Theoretical Study on the Photoinduced Electron Transfer Mechanisms

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Theoretical Study on the Photoinduced Electron Transfer Mechanisms of Different Peroxynitrite Probes Li Li, Minghui Zan, Xingwang Qie, Juan Yue, Peng Miao, Mingfeng Ge, Zhimin Chang, Zheng Wang, Fuquan Bai, Hong-Xing Zhang, James Kit Ferri, and Wen-Fei Dong J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b10716 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 18, 2017

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The Journal of Physical Chemistry

Theoretical Study on the Photoinduced Electron Transfer Mechanisms of Different Peroxynitrite Probes Li Li,a Minghui Zan,a Xingwang Qie,a Juan Yue,a Peng Miao,a Mingfeng Ge,a Zhimin Chang,a Zheng Wang,a Fu-Quan Bai,b Hong-Xing Zhang,b James K. Ferri,c and Wen-Fei Donga*

a

CAS Key Laboratory of Biomedical Diagnostics, Suzhou Institute of Biomedical Engineering

and Technology, Chinese Academy of Science (CAS), 88 Keling Road, Suzhou 215163, People’s Republic of China. b

Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People’s Republic of

China c

Department of Chemical and Life Science Engineering, Virginia Commonwealth University,

Richmond, 23219, Virginia, USA

* Corresponding author. E-mail: [email protected]

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Abstract The development of probes for rapid and selective detection of peroxynitrite in vivo is of great importance in biological science. We investigate different photoinduced electron transfer (PIET) processes of two generations of peroxynitrite probes. Each has fluorescein and phenol moieties; one is conjugated by an ether linkage while the other is conjugated via an amine linkage. Using theoretical calculations, we demonstrated that the PIET in the probe with an ether linkage occurs from the benzoic acid to the xanthene moiety. In contrast, the PIET in the probe with an amine linkage occurs from the phenol moiety to the fluorescein. This suggests that better sensitivity can be accomplished in probes with an amine linkage than with an ether linkage. Following this model, we designed two novel peroxynitrite probes and simulated their detection capabilities in the near infrared region.


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1 Introduction Peroxynitrite (ONOO-) is the product of a fast reaction between nitric oxide and superoxide radicals, and is a strong oxidant and cytotoxic agent in vivo.1-3 It can react with different biomolecules, including proteins, lipids, and DNA, and can alter their structures and functions, and even lead to cellular injury or death.4-6 Therefore, it is necessary to develop highly sensitive and specific methods for detecting ONOO-, and, more importantly, to make direct analysis of peroxynitrite production in vivo.7-9 Since the beginning of the 21st century, many fluorescent probes based on fluorescein,10-12 rhodamine,13 boron-dipyrromethene (BODIPY),14 and others,15 have been synthesized. The first generation of ONOO- probes (Probe 1, Tetsuo Nagano,16 Fig. 1) change optical behavior upon peroxynitrite-mediated oxidation. However, detection in vivo can easily have interference by other reactive oxygen species (ROS), such as H2O2, 1O2, •OH, NO, and -OCl.17 Recently, Yang and coworkers synthesized a novel ONOOprobe (Probe 2, Fig. 1) based on fluorescein, and claimed complete probe specificity and high sensitivity for detecting ONOO-.18 Superficially, the two probes have similar structures, in particular, each has fluorescein and phenol moieties. The principal difference is that Probe 1 is conjugated by an ether linkage while Probe 2 is conjugated via an amine linkage (Fig. 1). In this article, we focus on the different photoinduced electron transfer (PIET) mechanisms19-21 of these probes. Using theoretical calculations, we simulated the emission spectra, investigated the excited-state structural changes, analyzed the frontier molecular orbitals, and investigated the different mechanisms for changes in fluorescence emission. Finally, we offer designs for new probes based on the hypothesized mechanism of detection.

