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Accepted Article

Title: Discriminative Detection of Biothiols through Thiol-TrappingTechnique using Methanethiosulfonate Trityl Probe and ElectronParamagnetic Resonance Spectroscopy

Authors: Xiaoli Tan, Kaiyun Ji, Xing Wang, Ru Yao, Guifang Han,Frederick Villamena, Jay Zweier, Yuguang Song, AntalRockenbauer, and Yangping Liu

This manuscript has been accepted after peer review and appears as anAccepted Article online prior to editing, proofing, and formal publicationof the final Version of Record (VoR). This work is currently citable byusing the Digital Object Identifier (DOI) given below. The VoR will bepublished online in Early View as soon as possible and may be differentto this Accepted Article as a result of editing. Readers should obtainthe VoR from the journal website shown below when it is publishedto ensure accuracy of information. The authors are responsible for thecontent of this Accepted Article.

To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201912832Angew. Chem. 10.1002/ange.201912832

Link to VoR: http://dx.doi.org/10.1002/anie.201912832http://dx.doi.org/10.1002/ange.201912832

RESEARCH ARTICLE

Discriminative Detection of Biothiols through Thiol-Trapping

Technique using Methanethiosulfonate Trityl Probe and Electron

Paramagnetic Resonance Spectroscopy

Xiaoli Tan,[a] Kaiyun Ji,[a] Xing Wang,[a] Ru Yao,[a] Guifang Han,[a] Frederick A. Villamena,[c] Jay L.

Zweier,[d] Yuguang Song,*[a] Antal Rockenbauer,*[b] and Yangping Liu*[a]

Abstract: Biothiols such as glutathione (GSH) and cysteine (Cys)

generally coexist in biological systems with diverse biological roles.

Thus, analytical techniques that can simultaneously detect and

distinguish multiple biothiols are extremely desirable but challenging.

Herein, we propose a concept of electron paramagnetic resonance

(EPR)-“thiol trapping” for simultaneous detection and quantification

of multiple thiols using a trityl radical-based thiol probe (MTST) that

contains a thiol-reactive methanethiosulfonate group. The “trapping”

reaction of MTST with 7 thiols including GSH and Cys were

investigated. The resulting disulfide conjugates contain two stable

conformers and exhibit characteristic EPR spectra due to different

populations and interconversion rates of the two conformers as well

as various proton hyperfine splittings for each conformer, as

determined by EPR spectral simulation. The ability of MTST to

simultaneously quantitating multiple thiols by EPR was verified by

measuring the mixtures of GSH/Cys and GSH/Cys/Hcy in phosphate

buffer. By optimizing EPR simulation program, up to five species in

one sample can be simultaneously determined. To expand the

biological application of this thiol-trapping method, MTST was used

to investigate effects of L-buthionine sulfoximine (BSO) and

pyrrolidine dithiocarbamate (PDTC) on efflux of GSH and Cys from

HepG2 cells. The reliability of this method was confirmed by HPLC.

In summary, our present thiol-trapping technique represents the first

example of simultaneous and discriminative detection and

quantification of multiple biothiols by EPR.

