Photoinduced Electron Transfer between Anionic Corrole and DNA

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Photoinduced Electron Transfer between Anionic Corrole and DNA Li-Li Wang, Lei Zhang, Hui Wang, Yang Zhang, Jun-teng Huang, He Zhu, Xiao Ying, Liang-Nian Ji, and Hai-Yang Liu J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b11021 • Publication Date (Web): 11 Jan 2016 Downloaded from http://pubs.acs.org on January 12, 2016

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The Journal of Physical Chemistry

Photoinduced Electron Transfer between Anionic Corrole and DNA Li-Li Wang,1 Lei Zhang,1 Hui Wang,*,1 Yang Zhang,2 Jun-Teng Huang,2 He Zhu,3 Xiao Ying,3 Liang-Nian Ji,1,4 Hai-Yang Liu*,2 1

State Key Laboratory of Optoelectronics Materials and Technologies, Sun Yat-sen University, Guangzhou 510275, China

2

3

Department of Chemistry, South China University of Technology, Guangzhou 510641, China

Department of Applied Physics, South China University of Technology, Guangzhou 510641, China 4

School of Chemistry and Chemical Engineering/MOE Laboratory of Bioinorganic and Synthetic Chemistry, Sun Yat-sen University, Guangzhou 510275, China

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Abstract The interaction between a water-soluble anionic Ga(III) corrole [Ga(tpfc)(SO3Na)2] and calf thymus DNA (ct-DNA) has been investigated by using femtosecond transient absorption spectroscopy. A significant broadening from 570 nm to 585 nm of positive absorption band of the blend of Ga(tpfc)(SO3Na)2 and ct-DNA ( Ga(tpfc)(SO3Na)2-ctDNA ) has been observed from 0.15 ps to 0.50 ps after photoexcitation of Ga(tpfc)(SO3Na)2 into the Soret band. The control experiment has been performed on the model DNA ([poly(dG-dC)]2) rich in guanine bases, which exhibits a similar spectral broadening, whereas it is absent for [poly(dA-dT)]2 without guanine bases. The molecular orbital calculation shows that HOMO of Ga(tpfc)(SO3Na)2 is lower than that of guanine bases. The results of the electrochemical experiment show the reversible electron transfer (ET) between Ga(tpfc)(SO3Na)2 and guanine bases of ct-DNA are thermodynamically favorable. The dynamical analysis of the transient absorption spectra reveals that an ultrafast forward ET from the guanine bases to Ga(tpfc)(SO3Na)2 occurs within the pulse duration (156 fs), leading to the formation of an intermediate state. The following back ET to the ground state of Ga(tpfc)(SO3Na)2 may be accomplished in 520 fs.

Keywords: Corrole, Gallium, DNA, Photoinduced electron transfer, Femtosecond transient absorption


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Introduction Electron transfer (ET) is ubiquitous in biology, an essential component of natural photosynthesis and photochemical reactions. More importantly, ET processes play a crucial role in DNA damage and repair.1-3 Previous works have shown that photoinduced ET reactions can lead to oxidation damage which occurs preferentially at guanine sites possessing the lowest oxidation potential of all bases.4-5 Many photosensitizer molecules have been used to generate a radical cation (hole) in DNA by intercalating them into the base pairs.1, 6 Barton et al. have studied the photooxidation and long-range ET in DNA, and they have found that the introduction and transfer of a hole in DNA can promote the oxidative damage and repair in DNA.7-12 It has been verified that photoexcitation of an intercalated dye can induce ultrafast injection of holes into DNA, and the ET reactions occur usually on a femtosecond time scale.1, 6, 13 On the other hand, the interaction of photosensitizer molecules with DNA by an outside binding mode has not been as well studied, in particular anionic molecules, which is believed to be longer reaction path as compared to the cationic molecules, leading to less efficient photocleavage involving singlet oxygen and hydroxyl radicals.14-15 Several studies have shown that anionic porphyrins can indeed interact with DNA and selectively cleave the nucleic acid to destroy the cancer or tumor cells whereas they do not bring any changes in the original structure of DNA.14-17 Corrole is a tetrapyrrolic macrocycle having a direct link between adjacent pyrrole rings.18-20 Corrole derivatives represent a new kind of photosensitizers with intriguing potential applications in pharmaceuticals21-22, photodynamic therapy (PDT)23-25 and photodynamic detection (PDD)25. The conjugate of sulfonated gallium corrole with HerPBK10 exhibits tumor-targeted toxicity and intense fluorescence in vivo.24-25 Therefore, it can not only be used for tumor detection, but also for targeting the delivery in vivo, leading to tumor cell death without normal tissue damage.24-25 Previous studies 3 ACS Paragon Plus Environment