2 Computational methods All the calculations were performed using density functional theory (DFT)22-23 and the Gaussian09 program.24 The ground-state (S0) and the first-excited-state (S1) structures were optimized at the PBE0/6-31+G(d)25 and TD-PBE0/6-31+G(d) levels, without any symmetry constraints. The absorption spectra were simulated through the electronic vertical excitations on the ground-state structures, and the emissions were received directly from the excitation energy



based on the S1-structure. The solvent effect (water) was assessed using the polarizable continuum model (PCM) method.26-27 In the calculations of electronic vertical excitations,28-33 a series of DFT functionals were tested on the calculation of the emission of 2b, including PBE0, B3LYP, M06,34 and CAM-B3LYP.35 The calculated results are listed in Table 1, together with the experimental data.18 As shown in Table 1, the results from B3LYP and PBE0 are very similar, closer to the experimental data than the results from other methods. Moreover, some other researchers have tested these functionals in the excited-state calculation,36-37 and they considered that PBE0 would be a suitable functional in the simulation of the singlet-excited states of those dyes including fluorescein.38 So finally, the PBE0 functional was chosen to complete the absorption and emission calculations at 6-31+G(d) level.

3 Results and discussions 3.1 Mechanism of Photoinduced Electron Transfer (PIET) for Probe 1 We optimized the ground-state (S0) structures of Probe 1, before and after reaction with ONOO-, respectively denoted 1 and 1b in Fig. 1. 30 electronic vertical transitions were calculated on the S0 structures, and Fig. 2 shows the simulated absorption curves with Gaussian band shapes, the FWHM (full width at half maximum) is taken as 0.666 eV. The first excited state (S1) structures of 1 and 1b were optimized using the TD-DFT method, and the emissions were calculated on the optimized S1 structures. Table 2 gives the details (including wavelength λ, oscillator strength f, and major contribution) of the absorption and emission of 1 and 1b. In Nagano’s report,16 1 shows almost non-fluorescence, and 1b has a strong emission wavelength at 515 nm. In our calculations, the emission wavelength of 1 is at 647 nm with a tiny oscillator strength of 0.18, reflecting a slow rate for emission. The low oscillator strengths allow photochemical reactions to occur before fluorescence, and this is what lowers the fluorescence intensity. The calculated emission band of 1b is at 502 nm with a large oscillator strength at 1.07, indicating that 1b emits strong fluorescence. This result is in good agreement with the experimental emission of 1b, whose quantum yield is 0.85. We then analysed the frontier molecular orbitals and the electronic transitions. From


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Table 2 it can be seen that all of the largest-wavelength absorptions result in an electronic excitation from the HOMO to the LUMO of the S0 structures, and all of the emissions originate from the electronic transition from the LUMO to the HOMO of the S1 structures. An energy level diagram of these molecular orbitals is provided in Fig. 3, which shows that the energy gap between the HOMO and the LUMO of the S0 structure of 1 (3.561 eV) is larger than that of 1b (3.331 eV). This difference yields a longer wavelength for the calculated absorption band of 1b. In the excited state, 1 has a smaller energy gap than 1b, matching the calculated red-shift emission band of 1 compared to 1b. Fig. 4 gives the appearance of the molecular orbitals. Most of the molecular orbitals are localized to the central xanthene moiety, except for the HOMO of 1 in the S1 structure, which partly occupies the -COOH of the benzoic acid group (see Fig. 4). This produces a small overlap between the HOMO and LUMO, resulting in the weak fluorescence intensity of 1. We also investigated the structural changes of 1 in the S1 state before and after reaction with ONOO-. We divided molecule 1 into three moieties: xanthene, phenol, and benzoic acid (Fig. 5). The dihedral angle between the benzoic acid and xanthene moieties is labelled D1. The phenol moiety is an electron-donating group, and benzoic acid is an electron-withdrawing group. The phenol substituent affects the electronic structure of the benzoic acid moiety through the central xanthene linker. The difference in D1 between 1 (63.6°) and 1b (60.1°) in the S1 state signifies that the introduction of phenol induces a small rotation of the benzoic acid moiety, causing the HOMO to partly localize on the -COOH group. The hypothesized detection mechanism of Probe 1 is summarized in Fig.6. First, the excitation makes one electron jump to a high energy level in the xanthene part, leaving a half-empty orbital. The calculated energy gap (2.65 eV) well matches the absorption energy (406 nm) in its S0 state. And in the excited-state (S1), the HOMO locates on the benzoic acid part while the LUMO locates on the xanthene part. Here, the HOMO level corresponds to the Donor-based orbital, thus the donor photoinduced electron transfer (d-PIET) occurs from HOMO to the half-vacant orbital around xanthene. This results in the decay of the excited electron from the LUMO on the xanthene to the HOMO on the benzoic acid, and because of the small overlap between the two orbitals, the fluorescence intensity must be very low. When the probe reacts with the ONOO- anion, the phenol part is oxidized off, the HOMO and