Introduction

Low-molecular-weight biothiols including glutathione (GSH),

cysteine (Cys) and homocysteine (Hcy) play crucial roles in many physiological processes and diseases. Discriminative detection and quantitation of these thiols is of great significance and highly desirable, given their diverse biological roles.[1] To date, a large number of colorimetric and fluorescent probes have been developed with high selectivity to one or two of the biothiols.[2] Moreover, high performance liquid chromatography (HPLC)-related techniques have been established as reliable but invasive methods for the measurement of biothiols.[3] However, development of noninvasive methods enabling simultaneous discrimination and quantitation of biothiols in biological systems is still challenging.[4] Low-frequency electron paramagnetic resonance (EPR) technique in combination with paramagnetic probes is a powerful tool for in vivo applications due to moderate tissue penetration depth of the radiofrequency. This technique exhibits some advantages over NMR which includes high intrinsic sensitivity and the lack of endogenous background signals.[5] Previously, nitroxide disulfide biradicals were reported as EPR thiol probes. The thiol-disulfide exchange between the biradicals and thiols results in the formation of the corresponding nitroxide monoradical which can be monitored by EPR.[6] These probes enabled measurement of the thiol (typically GSH) levels in cells and mice.[7] However, facile bioreduction and moderately broad EPR triplet spectra of nitroxides limit the desired sensitivity and image resolution of EPR spectroscopy and imaging. To address these issues, we previously reported a trityl radical-based disulfide biradical which exhibits enhanced sensitivity and specificity to thiols.[8] Unfortunately, none of these biradical-based probes could distinguish biothiols due to indiscriminative EPR spectra for various thiol conjugates. In the past decade, TAM (also named as trityl) radicals have accepted intense attention in the field of EPR due to their unique properties including high biostability, water solubility, very sharp EPR single line and long relaxation time.[9] These TAM radicals have been functionalized to measure various physiological parameters such as superoxide radical anion,[10] pH,[11] as well as redox status.[12] Moreover, these radicals and their derivatives also have shown great promise as spin labels for distance measurements in proteins[13] and nucleic acids[14] and as efficient polarizing agents of high-field dynamic nuclear polarization.[15] In the present study, inspired by the principle of EPR spin-trapping technique whereby distinctive EPR spectra are generated upon trapping of free radicals by spin traps,[16] we propose a concept of EPR-“thiol trapping”, utilizing novel methanethiosulfonate-containing[17] TAM radical (named as MTST, Scheme 1) as a thiol probe. We hypothesize that MTST could rapidly “trap” thiol through its methanethiosulfonate group resulting in the corresponding disulfide conjugate which exhibits an EPR spectrum characteristic of the trapped thiol. Therefore, our results represent the first example for discriminative detection and quantitation of biothiols by EPR.

[a] Dr. X. L. Tan, K. Y. Ji, X. Wang, R. Yao, Prof. Y. G. Song, Prof. Y. P. Liu Tianjin Key Laboratory on Technologies Enabling Development of Clinical Therapeutics and Diagnostics, School of Pharmacy, Tianjin Medical University, Tianjin 300070, P. R. China

Email: liuyangping@tmu.edu.cn songyuguang@tmu.edu.cn

[b] Prof. A. Rockenbauer Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, 1117 Budapest, Hungary and Department of Physics, Budapest University of Technology and Economics, Budafoki ut 8, 1111 Budapest, Hungary

Email: rockenbauer.antal@ttk.mta.hu [c] Prof. F. A. Villamena

Department of Biological Chemistry and Pharmacology, College of Medicine, The Ohio State University, Columbus, Ohio 43210, United States

[d] Prof. J. L. Zweier Center for Biomedical EPR Spectroscopy and Imaging, The Davis Heart and Lung Research Institute, the Division of Cardiovascular Medicine, Department of Internal Medicine, The Ohio State University, Columbus, Ohio 43210, United States Supporting information for this article is given via a link at the end of the document.

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RESEARCH ARTICLE

SS

SS

S

SS

S

SS

SS

HO

O

OHO

SS

OO

R-SH

SS

SS

S

SS

S

SS

SS

HO

O

OHO

SS

R

H2H1

Disulfide conjugateMTST

H1 H2

Thiol-trapping

HONH

NH OH

O

O

O

NH2

SHO

GSH

HS OH

O

NH2

OH

O

NH2

HS

Cys Hcy

Scheme 1. Discriminative detection of biothiols using the novel thiol probe MTST. EPR spectrum of the disulfide conjugate is sensitive to the nature of thiols such as glutathione (GSH), cysteine (Cys) and homocysteine (Hcy).

Results and Discussion

Synthesis The synthesis of the methanethiosulfonate trityl

probe MTST was shown in Scheme 2. In brief, the triester 1[18]

was reduced partially by LiAlH4 to the corresponding benzyl

alcohol 2 in 50% yield. Then, the reaction of 2 with

methanesulfonyl chloride in the presence of DIPEA led to the

corresponding benzyl chloride 3 in 90% yield. Subsequently, the

SN2 reaction of 3 with sodium methanethiosulfonate afforded the

compound 4 in 48% yield. Finally, MTST was obtained in 50%

yield by treatment of 4 with BF3-Et2O/SnCl2. The thiol probe

MTST was characterized by HRMS (Figure S1) and its purity

was determined by HPLC (> 96%) (Figure S2).

