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Article

Scandium Ion-Promoted Electron-Transfer Disproportionation of 2-Phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-Oxide (PTIO•) in Acetonitrile and Its Regeneration Induced by Water

by
Yoshimi Shoji
1,
Yuri Terashima
2,
Kei Ohkubo
3,†,
Hiromu Ito
1,
Kouichi Maruyama
2,
Shunichi Fukuzumi
4,5,† and
Ikuo Nakanishi
1,*
1
Quantum RedOx Chemistry Team, Institute for Quantum Life Science (iQLS), Quantum Life and Medical Science Directorate (QLMS), National Institutes for Quantum Science and Technology (QST), Chiba-shi 263-8555, Chiba, Japan
2
Environmental Radiation Effects Research Group, Department of Radiation Measurement and Dose Assessment, Institute for Radiological Science (NIRS), Quantum Life and Medical Science Directorate (QLMS), National Institutes for Quantum Science and Technology (QST), Chiba-shi 263-8555, Chiba, Japan
3
Institute for Advanced Co-Creation Studies, Open and Transdisciplinary Research Initiatives, Osaka University, Suita 565-0871, Osaka, Japan
4
Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Republic of Korea
5
Department of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8571, Ibaraki, Japan
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Int. J. Mol. Sci. 2024, 25(8), 4417; https://doi.org/10.3390/ijms25084417
Submission received: 20 March 2024 / Revised: 9 April 2024 / Accepted: 16 April 2024 / Published: 17 April 2024
(This article belongs to the Section Biochemistry)
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Versions Notes

Abstract

:
2-Phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO•), a persistent nitronyl nitroxide radical, has been used for the detection and trapping of nitric oxide, as a redox mediator for batteries, for the activity estimation of antioxidants, and so on. However, there is no report on the reactivity of PTIO• in the presence of redox-inactive metal ions. In this study, it is demonstrated that the addition of scandium triflate, Sc(OTf)3 (OTf = OSO2CF3), to an acetonitrile (MeCN) solution of PTIO• resulted in an electron-transfer disproportionation to generate the corresponding cation (PTIO+) and anion (PTIO−), the latter of which is suggested to be stabilized by Sc3+ to form [(PTIO)Sc]2+. The decay of the absorption band at 361 nm due to PTIO•, monitored using a stopped-flow technique, obeyed second-order kinetics. The second-order rate constant for the disproportionation, thus determined, increased with increasing the Sc(OTf)3 concentration to reach a constant value. A drastic change in the cyclic voltammogram recorded for PTIO• in deaerated MeCN containing 0.10 M Bu4NClO4 was also observed upon addition of Sc(OTf)3, suggesting that the large positive shift of the one-electron reduction potential of PTIO• (equivalent to the one-electron oxidation potential of PTIO−) in the presence of Sc(OTf)3 may result in the disproportionation. When H2O was added to the PTIO•–Sc(OTf)3 system in deaerated MeCN, PTIO• was completely regenerated. It is suggested that the complex formation of Sc3+ with H2O may weaken the interaction between PTIO− and Sc3+, leading to electron-transfer comproportionation to regenerate PTIO•. The reversible disproportionation of PTIO• was also confirmed by electron paramagnetic resonance (EPR) spectroscopy.

Graphical Abstract

1. Introduction

Redox-inactive metal ions have attracted much attention because they are known to affect the redox behavior of redox active compounds acting as a Lewis acid [1,2,3,4,5,6,7,8,9]. Among such metal ions, scandium ion (Sc3+) shows the strongest Lewis acidity because of the small ionic radius with trivalent positive charge, leading to a significant positive shift of the reduction potentials of compounds [10,11,12,13,14,15,16,17,18,19,20,21,22,23], including metal–oxygen complexes [19,20,21] and radical species [22,23]. Of special interest is the fact that Sc3+ enables the electron-transfer reactions, which would otherwise never take place in the absence of Sc3+, to occur thermodynamically as well as kinetically [10,11,12,13,14,15,20,21]. We have reported that an electron-transfer disproportionation of 2,2-diphenyl-1-picrylhydrazyl radical (DPPH•) occurs upon the addition of scandium triflate, Sc(OTf)3 (OTf = OSO2CF3), to an acetonitrile (MeCN) solution of DPPH• to produce one-electron-oxidized and -reduced species of DPPH•, DPPH+, and DPPH−, respectively [23]. The spectral titration showed that four molecules of DPPH• react with Sc3+ to produce two molecules of DPPH+ and a complex between two molecules of DPPH− and Sc3+, [(DPPH)2Sc]+ [24]. Further, it is also reported that the addition of H2O to the DPPH•–Sc(OTf)3 system in MeCN resulted in the regeneration of DPPH• [24]. Very recently, a similar reversible disproportionation reaction has been reported for 2,2,6,6-tetramethylpiperidyl-1-oxyl (TEMPO) in the presence of Al(OTf)3 to develop an aqueous aluminum radical batteries [25]. Dedzo et al. have also reported drastic changes in the redox reactivity of DPPH• in the presence of metal cations, such as Cu2+ and Zn2+, as well as Brønsted acid, such as HClO4 and HNO3 [8].
On the other hand, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO•) (Scheme 1), a persistent nitronyl nitroxide radical, has been used for the detection and trapping of nitric oxide (•NO) for about 30 years [26,27,28,29,30,31,32,33,34]. Furthermore, PTIO• has also attracted much attention as a redox mediator for batteries [35,36,37]. Recently, it has been reported that PTIO• can be used to estimate the activity of antioxidants as a reactivity model of reactive oxygen species [38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57]. However, there is no report on the reactivity of PTIO• in the presence of redox-inactive metal ions. PTIO• shows a similar reversible one-electron redox behavior (Scheme 1) to DPPH•, although the separation between the one-electron oxidation and reduction potentials of PTIO• (1.73 V) [35] is much larger than that of DPPH• (0.52 V) [23]. We report herein that an electron-transfer disproportionation of PTIO• also occurs upon the addition of Sc(OTf)3 to a deaerated MeCN solution of PTIO•. Also studied was the regeneration of PTIO• upon the addition of H2O to the PTIO•–Sc(OTf)3 system in deaerated MeCN. The drastic change in the redox behavior of compounds due to the strong Lewis acidity of Sc3+ observed in this study provides valuable and fundamental information about the fine tuning of the redox reactions by the redox-inactive metal ions.