LUMO keep on the central xanthene part whenever the probe is in ground state or in excited state, so 1b appears a strong fluorescence. To verify our hypothesis, we designed another two molecules, 1m and 1p (Fig. 7). Compared to Probe 1, the new molecules alter the electron-donating group (the phenol substituent) into a methyl group (1m) and a pyridine group (1p). Neither molecule has any native fluorescence. The molecular orbital shapes are shown in Fig. 8. As with Probe 1, in the S1 structures of 1m and 1p, the HOMO localizes to the benzoic acid, and the LUMO localizes to the central xanthene. These observations indicate that when the ether linkage is oxidized, the fluorescence will re-emerge. All of the above calculations suggest that the benzoic acid moiety plays a very important role in detection. If the benzoic acid group is removed, and the hydroquinone substituent is conjugated to another fluorophore, such as Nile red (1N, Fig. 9a), the new molecule should not display specificity for ONOO-. This is because the phenol substituent does not alter the electronic transition of the Nile red fluorophore in the excited state (Fig. 9b). Thus, for further design of novel ONOO- probes, if the ether linkage is used, fluorescein (or its derivatives) with a benzoic acid group as the fluorophore should be employed.

3.2 Mechanism of Photoinduced Electron Transfer (PIET) for Probe 2 As with Probe 1, the structure of Probe 2 before and after reaction with ONOO- is denoted 2 and 2b in Fig. 1. Absorption properties were simulated using the optimized S0 structures, and the emission properties were simulated using the optimized S1 structures. Table 3 gives the data of absorption and emission of 2 and 2b. In our calculations, the absorptions of 2 and 2b both led to the electronic excitation from the HOMO to the LUMO, and the emissions of both originated from the electronic transition from the LUMO to the HOMO. Probe 2 had no fluorescence, and after it reacted with the ONOO- anion, the product 2b emitted strong fluorescence at 516 nm. Our calculation matched the experimental measurement18 of Yang et al. well. The shapes of the relevant molecular oribtals are given in Fig. 10. All of the HOMOs and LUMOs of 2 and 2b on their S0 and S1 structures are located on the central xanthene, except for the HOMO of the S1 structure of 1. This HOMO completely occupies the phenol group


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(Fig. 10). This prevents electronic relaxation from the LUMO to the HOMO and leads to the fluorescence quenching in the S1 state of 2. When the phenol group is oxidized by the ONOO- anion, the product 2b forms, and the excited-state electronic transition back to the central xanthene recovers strong fluorescence. The PIET process of Probe 2 is shown in Fig. 11, and the detailed hypothesized detection mechanism is summarized. For 2, upon irradiation, the electronic excitation occurs in the fluorescein part, then the electron occupying on the HOMO of the phenol will transfer to the lower half-vacant orbital on fluorescein part by the d-PIET process, which prevents the excited electron back to the fluorescein part directly and leads to the fluorescence quenching. And when the phenol part is oxidized off, the electronic transition occurs completely in the fluorescein moiety, and its strong fluorescence recovers. The different geometries in ground and excited state structures of Probe 2 result in different components of the HOMO and the LUMO. The dihedral angle denoted D2 (Fig. 12) is between the -N-CH3 group and the central xanthene part. In the S0 structure of 2, D2 is 0°, and in the S1 structure, the value of D2 is 90°. As the dihedral angle rotates from 0° to 90°， the HOMO component on the phenol moiety increases, and ultimately, it occupies the entire phenol group. In contrast, the LUMOs are always localized on the xanthene moiety. This indicates that rotation about D2 in the excited state blocks the electronic transition in the fluorescein and results in fluorescence quenching. Thus, newly designed probes should also be sensitive to the ONOO- anion if the phenol group is maintained and if the fluorescein moiety is replaced with fluorophores, such as Nile red or cyanine, providing that the phenolic HOMO is higher than that of the alternative fluorophore. To verify this hypothesis, we simulated the emission spectra of new probe designs. The two probe structures are provided in Fig. 13. In 2N, the fluorescein in Probe 2 was replaced with a Nile red fluorophore, and in 2Cy, the fluorescein in Probe 2 was replaced with a cyanine fluorophore. The calculated spectral properties of the probes before (2N and 2Cy) and after (2Nb and 2Cyb) reaction with ONOO- are given in Table 4, including the absorption and emission wavelength λ, the oscillator strength f, and the electronic transition details. Table 4 shows that all of the absorptions result in the electronic excitation from the HOMO to the LUMO, and all