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have shown that cationic corroles can interact with DNA and stabilize DNA G-quadruplex structure and induce formation of G-quadruplex structure.26-28 The study on the interaction between cationic tri-N-methylpyridyl corroles (TMPC) and nucleic acid homopolymers has demonstrated that TMPC may be a promising probe for discriminating between ss and ds DNA conformations.29 Anionic sulfonated metallocorroles is one of the most studied water-soluble corroles, they can be delivered into target tumor cells when binding with appropriate protein carriers.30 Recently, we found sulfonated gallium corrole Ga(tpfc)(SO3Na)2, an outside DNA binders, displayed the good photocleavage activity.31 However, the mechanism of DNA cleavage is not clear. In this article, we report our investigation of the interaction between a water-soluble anionic corrole Ga(tpfc)(SO3Na)2 and ct-DNA by means of femtosecond transient absorption spectroscopy. The ultrafast reversible ET between Ga(tpfc)(SO3Na)2 and guanine bases of DNA has been observed upon exciting Ga(tpfc)(SO3Na)2 into the Soret band. The experimental confirmation has been performed on [poly(dG-dC)]2 and [poly(dA-dT)]2 rich and absent in guanine bases, respectively. The forward and back ET rates have been estimated by dynamic fitting of the transient absorption spectra. The results of electrochemical experiment and density functional theory (DFT) calculation show that both forward and back driving forces are negative, indicating the ET between DNA and Ga(tpfc)(SO3Na)2 is thermodynamically favorable.

Experimental Section Sample preparation Ga(tpfc)(SO3Na)2 was synthesized according to the procedures described in the literatures.31-33 Figure 1 shows the molecular structure of Ga(tpfc)(SO3Na)2, in which the metal center binds with no axial ligand pyridine. ct-DNA, [poly(dG-dC)]2 and [poly(dA-dT)]2 were commercially available products from Sigma-Aldrich. ct-DNA contains 42% guanine-cytosine (GC) base pairs, and [poly(dG-dC)]2


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contains 100% GC base pairs.34 DNA-binding experiments were performed in Tris–HCl buffer solution (5 mM Tris, 50 mM NaCl, pH = 7.2).

Figure 1. Molecular structure of Ga(tpfc)(SO3Na)2. The concentrations of DNA were determined from their optical absorbance at 260 nm using the molar absorption coefficient of 6600 M-1cm-1 for ct-DNA, 7100 M-1cm-1 for [poly(dG-dC)]2, and 8300 M-1cm-1 for [poly(dA-dT)]2, respectively.35-36 Measurements Steady-state Spectra Measurements. Steady-state absorption and emission spectra of all samples were measured using a PerkinElmer Lambda 850 UV-vis Spectrometer and a PerkinElmer LS55 Luminescence Spectrometer (PE Company, USA), respectively. The excitation wavelength was 560 nm. Ga(tpfc)(SO3Na)2 (9.6 µM) in Tris-HCl buffer solution (pH = 7.2) was titrated by adding microliter portions of ct-DNA stock solution. Absorption titration experiments are widely used to determine the DNA-binding activities of small molecules. The steady-state absorption spectra of the blend of Ga(tpfc)(SO3Na)2 and ct-DNA at various ct-DNA concentrations show an intense Soret band (B band) at 424 nm and two weak Q bands around 611 nm and 588 nm, respectively, as shown in Figure 2, which are characteristics of Ga(tpfc)(SO3Na)2.37 The Q-band absorptions are more intense compared to porphyrins. Titration of ct-DNA into a solution of Ga(tpfc)(SO3Na)2 induces a 12.4% hypochromism, suggesting that Ga(tpfc)(SO3Na)2 binds with ct-DNA via an outside binding mode, which may be due to 5 ACS Paragon Plus Environment