2. Results and Discussion

When Sc(OTf)3 was added to a deaerated MeCN solution of PTIO•, the absorption bands at 238, 264, and 361 nm and a broad band at around 600 nm due to PTIO• decreased immediately with clear isosbestic points at 277, 334, 381, and 525 nm, as shown in Figure 1 (Video S1 of the Supplementary Materials). The broad absorption band at 450 nm is diagnostic of the one-electron-oxidized PTIO• (PTIO+) [33]. Thus, an electron-transfer disproportionation of PTIO• is suggested to take place upon addition of Sc(OTf)3 to produce PTIO+ and the one-electron-reduced PTIO• (PTIO−), as in the case of DPPH• [23]. The spectral titration (inset of Figure 1) shows the Sc(OTf)3/PTIO• molar ratio being 1:2. This suggests that one molecule of PTIO− may be stabilized by one Sc3+ (Scheme 2). When MeCN was replaced by methanol (MeOH) or ethanol (EtOH) as the solvent, such a spectral change was not observed. This suggests that the stronger solvation of Sc3+ in MeOH or EtOH compared to that in MeCN may preclude the disproportionation from occurring.
The spectral change after the addition of Sc(OTf)3 (7.9 × 10−3 M) to a deaerated MeCN solution of PTIO• (5.5 × 10−5 M) monitored by a stopped-flow technique is shown in Figure 2a. The time course change in the absorbance at 361 nm obeyed second-order kinetics (inset of Figure 2a). The observed second-order rate constant (kdisp, disp: disproportionation) was determined by a decrease in absorbance at 361 nm due to PTIO•. The kdisp value increases with increasing concentration of Sc(OTf)3 ([Sc(OTf)3]) to reach a constant value (Figure 2b). The limiting kdisp value (k∞) and the binding constant (K) between PTIO• and Sc3+ were determined from curve fitting based on Scheme 3 and Equation (1) to be 9.3 × 104 M−1 s−1 and 1.0 × 103 M−1, respectively.
According to Equation (1), when [Sc(OTf)3] is low (1 >> K[Sc(OTf)3]), the kdisp value increases with increasing [Sc(OTf)3]. On the other hand, when [Sc(OTf)3] is high enough to produce [(PTIO•)Sc]3+, the reaction between PTIO• and [(PTIO•)Sc]3+ becomes the rate-determining step and the kdisp value reaches k∞.
kdisp = k∞K[Sc(OTf)3]/(1 + K[Sc(OTf)3])
Cyclic voltammetry measurements were carried out to examine the effect of Sc3+ on the redox behavior of PTIO• in deaerated MeCN containing 0.10 M Bu4NClO4. Two well-defined reversible redox waves were observed at −0.98 and +0.70 V vs. the saturated calomel electrode (SCE) for the one-electron reduction and oxidation of PTIO• to produce PTIO− and PTIO+, respectively, in the absence of Sc3+ (Figure 3). Thus, the separation between the one-electron oxidation and reduction potentials obtained in this study (1.68 V) is slightly smaller than the literature value (1.73 V) [35]. Upon the addition of 10 equiv. of Sc(OTf)3, however, a drastic change was observed in the cyclic voltammogram (Figure 3). The reversible wave for the one-electron reduction of PTIO• and the oxidation wave of PTIO• disappeared, while a new oxidation peak appeared at +1.66 V vs. SCE. This new peak was assigned to the oxidation of [(PTIO)Sc]2+, which was generated by the disproportionation of PTIO• upon the addition of Sc3+, to produce PTIO• and Sc3+ (Scheme 4). Then, PTIO• was further oxidized to PTIO+ (Scheme 4). Although the reduction peak of PTIO• in the presence of Sc(OTf)3 could not be observed due to the disproportionation reaction, such a large (ca. 2.6 V) positive shift of the one-electron reduction potential of PTIO• (equivalent to the oxidation peak for PTIO−) upon the addition of Sc(OTf)3 enables the disproportionation to occur.
The reversibility of the disproportionation of PTIO• by the addition of Sc(OTf)3 has also been examined as in the case of DPPH• [24]. The addition of H2O to the PTIO•–Sc(OTf)3 system in deaerated MeCN resulted in the increase in the absorption band at 361 nm due to PTIO• (Figure 4) (Video S2 of the Supplementary Materials). This indicates that an electron-transfer comproportionation occurred to regenerate PTIO•. When H2O was replaced by EtOH or MeOH, the regeneration of PTIO• was also observed. However, the amount of the recovery was significantly lower compared to the case of H2O.
The spectral change after the addition of H2O (1.9 M) to a deaerated MeCN solution containing PTIO• (6.9 × 10−5 M) and Sc(OTf)3 (3.5 × 10−5 M) monitored by a stopped-flow technique is shown in Figure 5a. The time course change in the absorbance at 361 nm obeyed second-order kinetics (inset of Figure 5a), from which the observed second-order rate constant for the comproportionation (kcomp, comp: comproportionation) was determined to be 1.4 × 104 M−1 s−1. The comproportionation reactions between reduced and oxidized forms of nitroxyl radicals, hydroxyl amines, and oxoammonium cations, respectively, have been extensively studied [58,59,60,61,62,63,64,65,66]. Goldstein et al. determined the kcomp value between the hydroxylamine and oxoammonium cation derived from TEMPO to be 5.2 × 10 M−1 s−1 in a phosphate-buffered solution, while the deprotonation of the hydroxylamine resulted in a significant increase in the kcomp value (3.3 × 104 M−1 s−1) [59]. This suggests that the electron donor to PTIO+ in this study is PTIO−, rather than its protonated form (PTIOH). The kcomp value linearly increased with the increasing concentration of H2O, as shown in Figure 5b. Thus, the H2O-induced comproportionation reaction is shown in Scheme 5. It is suggested that the complex formation of Sc3+ with H2O may weaken the interaction between PTIO− and Sc3+, leading to the electron-transfer comproportionation regenerating PTIO•.
The reversible disproportionation of PTIO• was also confirmed by the electron paramagnetic resonance (EPR) spectroscopy. The well-resolved five lines with a g value of 2.0067 and a hyperfine coupling constant (aN) of 0.75 mT were observed in the EPR spectrum of PTIO• in deaerated MeCN (Figure 6a). After 0.5 equiv. of Sc(OTf)3 was added to the MeCN solution of PTIO•, the signal intensity was significantly decreased, as shown in Figure 6b, although a trace amount of PTIO• was observed. The addition of H2O to this PTIO•–Sc(OTf)3 system in deaerated MeCN resulted in the regeneration of PTIO•, which was confirmed by the increase in the EPR signal intensity due to PTIO• (Figure 6c).