OH

LiAlH4/THF

1 2

50%S

SS

S

OO

SS

SS

S

SS

S

OO

OH

HO CH3SO2Cl,DIPEA/DCM

90%

BF3.Et2O

SnCl250%

MTST

3

2

4

SS

SS

S

SS

S

OO

S

HO

2

SO O

3

SS

SS

S

SS

S

OO

Cl

HO

2

CH3SO2SNa48%

Scheme 2. Synthesis of the TAM-based thiol probe MTST.

EPR spectra of MTST Figure 1A shows experimental and simulated EPR spectra of MTST in phosphate buffer (PB) under anaerobic condition which exhibit a well-resolved quartet signal due to hyperfine splittings (αH) from two nonequivalent protons

on the benzyl group. Worth noting is that the two peaks at the central part are slightly broadened with weaker intensities relative to the other two peaks, characteristic of relatively slow chemical exchange between two conformers.[19] EPR spectral simulation suggests that the two conformers (T1 and T2) are exchanged at a rate of 0.14 μs (Table 1) with the values of αH1 = 1.41 G and αH2 = 1.35 G for T1 (30%) and, αH1 = 1.21 G and αH2 = 1.66 G for T2 (70%). Figure 1B shows the two-site exchange mechanism between two asymmetric conformers. The αH values which characterize the geometry of the conformer depend on the magnitude of the dihedral angles (θ1 and θ2) between the C-H1/C-H2 σ bonds on the benzyl group and 2pz orbit on the ipso carbon (Cipso) of the aromatic ring. This relationship can be described by the McConnell equation[20]:

αH(θ) = B·ρC·cos2(θ) (1) where B is spin delocalization parameter and ρC spin population at the Cipso atom. Considering that the values of B and ρC should be identical for the two conformers, the values of αH1 and αH2 are negatively related to the dihedral angles (θ1 and θ2).

Figure 1. (A) Experimental (solid line) and simulated (dotted line) EPR spectra of MTST (100 μM) in PB (20 mM, pH 7.4) under anaerobic condition. *, 13C satellites. (B) Newman projection showing the chemical exchange between the conformers T1 and T2. Note: in the case of MTST, the group SSR is replaced by SSO2CH3. For simplicity, the substituents on trityl radical are omitted.

Reaction of MTST with GSH Reactivity of MTST with GSH was

investigated by EPR. Addition of GSH (50 or 120 μM) to the

solution of MTST (100 μM) in PB led to fast formation of the

disulfide conjugate T-GSH and the reaction was completed

within 1 min (Figure S3). The use of slight excess of GSH (120

μM) guarantees complete conversion of MTST (100 μM) into T-

GSH. As shown in Figure 2A, the spectrum of T-GSH has a

narrower spectral width (2.53 G) along with more complicated

peaks in the internal region as compared to that of MTST (2.83

G). Since GSH is a chiral tripeptide and MTST contains a

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RESEARCH ARTICLE

racemate due to the propeller-like configuration of the trityl

moiety with the right-handed (P) and left-hand (M) helices,[21] the

thiol conjugate T-GSH should contain two diastereoisomers.

Delightedly, the two diastereoisomers (i.e., T-GSHa and T-GSHb)

were separable by HPLC (Figure S4 and S5). Electronic circular

dichroism (ECD) spectra of both T-GSHa and T-GSHb were

recorded (Figure S6) and the absolute configurations on the trityl

part were determined to be M for T-GSHa and P for T-GSHb

according to our recent work[15b] (see their molecular structures

in Figure S8). EPR spectra of T-GSHa and T-GSHb were also

recorded (Figure 2B). Spectral simulation suggests that a pair of

interconverting conformers with almost the same population

exist for each diastereoisomer (Table 1). The difference in

spectral profiles of T-GSHa and T-GSHb mainly stems from

distinct αH values and exchange rates of their two conformers.

Thus, the configuration of the trityl part has an important effect

on the αH values, i.e., the dihedral angles θ1 and θ2. Calculations

were also carried out to display effect of the exchange rate (~7

×10-5 to ~70 μs) on EPR spectra of T-GSHa and T-GSHb,

assuming that other parameters (e.g., αH values) remain

constant. As shown in Figure S9, EPR spectra of both isomers

are strongly dependent on the values of exchange rates.