of the emissions correspond to the electronic decay from the LUMO to the HOMO. Additionally, the calculations show that the original probes (2N and 2Cy) themselves have no fluorescence, and after they react with ONOO-, each has a strong near-infrared emitting band (at 645 nm and 678 nm for 2Nb and 2Cyb, respectively). The relevant molecular orbitals in the S1-state structures are shown in Fig. 14. Like Probe 2, the HOMOs of the two probes are localized to the phenol moiety while the LUMO and HOMO-1 orbitals remain local to the fluorophore moiety. Thus, the two designed probes also have the d-PIET process upon irradiation, suggesting that both have great potential for the application of ONOO- anion detection.

4 Conclusions We investigated the different photoinduced electron transfer processes of two generations of peroxynitrite probes. Each of the probes has a fluorescein group and a phenol substituent; one of the probes is attached by an ether linkage, and the other is conjugated via an amine linkage. Through theoretical calculations, we demonstrated that in the probe with an ether linkage, PIET occurs from the benzoic acid to the xanthene moiety. In contrast, in the probe with an amine linkage, PIET emerges from the phenol moiety to fluorescein. This suggests that probes with an amine linkage will achieve better sensitivity than those with an ether linkage, and hence, the emitting wavelength of the probe can be easily controlled. Thus, we conclude that the amine linkage has greater application potential for new generations of peroxynitrite probes, and more generally, that density functional theory can be used to enable molecular design in-silico.

Acknowledgements This work was supported by the National Key R&D Program of China (Grand No. 2017YFF0108600, 2017YFC0211900 and 2016YFF0103800), the National Natural Science Foundation of China (Grand No. 81771982, 61535010, 81771929, and 8160071152), Key Research Program of the Chinese Academy of Sciences (No. KFZD-SW-21), the Natural Science Foundation of Jiangsu Province (No. BE2015601) and the Science and Technology


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Department of Suzhou City (No. SS201539 and ZXY201434).



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Table and Figure Captions Table 1. The calculated emission of 2b using the vertical excitation on the optimized S1 structure, together with the experimental data (535 nm). Table 2. Absorption and emission details of 1 and 1b. Table 3. The data of absorption and emission of 2 and 2b. Table 4. The absorption and emission properties of 2N and 2Cy. Fig. 1 The two different peroxynitrite probes, 1 and 2. Fig. 2 The simulated absorption curves of Probe 1 before (1-ab) and after (1b-ab) reaction with ONOO-. Fig. 3 The energy levels grogram of the molecular orbitals of Probe 1. Fig. 4 Schematic of HOMO and LUMO orbitals in Probe 1. Fig. 5 The three moieties of Probe 1. Fig. 6 The PIET process of Probe 1. In the excited-state 1, the HOMO level corresponds to the Donor-based orbital, so that PIET is thermodynamically feasible. In 1b, the HOMO level is the A-based orbital, so that PIET is not plausible. Fig. 7 The molecular structures of 1m and 1p. Fig. 8 The shapes of HOMO and LUMO in 1m and 1p at S1 state. Fig. 9 The molecular structure (a) and the electronic transition (b) of 1N at S1 state. Fig. 10 The shapes of the relevant molecular oribtals of Probe 2. Fig. 11 The d-PIET process in Probe 2. In the excited-state 2, the HOMO level corresponds to the Donor-based orbital, so that PIET is thermodynamically feasible. In 2b, the Donor part is oxidized off, and the HOMO and LUMO both locate in the Acceptor part, so there is no PIET process and strong fluorescence recovers. Fig. 12 The molecular orbital shapes of HOMO and LUMO in different structures of Probe 2. Fig. 13 Molecular structure of new probe designs 2N and 2Cy. Fig. 14 The shapes of molecular orbitals of 2N and 2Cy at S1 state.