3. Materials and Methods

3.1. Materials

PTIO• was commercially obtained from Tokyo Chemical Industry Co., Ltd., Tokyo, Japan. Sc(OTf)3 was purchased from Sigma-Aldrich, St. Louis, MO, USA. MeCN, MeOH, and EtOH (spectral grade) used as solvents were commercially obtained from Nacalai Tesque, Inc., Kyoto, Japan, and used as received. Tetra-n-butylammonium perchlorate (Bu4NClO4), used as a supporting electrolyte for electrochemical measurements, was purchased from Tokyo Chemical Industry Co., Ltd., Tokyo, Japan, recrystallized from EtOH (spectral grade, Nacalai Tesque, Inc., Kyoto, Japan), and dried under vacuum at 313 K. The water used in this study was freshly prepared with a Milli-Q system (Millipore Direct-Q UV3) (Merch Millipore, Burlington, MA, USA).

3.2. Spectral and Kinetic Measurements

To avoid the effect of molecular oxygen (O2), the reactions were carried out under strictly deaerated conditions, where a continuous flow of argon (Ar) gas was bubbled through each MeCN solution. Typically, a 10 µL of aliquot of Sc(OTf)3 (1.3 × 10−3 M) in deaerated MeCN was added to a quartz cuvette (10 mm i.d.), which contained PTIO• (7.2 × 10−3 M) in deaerated MeCN. UV-vis spectral changes associated with the reaction were monitored using an Agilent 8453 photodiode array spectrophotometer thermostated with a Peltier temperature control at 298 K (Agilent Technologies, Santa Clara, CA, USA). The reaction rates were followed by monitoring the absorbance at 361 nm due to PTIO• after mixing of PTIO• in deaerated MeCN with a deaerated MeCN solution containing Sc(OTf)3 at a volumetric ratio of 1:1 using a stopped-flow technique on a UNISOKU RSP-1000-02NM stopped-flow spectrophotometer (UNISOK Co., Ltd., Osaka, Japan), which was thermostated with a Thermo Scientific NESLAB RTE-7 Circulating Bath (Thermo Fisher Scientific, Inc., Waltham, MA, USA) at 298 K. For the regeneration reaction of PTIO•, a deaerated MeCN solution containing H2O were mixed with a deaerated MeCN solution of PTIO• (1.4 × 10−4 M) and Sc(OTf)3 (7.0 × 10−5 M) at a volumetric ratio of 1:1 using a stopped-flow technique. The observed second-order rate constants (kdisp and kcomp) were obtained by a least-square curve fit using an Apple MacBook Pro personal computer (Apple Inc., Cupertino, CA, USA) or an HP EliteDesk 800 G4 SFF (HP Inc., Palo Alto, CA, USA). The plots of 1/(A − A∞) vs. time (A and A∞ are the absorbance at the reaction time and the final absorbance, respectively) were linear until three or more half-lives, with a correlation coefficient ρ > 0.999. The kdisp and kcomp values were calculated by Slope(A0 − A∞)/[PTIO•]0, where Slope is the slope of the linear plot of 1/(A − A∞) vs. time, and A0 and [PTIO•]0 are the initial absorbance at 361 nm and initial concentration of PTIO•, respectively. In each case, it was confirmed that the kdisp and kcomp values derived from at least three independent measurements agreed within experimental error of ±5%.

3.3. Electrochemical Measurements

The cyclic voltammetry measurements were performed on an ALS-630A electrochemical analyzer (BAS Co., Ltd., Tokyo, Japan) in deaerated MeCN containing 0.10 M Bu4NClO4 as a supporting electrolyte. The continuous flow of Ar gas was bubbled through each MeCN solution to avoid the effect of O2. The glassy carbon working electrode (3 mm diameter) (BAS Co., Ltd., Tokyo, Japan) was polished with polishing alumina suspension (BAS Co., Ltd., Tokyo, Japan) and an alumina polishing pad (BAS Co., Ltd., Tokyo, Japan) and rinsed with methanol (FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan) prior to each measurement. The counter electrode was a platinum wire (BAS Co., Ltd., Tokyo, Japan). The concentration of PTIO• and Sc(OTf)3 were 1.0 × 10−3 and 1.0 × 10−2 M, respectively. The measured potentials were recorded with respect to an Ag/AgNO3 (0.01 M) reference electrode (BAS Co., Ltd., Tokyo, Japan) with a sweep rate of 100 mV s−1 at 298 K. The potentials were converted to those vs. the saturated calomel electrode (SCE) by adding 0.29 V [67].

3.4. EPR Measurements

The EPR spectra of PTIO• (7.0 × 10−5 M) in the presence or absence of Sc(OTf)3 (3.5 × 10−5 M) and/or H2O (5.6 M) in deaerated MeCN were taken using a disposable RDC-60-S flat cell (inner size, 60 mm × 6 mm × 0.3 mm) (Flashpoint Ltd., Tokyo, Japan) on a JEOL X-band spectrometer (JES-RE1X) (JEOL Ltd., Tokyo, Japan) at room temperature under the following conditions: microwave frequency, 9.40 GHz; microwave power, 8 mW; center field, 333 mT; sweep width, 15 mT; sweep rate, 3 mT min−1; modulation frequency, 100 kHz; modulation amplitude, 0.2 mT; and time constant, 0.1 s. EPR data acquisition was controlled by the WIN-RAD ESR Sata Analyzer System (Radical Research, Inc., Tokyo, Japan). The g values were calibrated with an Mn2+ marker. The experimental EPR spectra were analyzed and simulated using the WinSim 2002 software [68].

4. Conclusions

The Sc3+ with a strong Lewis acidity induced the electron-transfer disproportionation of PTIO• in deaerated MeCN. The electrochemical measurements suggested that the significantly large positive shift of the one-electron reduction potential of PTIO• in the presence of Sc3+ enables the disproportionation to occur. The addition of H2O to the PTIO•–Sc(OTf)3 system in deaerated MeCN resulted in the regeneration of PTIO• because the complex formation of Sc3+ with H2O weakened the interaction between PTIO− and Sc3+. The drastic change in the redox reactivity of PTIO• in the presence of Sc3+ as a strong Lewis acid provides not only valuable and fundamental information about the effects of the reaction environments on the reactivity of radical species but an excellent opportunity to develop radical-based redox flow batteries.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/ijms25084417/s1.

