotides and Hg(NO3)2 is 10-fold that used in the fluorescence assay. In the presence of Hg(NO3)2, T-Hg2þ-T base pairs are formed between a thymine-rich sequence in the oligonucleotide to give rise to a hairpin structure. No new absorption band appears in the UV-vis spectra, and the absorption spectra of either FAM or TAMRA do not change almost, within error, over the entire range of Hg(NO3)2 concentrations, which implies that there is no ground-state complex to form between T-Hg2þ-T base pairs and FAM or between T-Hg2þ-T base pairs and TAMRA. 3.3. Measurement of Apparent FRET Efficiency by (ratio)A Method. In the presence of Hg2þ ions, formation of T-Hg2þT base pairs gives rise to a hairpin structure. The donor and acceptor at the termini of ssDNA1 are brought close to each other, which leads to an enhanced fluorescence resonance energy transfer (FRET) process between them. The FRET efficiency can be determined by using the (ratio)A method to minimize errors from sources such as instrumental noise and sample difference.34-38 The (ratio)A is an indicator of the enhanced acceptor emission intensity resulting from FRET and is a normalized measure of the enhanced acceptor emission intensity due to FRET by the directly excited acceptor emission intensity.

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Figure 3. Fluorescence spectra (λex = 540 nm) of ssDNA1 (0.1 μM) on addition of Hg(NO3)2. (Insert) Fluorescence intensity as a function of Hg(NO3)2 concentration. The concentration of Hg(NO3)2 is as follows: (A) 0, (B) 0.2, (C) 0.4, (D) 0.6, (E) 0.8, (F) 1.0, and (G) 10.0 μM.

There are many mechanisms for the fluorescence of a donor (FAM) to be quenched, but the enhancement of the acceptor (TAMRA) is probably the best evidence for FRET. The (ratio)A method is particularly advantageous for M-DNA studies.35 The (ratio)A is expressed as35

∑ FDA ðλDex , λem Þ - FD ðλDex , λem Þ ðratioÞA ¼ ∑ FA ðλAex , λemÞ λ λem

ð1Þ

em

where FDA, FD, and FA are the fluorescence intensity of the FRET pair, the donor only, and the directly excited acceptor’s fluorescence, respectively, for the emission wavelength λem. The A excitation wavelengths λD ex and λex correspond to direct excitation of the donor or acceptor molecules. The fluorescence spectra of FDA, FD, and FA correspond to Figures 1, 2 and 3, respectively. An example of fluorescence emission spectra for calculating the (ratio)A is shown in Figure 4. In eq 1, the notation ∑λem( 3 3 3 ) indicates that a discrete set of data points should be summed over an appropriate range of emission spectra. FDA(λD ex,λem) and FA(λAex,λem) are taken from the same sample of double-labeled oligonucleotide. FA is an internal normalization factor to correct the variations in the acceptor concentration and fluorescence quantum yield. When the acceptor emission is extracted by subtracting the normalized donor emission for the single-labeled sample (curve C in Figure 4) from the double-labeled sample emission excited at the donor excitation wavelength (curve A in Figure 4), some acceptor fluorescence resulting from direct excitation of the acceptor at the donor excitation wavelength at 480 nm is also included in the numerator of eq 1. Thus, this portion of fluorescence should be subtracted, as in eq 2, when calculating the FRET efficiency from the (ratio)A. The FRET efficiency E is expressed as35,36   εA ð540Þ εA ð480Þ ðratioÞA E ¼ ðdþ Þ-1 ð2Þ εD ð480Þ εA ð540Þ εD and εA are the extinction coefficients of the donor and acceptor. dþ is the fraction of DNA labeled with donor in the FRET system. In this study, donor labeling (in ssDNA1) is 100% 4839

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Figure 4. Example of fluorescence spectra (0.1 μM ssDNA1and 0.1 μM ssDNA2 in the presence of 0.4 μM Hg(NO3)2) used for calculating the (ratio)A. The y coordinate is fluorescence emission intensity, and the x coordiante is the emission wavelength. The fluorescence spectra for calculating the (ratio)A are as follows: (A) the double-labeled sample emission excited at 480 nm; (B) the donor emission for single-labeled sample excited at 480 nm; (C) the normalized donor emission for the single-labeled sample excited at 480 nm; (D) the extracted acceptor emission by subtracting C from A; (E) the acceptor emission for the double-labeled sample directly excited at 540 nm.