Figure 2. (A) EPR spectra showing reaction of MTST with GSH. GSH (0, 50 and 120 μM) was added to the solution of MTST (100 μM) in PB (20 mM, pH 7.4) and EPR spectra under anaerobic condition were recorded 1 min after mixing. (B) EPR spectra of the diastereoisomers T-GSHa and T-GSHb under anaerobic condition. Solid and dotted lines indicate experimental and simulated spectra, respectively.

Reaction of MTST with Other Thiols High reactivity of MTST to

GSH and characteristic EPR spectrum of T-GSH encouraged us

to further investigate the reaction of MTST with other thiols

including cysteine (Cys), homocysteine (Hcy), p-

mercaptobenzoic acid (MPA), mercapto acetic acid (TGA), (2S)-

thiolactic acid (TLA) and L-penicillamine (PCA). Similarly, the

reaction of MTST with these thiols afforded the corresponding

disulfide conjugates that exhibit distinctive EPR spectra

particularly in the internal regions (marked by red, Figure 3).

While the disulfide conjugates from the chiral thiols (e.g., Cys,

Hcy, TLA and PCA) have two diastereoisomers, the achiral

thiols (e.g., MPA and TGA) lead to the racemic conjugates.

Figure 3. Experimental (solid line) and simulated (dotted line) EPR spectra of the disulfide conjugates which were obtained by mixing biothiol (120 μM) with the solution of MTST (100 μM) in PB (20 mM, pH 7.4) at 298 K under aerobic condition.

In contrast to the case of T-GSH, the two diastereoisomers or

enantiomers of these conjugates are indistinguishable by EPR.

Thus, their EPR spectra can be well simulated assuming that the

two identical isomers contain a pair of interconverting

conformers. EPR parameters of the conjugates are given in

Table 1. Clearly, the values of αH1 and αH2, population and

exchange rate of the two conformers are strongly dependent on

the nature of the thiols. Worth noting is that these spectral

parameters, especially the exchange rate, may be sensitive to

experimental conditions such as temperature and viscosity. For

example, increasing temperature accelerated the chemical

exchange between the two conformers of T-Cys with the τ

values decreasing from 0.78 µs (298K) to 0.42 µs (315K) (Table

S2). However, increasing viscosity (0-50% glycerol in water)

slowed down the chemical exchange of T-Cys and T-Hcy to a

different extent (Table S3 and S4). In contrast to the chemical

exchange, neither the hyperfine splittings nor population of the

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conformers was significantly affected by temperature and

viscosity (Table S2-S4). Moreover, effect of pH on the spectral

parameters was also negligible (Figure S11). Thus, EPR

measurement of the individual spectra of different thiol

conjugates under the same conditions is often necessary for

unambiguous identification of the thiol(s) in the systems

investigated. Our results also imply that in spite of three σ bonds

away from the C-H1/C-H2 bonds to R group in the thiol conjugate

(Scheme 1), the difference in the R groups can still exert marked

effects on the bond rotations of C-H1/C-H2 and the dihedral

angles θ1 and θ2. Collectively, MTST can be used to

discriminatively detect thiols.

Table 1. EPR parameters of the disulfide conjugates of MTST with thiols in PB

(20 mM, pH 7.4) at 298 K.

Conjugates Isomers ConformersαH1 (G)

αH2 (G)

τ (µs)

tR (min)

MTST M, P T1 (30%) 1.41 1.35

0.14 14.4T2 (70%) 1.21 1.66

T-GSH

M (51%) T1 (50%) 1.75 1.11

0.63 10.9T2 (50%) 0.63 1.66

P (49%) T1 (45%) 1.46 1.05

0.49 10.6T2 (55%) 1.00 1.37

T-Cys M, P T1 (36%) 1.62 0.47

0.77 11.6T2 (64%) 0.79 1.96

T-Hcy M, P T1 (40%) 1.36 0.66

0.34 12.1T2 (60%) 1.23 1.74

T-MPA M, P T1 (33%) 1.64 0.51

0.63 13.5T2 (67%) 0.62 2.01

T-TGA M, P T1 (42%) 1.21 0.63

0.22 11.9T2 (58%) 1.25 1.83

T-TLA M, P T1 (38%) 1.21 0.49

0.24 12.0T2 (62%) 1.22 2.04

T-PCA M, P T1 (45%) 1.45 0.68

0.57 12.2T2 (55%) 0.93 2.07

Note: τ, chemical exchange rate; tR, retention time on HPLC.