Author Contributions

Conceptualization, I.N.; methodology, S.F. and I.N.; formal analysis, Y.S. and H.I.; investigation, Y.S. and H.I.; validation, Y.S. and H.I.; resources, I.N.; data curation, Y.S., Y.T. and H.I.; writing—original draft preparation, I.N.; writing—review and editing, K.O., H.I., K.M. and S.F.; visualization, Y.S. and Y.T.; supervision, K.O., K.M., S.F. and I.N.; project administration, I.N.; funding acquisition, K.O., S.F. and I.N. All authors have read and agreed to the published version of the manuscript.

Funding

This work was partially supported by Grant-in-Aid (No. JP18K06620 to I.N., JP20H02779, JP20H04819, JP18H04650, JP17H03010, and JP16H02268 to K.O., and JP23K04686 to S.F.) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

Al(OTf)3Aluminum triflate (OTf = OSO2CF3)
Bu4NClO4Tetra-n-butylammonium perchlorate
DPPH•2,2-Diphenyl-1-picrylhydrazyl
DPPH−One-electron-reduced species of DPPH•
DPPH+One-electron-oxidized species of DPPH•
EPRElectron paramagnetic resonance
EtOHEthanol
MeCNAcetonitrile
MeOHMethanol
•NONitric oxide
PTIO•2-Phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide
PTIO−One-electron-reduced species of PTIO•
PTIO+One-electron-oxidized species of PTIO•
PTIOHProtonated form of PTIO−
SCESaturated calomel electrode
Sc(OTf)3Scandium triflate (OTf = OSO2CF3)
TEMPO2,2,6,6-Tetramethylpiperidyl-1-oxyl