and dþ = 1. The extinction coefficient ratios were calculated by averaging together the data points throughout the entire range of Hg(NO3)2 concentration. We measured the extinction coefficient ratios εA(540)/εD(480) and εA(480)/εA(540) to be 1.67 ( 0.16 and 0.17 ( 0.02, respectively. The (ratio)A and the apparent FRET efficiencies were calculated and are shown in Figure 5. The values of (ratio)A are increasing gradually in the presence of Hg(NO3)2 until the concentration of Hg(NO3)2 is 10-fold that of ssDNA1. Whereas when a large amount of Hg(NO3)2 (100-fold concentration of ssDNA1) is present to ensure complete formation of the hairpin structure, the value of (ratio)A decreases on the contrary. A similar phenomenon can be observed for the apparent FRET efficiencies. When the ratio of CHg(NO3)2/CssDNA1 is less than 4, where there is not enough Hg2þ ions to coordinate completely with the thymine-thymine mismatch pairs to form the hairpin structure, the apparent FRET efficiencies are below zero. This situation indicates that the acceptor fluorescence at 580 nm is acquired mostly from direct excitation of the acceptor at the donor excitation wavelength of 480 nm rather than from FRET. In the presence of a large amount of Hg(NO3)2 (100-fold concentration of ssDNA1) for complete formation of hairpin structure, the calculated FRET efficiency is found to decrease instead. In theory, the distance-dependent FRET efficiency is defined as the quantum yield for the energy transfer process. When the donor and acceptor come closer, the FRET efficiency is unidirectional augmentation from zero to unity. The calculated (ratio)A and apparent FRET efficiencies imply that although the donor intensity decreases, the energy is not completely transferred to the acceptor via FERT upon formation of T-Hg2þ-T base pairs. An additional efficient quenching pathway has to be taken into account to explain the observed strong deviations of the experimental FRET efficiencies from theory. 3.4. Possible Quenching Mechanism. In the presence of Hg(NO3)2, Hg2þ ion binds directly to N3 of thymidine in place

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Figure 5. (ratio)A and FRET efficiency as a function of CHg(NO3)2/ CssDNA1.

of the imino proton and bridges two thymidine residues to form the T-Hg2þ-T base pairs. The fluorescence of either the donor or the acceptor single-labeled oligonucleotide was quenched upon formation of T-Hg2þ-T base pairs. In theory, fluorescence quenching in this system can occur by collisional quenching, formation of nonfluorescent ground-state complex, energy transfer, or electron transfer. The absorption band of T-Hg2þ-T base pairs (see Figure S1 in the Supporting Information) ranges between 240 and 300 nm,18 which does not overlap with the emission spectrum of FAM or TAMRA. Thus, the energy transfer from the fluorophore to T-Hg2þ-T base pairs can be ruled out. Furthermore, Hg2þ ions cannot quench the fluorescence of fluorescein and rhodamine B under the same condition (see Figures S5 and S6 in the Supporting Information); collisional quenching caused by heavy metal is also not responsible for the fluorescence quenching of single-labeled oligonucleotide. In addition, the absorption of the FAM or TAMRA single-labeled oligonucleotide did not change significantly in the presence of Hg(NO3)2; thereby a static quenching mechanism upon formation of ground-state complex can be eliminated as the possible mechanism too. Finally, the T-Hg2þ-T base pairs were not substituted directly into the fluorophore FAM or TAMRA. FAM and TAMRA were labeled at the 50 -termini and 30 -termini of oligonucleotide, respectively, with an intervening six-carbon spacer. Hg atoms were so far away from the fluorophore and hard to change the probability of T T S intersystem crossing processes of the excited fluorophore. Thus, the spin-orbital coupling mechanism could be neglected. Accordingly, the observed quenching of single-labeled oligonucleotide in the presence of Hg(NO3)2 is presumably due to electron transfer down the π-stacked DNA helix containing T-Hg2þ-T base pairs from the excited FAM or TAMRA. The fluorescence quenching of FAM or TAMRA singlelabeled oligonucleotide upon formation of T-Hg2þ-T base pairs involves electron injection into the mercury-mediated πstacked DNA helix from the excited fluorophore, and this process may be accelerated in the presence of T-Hg2þ-T base pairs. Several reasons can be proposed to support an electron transfer mechanism. First, in theory, although the metal orbitals of mercury play a minor role in mediating the hole transfer 4840

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formation of T-Hg2þ-T base pairs can accept and mediate electron transfer and provide an additional de-excitation process for the excited state of fluorophores via photoinduced electron transfer. This finding is consistent with Voityuk’s prediction that the interaction of neighboring T-Hg2þ-T base pairs plays an important role in excess electron transfer.29 We envision that our finding can be applicable to M-DNA materials based on THg2þ-T base pairs for nanoelectronic applications.

’ ASSOCIATED CONTENT

bS

Figure 6. Fluorescence quenching mechanism in double-labeled oligonucleotide: D, donor; A, acceptor; FRET, fluorescence resonance energy transfer; PET, photoinduced electron transfer.