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the interactions between the anionic corrole and the negative phosphodiester groups of DNA, resulting in the lower intercalation tendency.33, 38-39 The hypochromism should be not attributed to the change of the solvent polarity after the addition of ct-DNA because no appreciable bathochromic shift was observed.40-41 Furthermore, the critical concentration for aggregation of Ga(tpfc)(SO3Na)2 is about 25 µM.31 The much lower concentration of Ga(tpfc)(SO3Na)2 in our experiment should not induce the observed hypochromism as shown in Figure 2. The intrinsic binding constant (Kb) of Ga(tpfc)(SO3Na)2 to ct-DNA can be obtained according to eq 1,31, 38

{

}

[ DNA] / ( ε a − ε f ) = [ DNA] / ( ε b − ε f ) + 1 / K b ( ε b − ε f )

(1)

where εf and εb are the extinction coefficients of Ga(tpfc)(SO3Na)2 at the Soret-band absorption maximum of the free and bound species, respectively. εa is the apparent extinction coefficient of Ga(tpfc)(SO3Na)2 in the presence of ct-DNA, which is obtained by calculating ratio of observed absorbance of the mixed solution to the total corrole concentration. [DNA] denotes the concentration of ct-DNA in the base pairs. The plot of [DNA]/(|εa-εf|) versus [DNA] is shown in the inset of Figure 2. The value of Kb is the ratio of the slope to the intercept as 3.48 × 104 M-1, which is similar to the reported value (1.40 × 104 M-1) about the interaction of anionic porphyrin with ct-DNA.17


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Figure 2. Changes in the absorption spectra of Ga(tpfc)(SO3Na)2 upon the addition of ct-DNA in Tris-HCl buffer (pH = 7.2). The inset shows the plot of [DNA]/(|εa-εf|) versus [DNA], where the error bars indicate standard deviations. Quantum-chemical Calculations. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been used to elaborate the electronic structure and provide band assignments in UV-vis spectra of corroles.42-44 The calculations of the molecular orbital energy have been performed using Gaussian09. Geometry optimization of Ga(tpfc)(SO3Na)2 and DNA bases was performed with DFT using the B3LYP exchange-correlation function.44-45 The result is shown in Figure 3. For Ga(tpfc)(SO3Na)2, cc-pVDZ-pp basis set was used for the gallium atom46, 6-31G(d) for the C and H atoms, and 6-311G+(d, p) for the N, O, F, S atoms, while the correlation consistent basis set for DNA bases was 6-311G(d, p). Electronic transition energies and the corresponding oscillator strengths of Ga(tpfc)(SO3Na)2 were calculated with TDDFT at the same level of theory. The results are summarized in Table S1 (see in the SI). Similar to the earlier results42, 44, the two electronic transitions in the Q-band show that the transition S0 → SI is associated to a combination of an electron transfer between HOMO → LUMO and HOMO-1 → LUMO+1, and the transition S0 → SII corresponds to a 7 ACS Paragon Plus Environment


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combination of an electron transfer between HOMO-1 → LUMO and HOMO → LUMO+1. Taking into account a constant energy offset of 2070 cm-1 for the calculation, the computed wavelength positions agree well with the experimental values.