References

  1. Fukuzumi, S.; Ohkubo, K. Metal ion-coupled and decoupled electron transfer. Coord. Chem. Rev. 2010, 254, 372–385. [Google Scholar] [CrossRef]
  2. Ghosh, T.K.; Maity, S.; Ghosh, S.; Gomila, R.M.; Frontera, A.; Ghosh, A. Role of redox-inactive metal ions in modulating the reduction potential of uranyl Schiff base complexes: Detailed experimental and theoretical studies. Inorg. Chem. 2022, 61, 7130–7142. [Google Scholar] [CrossRef]
  3. Mukai, K.; Nakamura, A.; Nagaoka, S.; Ouchi, A.; Azuma, N. Notable effects of the metal salts on the quenching reaction of singlet oxygen by α-tocopherol in ethanol solution. Bull. Chem. Soc. Jpn. 2015, 88, 1503–1510. [Google Scholar] [CrossRef]
  4. Mukai, K.; Kohno, Y.; Ouchi, A.; Nagaoka, S. Notable effect of metal salts on UV-vis absorption spectra of α-, β-, γ-, and δ-tocopheroxyl radicals in acetonitrile solution. The complex formation between tocopheroxyls and metal cations. J. Phys. Chem. B 2012, 116, 8930–8941. [Google Scholar] [CrossRef]
  5. Mukai, K.; Oi, M.; Ouchi, A.; Nagaoka, S. Kinetic study of the α-tocopherol-regeneration reaction of ubiquinol-10 in methanol and acetonitrile solutions: Notable effect of the alkali and alkaline earth metal salts on the reaction rates. J. Phys. Chem. B 2012, 116, 2615–2621. [Google Scholar] [CrossRef]
  6. Kohno, Y.; Fujii, M.; Matsuoka, C.; Hashimoto, H.; Ouchi, A.; Nagaoka, S.; Mukai, K. Notable effects of the metal salts on the formation and decay reactions of α-tocopheroxyl radical in acetonitrile solution. The complex formation between α-tocopheroxyl radical and metal cations. J. Phys. Chem. B 2011, 115, 9880–9888. [Google Scholar] [CrossRef]
  7. Ouchi, A.; Nagaoka, S.; Abe, K.; Mukai, K. Kinetic study of the aroxyl radical-scavenging reaction of α-tocopherol in methanol solution: Notable effect of the alkali and alkaline earth metal salts on the reaction rates. J. Phys. Chem. B 2009, 113, 13322–13331. [Google Scholar] [CrossRef]
  8. Ngueumaleu, Y.; Deutchoua, A.D.D.; Hanga, S.S.P.; Liendji, R.W.; Dedzo, G.K.; Ngameni, E. Probing the reactivity of 2,2-diphenyl-1-picrylhydrazyl (DPPH) with metal cations and acids in acetonitrile by electrochemistry and UV-vis spectroscopy. Phys. Chem. Chem. Phys. 2023, 25, 5282–5290. [Google Scholar] [CrossRef]
  9. Deutchoua, A.D.D.; Ngueumaleu, Y.; Liendji, R.W.; Sonita, S.; Hanga, P.; Nguelo, B.B.; Dedzo, G.K.; Ngameni, E. Unusual reactivity of 2,2-diphenyl-1-picrylhydrazyl (DPPH) with Fe3+ controlled by competing reactions. RSC Adv. 2024, 14, 1354–1359. [Google Scholar] [CrossRef]
  10. Yuasa, J.; Fukuzumi, S. Off-off-on switching of fluorescence and electron transfer depending on stepwise complex formation of a host ligand with guest metal ions. J. Am. Chem. Soc. 2008, 130, 566–575. [Google Scholar] [CrossRef]
  11. Fukuzumi, S.; Yuasa, J.; Satoh, N.; Suenobu, T. Scandium ion-promoted photoinduced electron transfer from electron donors to acridine and pyrene. Essential role of scandium ion in photocatalytic oxygenation of hexamethylbenzene. J. Am. Chem. Soc. 2004, 126, 7585–7594. [Google Scholar] [CrossRef]
  12. Fukuzumi, S.; Fujii, Y.; Suenobu, T. Metal ion-catalyzed cycloaddition vs. hydride transfer reactions of NADH analogues with p-benzoquinone. J. Am. Chem. Soc. 2001, 123, 10191–10199. [Google Scholar] [CrossRef]
  13. Zeng, Y.; Zhang, G.; Zhang, D.; Zhu, D. A new tetrathiafulvalene–quinone–tetrathiafulvalene triad: Modulation of the intramolecular charge transfer by the electron-transfer process promoted by metal ions. J. Org. Chem. 2009, 74, 4375–4378. [Google Scholar] [CrossRef]
  14. Zhao, B.-T.; Peng, Q.-M.; Zhu, X.-M.; Yan, Z.-N.; Zhu, W.-M. Metal-promoted intermolecular electron transfer in tetrathiafulvalene–thiacalix[4]arene conjugates and tetrachlorobenzoquinone. J. Org. Chem. 2015, 80, 1052–1058. [Google Scholar] [CrossRef]
  15. Mühldorf, B.; Wolf, R. Photocatalytic benzylic C–H bond oxidation with a flavin scandium complex. Chem. Commun. 2015, 51, 8425–8428. [Google Scholar] [CrossRef]
  16. Sankaralingam, M.; Lee, Y.-M.; Pineda-Galvan, Y.; Karmalkar, D.G.; Seo, M.S.; Jeon, S.H.; Pushkar, Y.; Fukuzumi, S.; Nam, W. Redox reactivity of a mononuclear manganese-oxo complex binding calcium ion and other redox-inactive metal ions. J. Am. Chem. Soc. 2019, 141, 1324–1336. [Google Scholar] [CrossRef]
  17. Kojima, T.; Kobayashi, R.; Ishizuka, T.; Yamakawa, S.; Kotani, H.; Nakanishi, T.; Ohkubo, K.; Fukuzumi, S. Binding of scandium ions to metalloporphyrin–Flavin complexes for long-lived charge separation. Chem. Eur. J. 2014, 20, 15518–15532. [Google Scholar] [CrossRef]
  18. Kaur, S.; Bera, M.; Santra, A.; Munshi, S.; Sterbinsky, G.E.; Wu, T.; Moonshiram, D.; Paria, S. Effect of redox-inactive metal ion–nickel(III) interactions on the redox properties and proton-coupled electron transfer reactivity. Inorg. Chem. 2022, 61, 14252–14266. [Google Scholar] [CrossRef]
  19. Devi, T.; Lee, Y.-M.; Nam, W.; Fukuzumi, S. Metal ion-coupled electron-transfer reactions of metal-oxygen complexes. Coord. Chem. Rev. 2020, 410, 213219. [Google Scholar] [CrossRef]
  20. Liu, U.; Lau, T.-C. Activation of metal oxo and nitrido complexes by Lewis acids. J. Am. Chem. Soc. 2019, 141, 3755–3766. [Google Scholar] [CrossRef]
  21. Pfaff, F.F.; Kundu, S.; Risch, M.; Pandian, S.; Heims, F.; Pryjomska-Ray, I.; Haack, P.; Metzinger, R.; Bill, E.; Dau, H.; et al. An oxocobalt(IV) complex stabilized by Lewis acid interactions with scandium(III) ions. Angew. Chem. Int. Ed. 2011, 50, 1711–1715. [Google Scholar] [CrossRef]
  22. Fukuzumi, S.; Ohkubo, K. Quantitative evaluation of Lewis acidity of metal ions derived from the g values of ESR spectra of superoxide: Metal ion complexes in relation to the promoting effects in electron transfer reactions. Chem. Eur. J. 2000, 6, 4532–4535. [Google Scholar] [CrossRef]