Selectivity of MTST toward thiols The selectivity of MTST

toward thiols was investigated by HPLC (Figure 4A). MTST was

completely converted into the corresponding conjugates upon

reaction with GSH, Cys or Hcy (1.2 equiv.) but no reaction was

observed for other amino acids (10 equiv.) including tyrosine

(Tyr), lysine (Lys), histidine (His), glycine (Gly), proline (Pro).

Thus, MTST shows high specificity toward biothiols.

The thiol quantification by MTST may be interfered from the

cleavage of the disulfide bond in the thiol conjugates by free

thiols in biological systems.[22] Accordingly, the stability of T-GSH,

T-Cys and T-Hcy toward GSH or Cys was studied by HPLC

(Figure 4B). Under our HPLC conditions, these conjugates were

well separated with different retention times (11.8 min for T-Cys,

12.3 min for T-Hcy, 10.6min/10.9 min for T-GSHa/T-GSHb).

Incubation of T-Cys or T-Hcy with GSH for 30 min did not lead to

any significant production of T-GSH (Figure 4B). Likewise, T-

Cys was undetectable when mixing T-GSH with Cys. Thus,

these disulfide conjugates are resistant to the thiol-disulfide

exchange, possibly due to the large steric hindrance from the

trityl part.[22a] Moreover, the GSH-induced decay was not

observed for these disulfide conjugates (Figure S15).[23] Taken

together, MTST can be used to quantitate biothiols.

Figure 4. (A) Selectivity of MTST to thiols. The reactivity with thiols is expressed as percentage of the conversion of the corresponding conjugates. Thiols (60 μM) or amino acids (500 μM) were mixed with MTST (50 μM) in PB (20 mM, pH 7.4) and HPLC analyses were carried out 30 min after mixing. (B) Resistance of the disulfide conjugates to the thiol-disulfide exchange. Thiol (250 μM) was mixed with the disulfide conjugate (50 μM) in PB (20 mM, pH 7.4) and HPLC analysis was carried out after 30-min incubation.

Simultaneous detection of GSH/Cys or GSH/Cys/Hcy with MTST The ability of MTST to detect multiple thiols in biological systems was initially tested by measuring various concentrations of GSH/Cys mixtures. EPR spectra were recorded under anaerobic conditions 2 min after mixing excess of MTST (100 μM) with the solutions of GSH (30-40 μM) and Cys (30-40 μM) in PB. We developed a new EPR program (ROKI/EPRMULTI) which enables simulating superimposed spectra consisting of multiple species (up to five species) with chemical exchange phenomenon (see the section of “EPR Simulation” in SI). To

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prevent any potential interference from variations of experimental conditions, the individual spectra of MTST and the thiol conjugates (T-GSH and T-Cys) were also recorded under the same conditions and used for the simulations of the superimposed spectra. As shown in Figure 5A, the superimposed spectra were satisfactorily simulated and the measured concentrations of GSH and Cys were well consistent with the actual concentrations (Figure 5B) with a correlation coefficient of > 0.97 (See Table S5). Furthermore, this thiol-trapping technique was also practical for the systems containing the mixtures of Hcy/Cys (Figure S16) and even the mixtures of three thiols (GSH, Cys and Hcy) (Figure 5A and 5B). The slightly higher standard deviations of the thiol concentrations were observed for the systems containing more species (see the section of “EPR Simulation” in SI).

Figure 5. Measurement of GSH/Cys and GSH/Cys/Hcy in the mixtures using MTST. (A) Experimental (solid line) and simulated (dashed line) EPR spectra obtained by mixing MTST (100 μM) with the solution of GSH/Cys and GSH/Cys/Hcy in PB (20 mM, 7.4). Sample A, GSH (40 μM)/Cys (40 μM); Sample B, GSH (30 μM)/Cys (30 μM); Sample C, GSH (15 μM)/Cys (30 μM)/ Hcy (25 μM); Sample D, GSH (30 μM)/Cys (25 μM)/ Hcy (30 μM). (B) Measured concentrations of the biothiols in the mixtures through EPR simulation (red bar) and their actual concentrations (black bar). Each experiment was conducted three times.