Figure 3. Representations of the HOMO and LUMO of Ga(tpfc)(SO3Na)2 and four DNA bases (guanine, adenine, cytosine and thymine). Electrochemical Experiment. Cyclic and differential pulse voltammograms of Ga(tpfc)(SO3Na)2 were collected using a CH Instruments Model-CHI660E electrochemical analyzer under nitrogen atmosphere at ambient temperature. The three-electrode configuration was consisted of a Glassy Carbon working electrode (diameter = 1 mm), a Pt auxiliary electrode and an Ag/AgCl (saturated KCl) reference electrode. The values (vs Ag/AgCl) were converted to those versus the standard hydrogen electrode (NHE) by adding 0.29 V.47 The scanning of each cyclic voltammograms had been repeated three times. Figure 4 shows the voltammograms of Ga(tpfc)(SO3Na)2 in Tris-HCl buffer (pH = 7.2). Around 1.21 V, there is a single irreversible oxidation peak, which corresponds to the formation of 8 ACS Paragon Plus Environment

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cation. And the single irreversible reduction peak at around -1.01 V is assigned to the formation of anion. Therefore, the oxidation (Eox) and reduction (Ered) potentials of Ga(tpfc)(SO3Na)2 are 1.21 V and -1.01 V (vs NHE), respectively, which can be used to estimate the energies of the HOMO (-5.81 eV) and LUMO (-3.59 eV).48 The results are similar to those derived from DFT calculation shown in Figure 3.

Figure 4. Cyclic voltammograms of Ga(tpfc)(SO3Na)2 in Tris-HCl buffer (0.3 mM, pH = 7.2) at a scan rate of 100 mV/s. The insets are the corresponding differential pulse voltammograms showing the oxidation and reduction potentials of Ga(tpfc)(SO3Na)2, respectively. Femtosecond Transient Absorption Experiment. Femtosecond transient absorption spectra of free Ga(tpfc)(SO3Na)2 and Ga(tpfc)(SO3Na)2 complexed to DNA (Ga(tpfc)(SO3Na)2-DNA) were measured by using the apparatus described in the Supporting Information (SI). Briefly, an amplified Ti: sapphire laser (Hurricane, Spectra Physics Inc.) produced 156 fs pulses at 800 nm with a repetition rate of 1 kHz. This output was divided into two parts by an optical wedge (W1). One was used for generating pump pulses through a BBO frequency doubling crystal. Another was focused into a flowing water cell to generate the white light continuum used as the probe and reference. The relative polarization of the 9 ACS Paragon Plus Environment


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pump and probe was set to 54.7º (“magic angle”) to remove any contributions from orientational effects.49-50 The relative delay between pump and probe was controlled by an optical delay line with micrometric precision (0.3 µm → 1 fs). The pump spot diameter on the sample was ~1 mm, which is much larger than the probe one. And the pump energy at the sample was 2 µJ/pulse. The sample solutions were placed in a 2-mm-thick quartz cell. The sample was exchanged in a stirring cell between two consecutive pump pulses. The pump beam was blocked after the sample, while the probe and reference beams were coincided into a monochromator (Andor SR-500-B2) and detected by a CCD detector (Andor DU971N-BV). All spectra were corrected for the chirp of the white-light probe pulses. The excitation wavelength of 400 nm corresponded to the transition of Ga(tpfc)(SO3Na)2 from the ground state S0 to the B band. The concentration of Ga(tpfc)(SO3Na)2 was 0.2 mM, and the molar ratio of corrole to DNA was about 0.2. The excited-state absorption spectra of all-trans-β-Apo-8’-carotenal in n-hexane (as the standard sample) were measured by using the same experimental apparatus, and the result (Figure S3 in the SI) is in agreement with the previous reports.51-52 In our experiment, in order to eliminate the instrument response, the kinetics data were analyzed by fitting to the exponential function convoluted with a Gaussian response function. We estimated the error on the measured lifetimes to be within ± 0.05 ps. All measurements were performed at room temperature (296 K) and aerated conditions. The spectral data of the ground and excited states of Ga(tpfc)(SO3Na)2 in Tris-HCl buffer are similar to those of Ga(tpfc)(SO3Na)2 in doubly distilled water, which suggest that Tris-HCl buffer has no obvious impact on the spectroscopic properties of Ga(tpfc)(SO3Na)2. The sample was exchanged completely after each experiment. No significant photo-degradation of the samples was observed in the spectrometers used in this study.