  23. Nakanishi, I.; Kawashima, T.; Ohkubo, K.; Waki, T.; Uto, Y.; Kamada, T.; Ozawa, T.; Matsumoto, K.; Fukuzumi, S. Disproportionation of a 2,2-diphenyl-1-picrylhydrazyl radical as a model of reactive oxygen species catalysed by Lewis and/or Brønsted acids. Chem. Commun. 2014, 50, 814–816. [Google Scholar] [CrossRef]
  24. Nakanishi, I.; Shoji, Y.; Ohkubo, K.; Ito, H.; Fukuzumi, S. Water-induced regeneration of a 2,2-diphenyl-1-picrylhydrazyl radical after its scandium ion-promoted electron-transfer disproportionation in an aprotic medium. Molecules 2023, 28, 5002. [Google Scholar] [CrossRef]
  25. Jiang, S.; Xie, Y.; Xie, Y.; Yu, L.-J.; Yan, X.; Zhao, F.-G.; Mudugamuwa, C.J.; Coote, M.L.; Jia, X.; Zhang, K. Lewis acid-induced reversible disproportionation o TEMPO enables aqueous aluminum radical batteries. J. Am. Chem. Soc. 2023, 145, 14519–14528. [Google Scholar] [CrossRef]
  26. Akaike, T.; Maeda, H. Quantitation of nitric oxide using 2-phenyl-4,4,5,5-tetramethylimizazoline-1-oxyl 3-oxide (PTIO). Methods Enzymol. 1996, 268, 211–221. [Google Scholar]
  27. Akaike, T.; Yoshida, M.; Miyamoto, Y.; Sato, K.; Kohno, M.; Sasamoto, K.; Miyazaki, K.; Ueda, S.; Maeda, H. Antagonistic action of imidazolineoxyl N-oxide against endothelium-derived relaxing factor/•NO through a radical reaction. Biochemistry 1993, 32, 827–832. [Google Scholar] [CrossRef]
  28. Sueishi, Y.; Hori, M.; Kita, M.; Kotake, Y. Nitric oxide (NO) scavenging capacity of natural antioxidants. Food Chem. 2011, 129, 866–870. [Google Scholar] [CrossRef]
  29. Fricker, S.P. Nitric oxide scavengers as a therapeutic approach to nitric oxide mediated disease. Expert Opin. Invesitig. Drugs 1999, 8, 1209–1222. [Google Scholar] [CrossRef]
  30. Kovaleva, V.D.; Uzdensky, A.B. Photodynamic therapy-induced nitric oxide production in neuronal and glial cells. J. Biomed. Opt. 2016, 21, 105005. [Google Scholar] [CrossRef]
  31. Perlikowski, D.; Lechowicz, K.; Pawłowicz, I.; Arasimonwicz-Jelonek, M.; Kosmala, A. Scavenging of nitric oxide up-regulates photosynthesis under drought in Festuca arundinacea and F. glaucescens but reduces their drought tolerance. Sci. Rep. 2022, 12, 6500. [Google Scholar] [CrossRef]
  32. Fu, J.; Chu, X.; Sun, Y.; Miao, Y.; Xu, Y.; Hu, T. Nitric oxide mediates 5-aminolevulinic acid-induced antioxidant defense in leaves of Elymus nutans griseb. exposed to chilling stress. PLoS ONE 2015, 10, e0130367. [Google Scholar] [CrossRef]
  33. Goldstein, S.; Russo, A.; Samuni, A. Reactions of PTIO and carboxy-PTIO with •NO, •NO2, and O2•−. J. Biol. Chem. 2003, 278, 50949–50955. [Google Scholar] [CrossRef]
  34. Joseph, J.; Kalyanaraman, B.; Hyde, J.S. Trapping of nitric oxide by nitronyl nitroxides: An electron spin resonance investigation. Biochem. Biophys. Res. Commun. 1994, 192, 926–934. [Google Scholar] [CrossRef]
  35. Duan, W.; Vemuri, R.S.; Milshtein, J.D.; Laramie, S.; Dmello, R.D.; Huang, J.; Zhang, L.; Hu, D.; Vijayakumar, M.; Wang, W.; et al. A symmetric organic-based nonaqueous redox flow battery and its state of charge diagnostic by FTIR. J. Mater. Chem. A 2016, 4, 5448–5456. [Google Scholar] [CrossRef]
  36. Tkacheva, A.; Sun, B.; Zhang, J.; Wang, G.; McDonagh, M. Nitronyl nitroxide-based redox mediators for Li-O2 batteries. J. Phys. Chem. C 2021, 125, 2824–2830. [Google Scholar] [CrossRef]
  37. Sinclair, N.S.; Poe, D.; Savinell, R.F.; Maginn, E.J.; Wainright, J.S. A nitroxide containing organic molecule in a deep eutectic solvent for flow battery applications. J. Electrochem. Soc. 2021, 168, 020527. [Google Scholar] [CrossRef]
  38. Zou, X.; Wu, Q.; Li, Z.; Zhang, S.; Dong, L.; Chen, Y.; Dai, Y.; Ji, C.; Liang, H.; Lin, X. Lactiplantibacillus plantarum A72, a strain with antioxidant properties, obtained through ARTP mutagenesis, affects Caenorhabditis elegans anti-aging. Foods 2024, 13, 924. [Google Scholar] [CrossRef]
  39. Li, X.; Han, W.; He, G.; Yang, J.; Li, J.; Ma, H.; Wang, S. Hydrogel-transformable antioxidant poly-γ-glutamic acid/polyethyleneimine hemostatic powder for efficient wound hemostasis. Gels 2024, 10, 68. [Google Scholar] [CrossRef]
  40. Ke, Q.; Wang, H.; Xiao, Y.; Kou, X.; Chem, F.; Meng, Q.; Gao, W. A novel water-soluble polysaccharide from daylily (Hemerocallis citrina baroni): Isolation, structure analysis, and probiotics adhesion promotion effect. Food 2024, 13, 721. [Google Scholar] [CrossRef]
  41. Li, X.; Ouyang, X.; Chen, B.; Liu, S.; Zeng, J. Linkage and stereochemistry characters of phenolic antioxidant product formation. J. Agric. Food Chem. 2023, 71, 5382–5390. [Google Scholar]
  42. Lu, H.; Wei, J.; Liu, K.; Li, Z.; Xu, T.; Yang, D.; Gao, Q.; Xiang, H.; Li, G.; Chen, Y. Radical-scavenging and subchondral bone-regenerating nanomedicine for osteoarthritis treatment. ACS Nano 2023, 17, 6131–6146. [Google Scholar] [CrossRef]
  43. Wang, J.; Xue, X.; Miao, X. Antioxidant effects of quercetin nanocrystals in nanosuspension against hydrogen peroxide-induced oxidative stress in a zebrafish model. Pharmaceuticals 2023, 16, 1209. [Google Scholar] [CrossRef]
  44. Qi, Y.; Huang, Y.; Dong, Y.; Zhang, W.; Xia, F.; Bai, H.; Stevanovic, Z.D.; Li, H.; Shi, L. Effective improvement of the oxidative stability of Acer truncatum bunge seed oil, a new woody oil food resource, by rosemary extract. Antioxidants 2023, 12, 889. [Google Scholar] [CrossRef]
  45. Gong, J.; Liu, Q.; Cai, L.; Yang, Q.; Tong, Y.; Chen, X.; Kotha, S.; Mao, X.; He, W. Multimechanism collaborative superior antioxidant CDzymes to alleviate salt stress-induced oxidative damage in plant growth. ACS Sustain. Chem. Eng. 2023, 11, 4237–4247. [Google Scholar]
  46. Wang, Y.; Huo, L.; Jia, Q.Q.; Jin, L. Preparation and evaluation of antioxidant activity of citric acid-polyethylene glycol-tannin elastomers. Colloid Polym. Sci. 2023, 301, 1351–1363. [Google Scholar]