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Table 1. The calculated emission of 2b using the vertical excitation on the optimized S1 structure, together with the experimental data (535 nm). functionals λcal/nm f a major contribution λexptb/nm PBE0 516 0.86 LUMO→HOMO(98%) 535 B3LYP 527 0.84 LUMO→HOMO(98%) CAM-B3LYP 473 1.16 LUMO→HOMO(95%) M06 497 0.97 LUMO→HOMO(98%) M06-2X 486 1.12 LUMO→HOMO(95%) a f is the oscillator strength of emission coefficient. b The experiment data from the reference 18. Table 2. Absorption and emission details of 1 and 1b. absorption λcal/nm

f

1

406

1b

426

a

emission b

c

Φd

major contribution

λexpt b/nm

0.18

LUMO→HOMO(95%)

-

1.07

LUMO→HOMO(98%)

515

0.85

major contribution

λexpt b/nm

Φd

major contribution

λexpt /nm

λcal/nm

f

0.66

HOMO→LUMO(95%)

490

647

0.82

HOMO→LUMO(98%)

490

502

a

f is the oscillator strength of absorption coefficient. The experiment data from the reference 16. c f is the oscillator strength of emission coefficient. d Φ is the fluorescence quantum yield from the reference 16. b

Table 3. The data of absorption and emission of 2 and 2b. absorption λcal/nm

f

2

433

2b

422

a

emission b

c

major contribution

λexpt /nm

λcal/nm

f

0.95

HOMO→LUMO(98%)

517

851

0

LUMO→HOMO(98%)

-

0.76

HOMO→LUMO(98%)

517

516

0.86

LUMO→HOMO(98%)

535

a

f is the oscillator strength of absorption coefficient. The experiment data from the reference 18. c f is the oscillator strength of emission coefficient. d Φ is the fluorescence quantum yield from the reference 18. b

Table 4. The absorption and emission properties of 2N and 2Cy. absorption a λ/nm f major contribution λ/nm f b 534 0.30 HOMO→LUMO(94%) 922 0 2N 2Nb 517 0.48 HOMO→LUMO(96%) 645 0.56 591 1.81 HOMO→LUMO(94%) 1157 0 2Cy 2Cyb 556 2.24 HOMO→LUMO(99%) 678 2.52 a f is the oscillator strength of absorption coefficient. b f is the oscillator strength of emission coefficient.


emission major contribution LUMO→HOMO(99%) LUMO→HOMO(93%) LUMO→HOMO(95%) LUMO→HOMO(98%)

0.73

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Fig. 1 The two different peroxynitrite probes, 1 and 2.



Fig. 2 The simulated absorption curves of Probe 1 before (1-ab) and after (1b-ab) reaction with ONOO-.


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Fig. 3 The energy levels grogram of the molecular orbitals of Probe 1.



Fig. 4 Schematic of HOMO and LUMO orbitals in Probe 1.


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Fig. 5 The three moieties of Probe 1.



Fig. 6 The PIET process of Probe 1. In the excited-state 1, the HOMO level corresponds to the Donor-based orbital, so that PIET is thermodynamically feasible. In 1b, the HOMO level is the A-based orbital, so that PIET is not plausible.


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Fig. 7 The molecular structures of 1m and 1p.



Fig. 8 The shapes of HOMO and LUMO in 1m and 1p at S1 state.


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Fig. 9 The molecular structure (a) and the electronic transition (b) of 1N at S1 state.



Fig. 10 The shapes of the relevant molecular oribtals of Probe 2.


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Fig. 11 The d-PIET process in Probe 2. In the excited-state 2, the HOMO level corresponds to the Donor-based orbital, so that PIET is thermodynamically feasible. In 2b, the Donor part is oxidized off, and the HOMO and LUMO both locate in the Acceptor part, so there is no PIET process and strong fluorescence recovers.



Fig. 12 The molecular orbital shapes of HOMO and LUMO in different structures of Probe 2.


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Fig. 13 Molecular structure of new probe designs 2N and 2Cy.



Fig. 14 The shapes of molecular orbitals of 2N and 2Cy at S1 state.


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Graphic for manuscript



338x192mm (150 x 150 DPI)


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Theoretical Study on the Photoinduced Electron Transfer Mechanisms

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