Results 10 ACS Paragon Plus Environment

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The


evolutions

of

the

transient

absorption

spectrum

of

Ga(tpfc)(SO3Na)2

and

Ga(tpfc)(SO3Na)2-ctDNA were monitored between 420 nm and 700 nm, following the excitation of Ga(tpfc)(SO3Na)2 at 400 nm. Figure 5a exhibits the transient absorption spectra of free Ga(tpfc)(SO3Na)2 at different time delays. At early time delays (middle panel), a broad positive absorption band extends between 450 nm and 570 nm, corresponding to the absorption of the population in excited states. Meanwhile, an intense negative band in 420 ~ 450 nm region (B-band) and two weak negative bands in 570 ~ 620 nm region (Q-band) arise, which are caused by the ground-state bleaching (GB) by compared with the steady-state absorption spectrum of the free Ga(tpfc)(SO3Na)2 shown in the upper panel. The GB signals in B-band region become blue-shifted with the time evolution, which may be due to the decay of the stimulated emission from the high excited-state (B-band) and the absorption of the excited states.53-54 A sharp minus peak at 460 nm appears around the delay time of 0.15 ps and a red-shift of about 60 nm with respect to the excitation wavelength (400 nm) occurs. It should be the stimulated Raman gain (SRG) signal assigned to the OH stretching mode of the solvent,55-56 because the fluorescence signals are always removed from the detected signals. The evolution of the transient spectra has not shown significant change in spectral structure, the intensity of the positive and negative signals increase at the earlier delay time (middle panel), then, decrease gradually (lower panel). For Ga(tpfc)(SO3Na)2-ctDNA, the positive absorption band become more intensive, and the absorption range is slightly broadened from 570 nm to 585 nm as shown in Figures 5b and 6a. Simultaneously, the Q-band negative signals become much weaker. At longer delay times, the broadening of the positive signals gradually disappears as shown in Figure 6b. Both positive and negative signals decrease with the time evolution as shown in the lower panel of Figure 5b, which may be due to the hypochromism of ct-DNA.



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Figure

5.

Femtosecond

transient

absorption

spectra

of

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Ga(tpfc)(SO3Na)2

(a)

and

Ga(tpfc)(SO3Na)2-ctDNA (b) in Tris-HCl buffer (pH = 7.2) monitored from 0.15 ps to 40 ps. The negative spectra are comparable to the steady-state absorption and emission spectra of Ga(tpfc)(SO3Na)2 shown in the upper panel.


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Figure 6. Femtosecond transient absorption spectra of Ga(tpfc)(SO3Na)2-ctDNA at different delay times and Ga(tpfc)(SO3Na)2 at 0.15 ps delay. As shown in Figure 5, the interaction between Ga(tpfc)(SO3Na)2 and ct-DNA is significant. The changes on the dynamic processes of the excited-states and ground state absorptions have been analyzed by the global fitting method. Figure 7 presents seven kinetic curves at the representative wavelengths of the ground and excited states absorptions. At 430 nm (B-band absorption), the GB exhibits an ultrafast initial recovery process (~ 235 fs), which is mainly due to the relaxation from the higher excited state (B-band) to the ground state. A following long recovery may be attributed to the decay from the Q-band to the ground state. The interaction with ct-DNA leads to a slightly decrease of the GB intensity around zero delay time and significantly faster GB recovery at longer delay time. Similar phenomenon has been 13 ACS Paragon Plus Environment