  47. Chen, B.; Li, X.; Ouyang, X.; Liu, J.; Liu, Y.; Chen, D. Comparison of ferroptosis-inhibitory mechanisms between ferrostatin-1 and dietary stilbene (piceatannol and astringin). Molecules 2021, 26, 1092. [Google Scholar] [CrossRef]
  48. Zhang, W.; Li, X.; Hua, Y.; Li, Z.; Chen, B.; Liu, A.; Lu, W.; Zhao, X.; Diao, Y.; Chen, D. Antioxidant product analysis of Hulu Tea (Tadehagi triquetrum). New J. Chem. 2021, 45, 20257–20265. [Google Scholar]
  49. Li, X.; Zeng, J.; Liu, Y.; Liang, M.; Liu, Q.; Li, Z.; Zhao, X.; Chen, D. Inhibitory effect and mechanism of action of quercetin and quercetin Diels-Alder anti-dimer on erastin-induced ferroptosis in bone marrow-derived mesenchymal stem cells. Antioxidants 2020, 9, 205. [Google Scholar] [CrossRef] [PubMed]
  50. He, H.; Shi, X.; Wang, J.; Wang, X.; Wang, Q.; Yu, D.; Ge, B.; Zhang, X.; Huang, F. Reactive oxygen species-induced aggregation of nanoqymes for neuron injury. ACS Appl. Mater. Interfaces 2020, 12, 209–216. [Google Scholar] [CrossRef] [PubMed]
  51. Ouyang, X.; Li, X.; Liu, J.; Liu, Y.; Xie, Y.; Du, Z.; Xie, H.; Chen, X.; Lu, W.; Chen, D. Structure-activity relationship and mechanism of four monostilbenes with respect to ferroptosis inhibition. RSC Adv. 2020, 10, 31171–31179. [Google Scholar] [CrossRef]
  52. Li, X.; Liu, J.; Chen, B.; Chen, Y.; Dai, W.; Li, Y.; Zhu, M. Covalent bridging of corilagin improves antiferroptosis activity: Comparison with 1,3,6-tri-O-galloyl-β-D-glucopyranose. ACS Med. Chem. Lett. 2020, 11, 2232–2237. [Google Scholar] [CrossRef]
  53. Li, X.; Chen, B.; Zhao, X.; Chen, D. 2-Phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO•) trapping activity and mechanisms of 16 phenolic xanthones. Molecules 2018, 23, 1692. [Google Scholar] [CrossRef]
  54. Li, X.; Chen, B.; Xie, H.; HE, Y.; Zhong, D.; Chen, D. Antioxidant structure-activity relationship analysis of five dihydrochalcones. Molecules 2018, 23, 1162. [Google Scholar] [CrossRef]
  55. Li, X.; Li, K.; Xie, H.; Xie, Y.; Li, Y.; Zhao, X.; Jiang, X.; Chen, D. Antioxidant and cytoprotective effects of the di-O-caffeoylqunic acid family: The mechanism, structure–activity relationship, and conformational effect. Molecules 2018, 23, 222. [Google Scholar] [CrossRef]
  56. Li, X. 2-Phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO•) radical scavenging: A new and simple antioxidant assay in vitro. J. Agric. Food Chem. 2017, 65, 6288–6297. [Google Scholar] [CrossRef]
  57. Zhu, M.; Wang, S.; Li, Z.; Li, J.; Xu, Z.; Liu, X.; Huang, X. Tyrosine residues initiated photopolymerization in living organisms. Nat. Commun. 2023, 14, 3598. [Google Scholar] [CrossRef]
  58. Samuni, U.; Samuni, Y.; Goldstein, S. On the distinction between nitroxyl and nitric oxide using nitronyl nitroxides. J. Am. Chem. Soc. 2010, 132, 8428–8432. [Google Scholar] [CrossRef]
  59. Israeli, A.; Pratt, M.; Oron, M.; Samuni, A.; Kohen, R.; Goldstein, S. Kinetics and mechanism of the comproportionation reaction between oxoammonium cation and hydroxylamine derived from cyclic nitroxides. Free Radic. Biol. Med. 2005, 38, 317–324. [Google Scholar] [CrossRef]
  60. Zhang, R.L.; Goldstein, S.; Samuni, A. Kinetics of superoxide-induced exchange among nitroxide antioxidants and their oxidized and reduced forms. Free Radic. Biol. Med. 1999, 26, 1245–1252. [Google Scholar] [CrossRef]
  61. Zhang, R.; Pinson, A.; Samuni, A. Both hydroxylamine and nitroxide protect cardiomyocytes from oxidative stress. Free Radic. Biol. Med. 1998, 24, 66–75. [Google Scholar] [CrossRef]
  62. Golubev, V.A.; Sen, V.D.; Kulyk, I.V.; Aleksandrov, A.L. Mechanism of the oxygen disproportionation of di-tert-alkylnitroxyl radicals. Izv. Akad. Nauk SSSR Ser. Khim. 1975, 10, 2235–2243. [Google Scholar]
  63. Zauche, T.H.; Espenson, J.H. Kinetics of oxidation and comproportionation reactions involving an oxoammonium ion, benzyl alcohol and methyltirioxorhenium(VII). Int. J. Chem. Kinet. 1999, 31, 381–385. [Google Scholar] [CrossRef]
  64. Dikalov, S.I.; Dikalova, A.E.; Mason, R.P. Noninvasive diagnostic tool for inflammation-induced oxidative stress using electron spin resonance spectroscopy and an extracellular cyclic hydroxylamine. Arch. Biochem. Biophys. 2002, 402, 218–226. [Google Scholar] [CrossRef]
  65. Sen’, V.D.; Tikhonov, I.V.; Borodin, L.I.; Pliss, E.M.; Golubev, V.A.; Syroeshkin, M.A.; Rusakov, A.I. Kinetics and thermodynamics of reversible disproportionation–comproportionation in redox triad oxoammonium cations-nitroxyl radicals-hydroxylamines. J. Phys. Org. Chem. 2015, 28, 17–24. [Google Scholar] [CrossRef]
  66. Sen, V.D.; Golubev, V.A. Kinetics and mechanism for acid-catalyzed disproportionation of 2,2,6,6-tetramethylpiperidine-1-oxyl. J. Phys. Org. Chem. 2009, 22, 138–143. [Google Scholar] [CrossRef]
  67. Mann, C.K.; Barnes, K.K. Electrochemical Reactions in Nonaqueous Systems; Marcel Dekker Inc.: New York, NY, USA, 1990. [Google Scholar]
  68. Duling, D.R. Simulation of multiple isotropic spin-trap EPR spectra. J. Magn. Reson. Ser. B 1994, 104, 105–110. [Google Scholar] [CrossRef]
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Scheme 1. Redox behavior of PTIO•.
Scheme 1. Redox behavior of PTIO•.
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Scheme 2. Sc3+-promoted electron-transfer disproportionation of PTIO•.
Scheme 2. Sc3+-promoted electron-transfer disproportionation of PTIO•.
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Figure 1. Spectral change observed upon addition of Sc(OTf)3 (4.4 × 10−6 M each) to PTIO• (7.2 × 10−5 M) in deaerated MeCN. Blue and orange lines are the initial and final spectra, respectively. Arrows denote the direction of absorption changes. Inset: plot of the absorbance at 361 nm vs. [Sc(OTf)3]/[PTIO•].