coupling, they may play an important role in excess electron transfer because of a considerable contribution of the metal orbitals in the lowest unoccupied molecular orbital (LUMO) of [T-Hg2þ-T] base pairs.29 Second, our group demostrated that oligonucleotides with T-T mismatches would block the electron transfer through the DNA. However, formation of a stable T-Hg2þ-T base pair on addition of Hg2þ ions would make the noncomplementary oligonucleotides switch to complementary dsDNA and switch on the electron transfer through the DNA duplex.39 We also observed an additional decay component when single-labeled oligonucleotides containing consecutive thymine formed π-stacked T-Hg2þ-T base pairs with “sandwich” structure in the presence of Hg2þ ions.40 The lifetime was recorded in the 100 ps range, and the fast electron transfer dominated the deexcitation process. Third, Hg2þ ion binds to N3 of thymidine in place of the imino proton during formation of T-Hg2þ-T base pairs, Hg2þ ion has free orbitals, and the electron could be delocalized over the pyrimidine aromatic ring and metal ions; thus, the hardly or weakly electron-deficient oligonucleotides were turned into rather strong PET acceptors. Electron transfer into the T-Hg2þ-T base pairs would be energetically more favorable than electron transfer out of the hole. Accordingly, formation of T-Hg2þ-T base pairs, which can accept and mediate the electron transfer, provides an additional decay component for the excited state of fluorophore via photoinduced electron transfer, resulting in fluorescence quenching. According to the above discussion, the possible quenching mechanism in double-labeled oligonucleotide can be shown schematically in Figure 6. In the presence of Hg2þ ions, formation of T-Hg2þ-T base pairs makes the donor and acceptor at the termini of ssDNA1 close to each other, which results in an enhanced fluorescence resonance energy transfer (FRET) process. At the same time, the PET process from the excited FAM and TAMRA down to the π-stacked DNA helix containing THg2þ-T base pairs de-excites the fluorophore. The FRET process and PET process facilitate the donor fluorescence to decrease. However, when the PET effect surpasses the FRET effect, the acceptor fluorescence decreases rather than increases.

4. CONCLUSION The effect of T-Hg2þ-T base pairs on the DNA-mediated charge transfer efficiency was studied by the fluorescence technique in this study. The available evidence indicate that

Supporting Information. Absorption spectra of ssDNA1 in the prescence of Hg2þ, fluorescence spectra of ssDNA4 in the prescence of Hg2þ, absorption spectra of ssDNA2 and ssDNA3 in the presence of Hg2þ, and effect of Hg2þ on the fluorescence of fluorescein and rhodamine B. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: 86-591-22866135. Fax: 86-591-83730260. E-mail: [email protected].

’ ACKNOWLEDGMENT This project was financially supported by the National Basic Research Program of China (No.2010CB732403), the NSFC (50503006, 20735002, 40940026), and the Science foundation of Fujian Province (2008J0149, JA10017) of China. ’ REFERENCES (1) Lee, J. S.; Latimer, L. J. P.; Reid, R. S. Biochem. Cell Biol. 1993, 71, 162–168. (2) Aich, P.; Labiuk, S. L.; Tari, L. W.; Delbaere, L. J. T.; Roesler, W. J.; Falk, K. J.; Steer, R. P.; Lee, J. S. J. Mol. Biol. 1999, 294, 477–485. (3) Aich, P.; Skinner, R. J. S.; Wettig, S. D.; Steer, R. P.; Lee, J. S. J. Biom. Struct. Dyn. 2002, 20, 93–98. (4) Wettig, S. D.; Wood, D. O.; Lee, J. S. J. Inorg. Biochem. 2003, 94, 94–99. (5) Fuentes-Cabrera, M.; Sumpter, B. G.; Sponer, J. E.; Sponer, J.; Petit, L.; Wells, J. C. J. Phys. Chem. B 2007, 111, 870–879. (6) Rakitin, A.; Aich, P.; Papadopoulos, C.; Kobzar, Y.; Vedeneev, A. S.; Lee, J. S.; Xu, J. M. Phys. Rev. Lett. 2001, 86, 3670–3673. (7) Long, Y. T.; Li, C. Z.; Kraatz, H. B.; Leey, J. S. Biophys. J. 2003, 84, 3218–3225. (8) Li, C. Z.; Long, Y. T.; Kraatz, H. B.; Lee, J. S. J. Phys. Chem. B 2003, 107, 2291–2296. (9) Xu, Y.; Jiang, Y.; Cai, H.; He, P. G.; Fang, Y. Z. Anal. Chim. Acta 2004, 516, 19–27. (10) Long, Y. T.; Li, C. Z.; Sutherland, T. C.; Kraatz, H. B.; Lee, J. S. Anal. Chem. 2004, 76, 4059–4065. (11) Dinsmore, M. J.; Lee, J. S. J. Electroanal. Chem. 2008, 617, 71–77. (12) Wettig, S. D.; Bare, G. A.; Skinner, R. J. S.; Lee, J. S. Nano Lett. 2003, 3, 617–622. (13) Wettig, S. D.; Wood, D. O.; Aich, P.; Lee, J. S. J. Inorg. Biochem. 2005, 99, 2093–2101. (14) Spring, B. Q.; Clegg, R. M. J. Phys. Chem. B 2007, 111, 10040–10052. (15) Katz, S. J. Am. Chem. Soc. 1952, 74, 2238–2245. (16) Yamane, T.; Davidson, N. J. Am. Chem. Soc. 1961, 83, 2599–2607. (17) Kuklenyik, Z.; Marzilli, L. G. Inorg. Chem. 1996, 35 (19), 5654–5662. 4841

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