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observed at 611 nm (Q-band absorption) except a smaller GB intensity around zero delay time. For free Ga(tpfc)(SO3Na)2, the relaxation processes of the positive signals (480-560 nm) also show an initial ultrafast decay (~ 200 fs), which is corresponding to the internal conversion (IC) from the B-band to the Q-band, and a following slow process (~ 4.50 ps) attributing to the vibration relaxation (VR) from the higher vibration state to the lowest vibration state of the Q-band.57-58 For Ga(tpfc)(SO3Na)2-ctDNA, a new positive absorption signal (480-611 nm) is obvious. The dynamic processes are well fitted by a triple exponential function, and the time constants are τ1 (~ 200 fs), τ2 (~ 520 fs) and τ3 (~ 3.10 ps), respectively. The τ1 and τ3 should be corresponding to the IC and VR processes of Ga(tpfc)(SO3Na)2, while τ2 may arise from the relaxation of a new species. The smaller value of the τ3 may be due to the change of the solvent polarity after the addition of ct-DNA.40 The gap between the red and blue line observed for the last three panel should be due to the overlap of the new positive and negative signals during the same delay times in Ga(tpfc)(SO3Na)2-ctDNA. At 560 and 580 nm, the positive signal is dominant, while the negative signal is stronger at 611 nm.


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Figure 7. Time profiles at seven selected wavelengths of Ga(tpfc)(SO3Na)2 (blue) and Ga(tpfc)(SO3Na)2-ctDNA (red) in Tris-HCl buffer (pH = 7.2). Solid lines are fits of the experimental data.

Discussion The previous works have confirmed that the interaction between photosensitizers (ruthenium complex, porphyrin, etc.) and DNA can lead to the occurrence of electron transfer (ET) with the rate of 108 ~ 1013 s-1.1, 6, 59 The donor (D) is usually the guanine bases because they possess the lowest oxidation potential of DNA, whereas photosensitizer molecules are the acceptors (A). In the cases of photoexcitation of A, the ET should occur from the HOMO of the guanine bases to the HOMO of A. For 15 ACS Paragon Plus Environment


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Ga(tpfc)(SO3Na)2-ctDNA, the calculation of the molecular orbital has shown that the HOMO of the guanine bases is higher than that of Ga(tpfc)(SO3Na)2 as shown in Figure 3. The driving forces for the forward (∆Gf) and back (∆Gb) ET between Ga(tpfc)(SO3Na)2 and the guanine bases can be calculated according to the Rehm-Weller equation60-61 and the reported value of the oxidation potential of the guanine base (1.15 V )13, which are -0.77 eV and -2.16 eV, respectively, indicating that the reversible ET between Ga(tpfc)(SO3Na)2 and guanine bases of ct-DNA are thermodynamically favorable.62 The control experiments have been performed on two kinds of the model DNAs, [poly(dG-dC)]2 and [poly(dA-dT)]2 rich and absent in guanine bases, respectively. Ga(tpfc)(SO3Na)2-[poly(dG-dC)]2 exhibits a similar spectral broadening within the earlier delay time of 0.50 ps and dynamic processes, which is absent for Ga(tpfc)(SO3Na)2-[poly(dA-dT)]2 as shown in Figure S4 and S5. The transient absorption spectrum of Ga(tpfc)(SO3Na)2-ctDNA within 0.50 ps delay times as shown in Figure 6a should be a combination of the ground-state bleaching, the absorptions of the excited state of Ga(tpfc)(SO3Na)2 and new transient species. Therefore, the pure absorption spectrum as shown in Figure 8 of the new species can be achieved by adding ground-state absorption and subtracting excited-state absorption signals of the free Ga(tpfc)(SO3Na)2, which exhibits an intense absorption below 450 nm and a weak absorption from 450 nm to 700 nm. Similar phenomenon has been observed for Ga(tpfc)(SO3Na)2-[poly(dG-dC)]2. However, it is absent for Ga(tpfc)(SO3Na)2-[poly(dA-dT)]2. The absorption spectrum of the new specie is consistent with the reported absorption spectrum of guanine cation radical (G+).63-65 With the evolution of the time, the transient absorption spectrum should tend to the steady-state absorption spectrum of G+.66