Figure 1. Spectral change observed upon addition of Sc(OTf)3 (4.4 × 10−6 M each) to PTIO• (7.2 × 10−5 M) in deaerated MeCN. Blue and orange lines are the initial and final spectra, respectively. Arrows denote the direction of absorption changes. Inset: plot of the absorbance at 361 nm vs. [Sc(OTf)3]/[PTIO•].
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Figure 2. (a) Spectral change (interval: 70 ms) observed during the reaction of PTIO• (5.5 × 10−5 M) with Sc(OTf)3 (7.9 × 10−3 M) in deaerated MeCN at 298 K. Blue and orange lines are the initial and final spectra, respectively. Arrows denote the direction of absorption changes. Inset: the second-order plot of the absorbance at 361 nm. (b) Plot of kdisp vs. [Sc(OTf)3] for the disproportionation of PTIO• (5.5 × 10−5 M) in the presence of Sc(OTf)3 in deaerated MeCN at 298 K.
Figure 2. (a) Spectral change (interval: 70 ms) observed during the reaction of PTIO• (5.5 × 10−5 M) with Sc(OTf)3 (7.9 × 10−3 M) in deaerated MeCN at 298 K. Blue and orange lines are the initial and final spectra, respectively. Arrows denote the direction of absorption changes. Inset: the second-order plot of the absorbance at 361 nm. (b) Plot of kdisp vs. [Sc(OTf)3] for the disproportionation of PTIO• (5.5 × 10−5 M) in the presence of Sc(OTf)3 in deaerated MeCN at 298 K.
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Scheme 3. Proposed mechanism of the Sc3+-promoted electron-transfer disproportionation of PTIO•.
Scheme 3. Proposed mechanism of the Sc3+-promoted electron-transfer disproportionation of PTIO•.
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Figure 3. Cyclic voltammograms of PTIO• (1.0 × 10−3 M) before (blue line) and after (orange line) the addition of Sc(OTf)3 (1.0 × 10−2 M) in deaerated MeCN containing 0.10 M Bu4NClO4 recorded at the scan rate of 100 mV s−1 on a glassy carbon working electrode at 298 K. Arrows denote starting points and the direction of scanning.
Figure 3. Cyclic voltammograms of PTIO• (1.0 × 10−3 M) before (blue line) and after (orange line) the addition of Sc(OTf)3 (1.0 × 10−2 M) in deaerated MeCN containing 0.10 M Bu4NClO4 recorded at the scan rate of 100 mV s−1 on a glassy carbon working electrode at 298 K. Arrows denote starting points and the direction of scanning.
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Scheme 4. Electrochemical oxidation of [(PTIO)Sc]2+ at +1.66 V vs. SCE.
Scheme 4. Electrochemical oxidation of [(PTIO)Sc]2+ at +1.66 V vs. SCE.
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Figure 4. Spectral change upon the addition of H2O to a deaerated MeCN solution of PTIO• (7.3 × 10−5 M) and Sc(OTf)3 (3.7 × 10−5 M) at 298 K. Orange and blue lines are the initial and final spectra, respectively. Arrows denote the direction of absorption changes.
Figure 4. Spectral change upon the addition of H2O to a deaerated MeCN solution of PTIO• (7.3 × 10−5 M) and Sc(OTf)3 (3.7 × 10−5 M) at 298 K. Orange and blue lines are the initial and final spectra, respectively. Arrows denote the direction of absorption changes.
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Figure 5. (a) Spectral change (interval: 0.1 s) observed after the addition of H2O (1.9 M) to a deaerated MeCN solution of PTIO• (6.9 × 10−5 M) and Sc(OTf)3 (3.5 × 10−5 M) at 298 K. Orange and blue lines are the initial and final spectra, respectively. Arrows denote the direction of absorption changes. Inset: the second-order plot of the absorbance at 361 nm. (b) Plot of kcomp vs. [H2O].
Figure 5. (a) Spectral change (interval: 0.1 s) observed after the addition of H2O (1.9 M) to a deaerated MeCN solution of PTIO• (6.9 × 10−5 M) and Sc(OTf)3 (3.5 × 10−5 M) at 298 K. Orange and blue lines are the initial and final spectra, respectively. Arrows denote the direction of absorption changes. Inset: the second-order plot of the absorbance at 361 nm. (b) Plot of kcomp vs. [H2O].
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Scheme 5. H2O-induced regeneration of PTIO•.
Scheme 5. H2O-induced regeneration of PTIO•.
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Figure 6. EPR spectra of (a) PTIO• (7.0 × 10−5 M), (b) PTIO• (7.0 × 10−5 M) after the addition of Sc(OTf)3 (3.5 × 10−5 M), and (c) PTIO• (7.0 × 10−5 M) after the addition of Sc(OTf)3 (3.5 × 10−5 M) and H2O (5.6 M) in deaerated MeCN at room temperature.
Figure 6. EPR spectra of (a) PTIO• (7.0 × 10−5 M), (b) PTIO• (7.0 × 10−5 M) after the addition of Sc(OTf)3 (3.5 × 10−5 M), and (c) PTIO• (7.0 × 10−5 M) after the addition of Sc(OTf)3 (3.5 × 10−5 M) and H2O (5.6 M) in deaerated MeCN at room temperature.
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Shoji, Y.; Terashima, Y.; Ohkubo, K.; Ito, H.; Maruyama, K.; Fukuzumi, S.; Nakanishi, I. Scandium Ion-Promoted Electron-Transfer Disproportionation of 2-Phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-Oxide (PTIO•) in Acetonitrile and Its Regeneration Induced by Water. Int. J. Mol. Sci. 2024, 25, 4417. https://doi.org/10.3390/ijms25084417

AMA Style

Shoji Y, Terashima Y, Ohkubo K, Ito H, Maruyama K, Fukuzumi S, Nakanishi I. Scandium Ion-Promoted Electron-Transfer Disproportionation of 2-Phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-Oxide (PTIO•) in Acetonitrile and Its Regeneration Induced by Water. International Journal of Molecular Sciences. 2024; 25(8):4417. https://doi.org/10.3390/ijms25084417

Chicago/Turabian Style

Shoji, Yoshimi, Yuri Terashima, Kei Ohkubo, Hiromu Ito, Kouichi Maruyama, Shunichi Fukuzumi, and Ikuo Nakanishi. 2024. "Scandium Ion-Promoted Electron-Transfer Disproportionation of 2-Phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-Oxide (PTIO•) in Acetonitrile and Its Regeneration Induced by Water" International Journal of Molecular Sciences 25, no. 8: 4417. https://doi.org/10.3390/ijms25084417

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