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Figure 8. Absorption spectra of new transient species for Ga(tpfc)(SO3Na)2 complexed to DNA: ct-DNA (blue), [poly(dG-dC)]2 (magenta) and [poly(dA-dT)]2 (green). The inset shows the formation of guanine cation radical (G+) upon a photoinduced ET, where G and Cor* respectively represent the electron donor guanine and the excited electron acceptor Ga(tpfc)(SO3Na)2. A schematic illustration of the states and processes involved is shown in Figure 9. In comparison with other photosensitizer molecules (such as porphyrin6 and thionine13), Ga(tpfc)(SO3Na)2 has a lower reduction potential. Therefore, the higher excited-state energy of Ga(tpfc)(SO3Na)2 (~ 2.93 eV) is required to ensure that ET is energetically favorable. The forward ET from the guanine (G) to the excited Ga(tpfc)(SO3Na)2 should occur within the pulse duration (< 156 fs) as shown in Figure 6a, and the backward ET may be accomplished within ~ 520 fs by fitting the relaxation of the positive signals shown in Figure 7. The ultrafast back ET may lead to an occupation of the higher vibration state of the ground state (S0), which decays to the lowest vibration state in several picoseconds67-68, inducing a faster GB ( both B and Q bands) recovery as shown in Figure 7.



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Figure 9. Schematic diagram showing the assumed relaxation channels of Ga(tpfc)(SO3Na)2 (a) and Ga(tpfc)(SO3Na)2-ctDNA (b) after excitation to the B-band of Ga(tpfc)(SO3Na)2. The solid horizontal lines represent active vibrational levels of the B, Q and S0 electronic states. IC, VR and FL represent the internal conversion, vibrational relaxation and fluorescence processes, respectively. Classical Marcus theory has well explained many ET dynamics occur on time scales of nanoseconds or longer, for which the systems are in equilibrium with local environments.68-69 In our experiment, the ET processes (τf < 156 fs and τb ~ 520 fs) observed in Ga(tpfc)(SO3Na)2-ctDNA may be much faster than the local environment relaxation, which should occur in non-equilibrium configurations.68, 70 The classical Marcus theory may not be suitable to explain the ultrafast ET in Ga(tpfc)(SO3Na)2-ctDNA. Moreover, the backward ET involves a -∆Gb value of 2.16 eV, pointing to the absence of Marcus inverted region. Therefore, another ET model is required to explain the experimental results. Ultrafast highly-exergonic ET reactions were observed in other reaction systems, and various hypotheses have been proposed to explain the deviations from Marcus theory for these ET.60,

67-68, 71-73

For


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Ga(tpfc)(SO3Na)2-ctDNA, a ground-state vibration coupling may be involved in the backward ET, leading to a reduction of ET barrier and an enhancement of ET rate.67-68

Conclusion The femtosecond transient absorptions of the water-soluble anionic Ga(III) corrole Ga(tpfc)(SO3Na)2 and the blend of Ga(tpfc)(SO3Na)2 and ct-DNA have been measured after photoexcitation of Ga(tpfc)(SO3Na)2 into the B band. The spectral analysis and control experiments suggest that the ultrafast ET between Ga(tpfc)(SO3Na)2 and the guanine bases of ct-DNA occurred within the pulse duration (156 fs). The electrochemical experiment and the calculation confirm that the ET is reversible and thermodynamically favorable. The back ET may be accomplished in 520 fs according to the kinetic fitting. The further study on the mechanism of the ultrafast ET between Ga(tpfc)(SO3Na)2 and ct-DNA is on the way.

Acknowledgments This work was financially supported by National Natural Science Foundation of China (Nos. 61178037, 11004256, 21171057 and 21371059), National Basic Research Program (973 Program) of China under Grant 2013CB922403, and the Open Fund of the State Key Laboratory of Optoelectronic Materials and Technologies of Sun Yat-sen University (No. 2012-KF-MS2). We would like to thank Professor Yi Zhao for helpful discussions concerning this work.

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Photoinduced Electron Transfer between Anionic Corrole and DNA

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