Measurement of thiol efflux from HepG2 cells using MTST Finally, MTST was used to monitor efflux of thiols from HepG2 cells. The formation of γ-glutamylcysteine catalyzed by γ-

glutamylcysteine synthetase (γ-GCS) is the rate-limiting step for the intracellular synthesis of GSH.[24] To investigate the effect of the intracellular synthesis of GSH on the efflux of thiols, HepG2 cells were treated with BSO (an inhibitor of γ-GCS)[25] or PDTC[26] that up-regulates the expression of γ-GCS. As shown in Figure 6, under untreated conditions, extracellular levels of Cys rapidly increased with time and reached to a plateau (~ 153 nmol/mg protein) at 8 h. Extending the incubation time slightly attenuated the levels of Cys possibly due to air oxidation of Cys in the medium.[27] Treatment of the cells with BSO inhibited the intracellular synthesis of GSH, accordingly reduced the consumption of Cys and thus significantly increased the extracellular levels of Cys with a peak level of ~186 nmol/mg protein. In comparison, the treatment with PDTC showed an opposite effect and decreased the efflux of Cys with the peak level of ~115 nmol/mg protein. In contrast, the efflux of GSH from the cells was minimal and detectable (< 10 nmol/mg protein) only after 10 h treatment with PDTC, consistent with the previous results that Cys has much higher extracellular levels than GSH.[28] These EPR results were verified by HPLC studies (Figure 6). Therefore, MTST was able to simultaneously monitor the dynamic changes of the GSH and Cys effluxes from HepG2 cells.

Figure 6. Monitoring effluxes of GSH/Cys from HepG2 cells by MTST. HepG2 cells were treated with phosphate buffered saline, BSO (200 μM) or PDTC (200 μM) for 1h. Then, aliquot of the medium (40 μL) was taken at different time points and mixed with MTST (10 μL, 1 mM). Then, EPR spectra were recorded and the levels of GSH/Cys were determined. Note: “filled icons” represent the EPR results while “unfilled icons” represent the results from HPLC.

Conclusion

In summary, using the present “thiol-trapping” technique, we have achieved discriminative detection and quantitation of biothiols by EPR. To the best of our knowledge, this is the first study showing simultaneous measurement of more than two biothiols by spectroscopic method without a need of chromatographic separations.[4] For future in vivo applications, the properties of MTST such as biocompatibility needs to be further improved possibly through PEGylated dendritic

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encapsulation.[29] Moreover, new probes having moderate reaction rates with thiols are desired in order not to alter redox status in biological systems. Overall, our results support the concept of EPR-“thiol-trapping” which allows for simultaneous quantitation of multiple biothiols in a single analysis. The newly synthesized probe MTST could also act as an excellent spin label due to its unique properties including fast labeling, high labeling efficiency and much shorter linker as compared to other trityl spin labels.[13]

Experimental Section

General information All air- and moisture-sensitive reactions were carried out under an atmosphere of argon using standard Schlenk technique. Solvents were dried using appropriate drying agents before use. All commercially available reagents were used as purchased without further purification. Phosphate buffer (PB) was prepared from NaH2PO4 and Na2HPO4 and its pH was adjusted by adding aqueous solution of NaOH (1 M) or HCl (1 M). 1H and 13C Nuclear Magnetic Resonance (NMR) spectra were recorded on a Bruker AVANCE III 400 instrument (1H: 400 MHz and 13C: 100 MHz). Chemical shifts (δ) were given in parts per million (ppm) and referenced to TMS or NMR solvents used (δH = 7.26 ppm for CDCl3). The following abbreviations were used: s, singlet; d, doublet; t, triplet; m, multiplet. Coupling constants (J) were reported in Hz. UV-vis spectra were recorded on a U-3900 spectrometer (HITACHI). High resolution mass spectra (HRMS) were obtained on an FTICR-MS instrument (Ionspec 7.0). High performance liquid chromatography (HPLC) spectra were recorded on an Agilent 1100 spectrometer.

Materials L-Glutathione (L-GSH), L-cysteine (L-Cys), L-homocysteine (L-Hcy), p-mercaptobenzoic acid (MPA), mercapto acetic acid (TGA), thiolacticacid (TLA), L-penicillamine (L-PCA), tyrosine (Tyr), lysine (Lys), histidine (His), glycine (Gly), proline (Pro), L-buthionine sulfoximine (BSO) and pyrrolidine dithiocarbamate (PDTC) were purchased from Sigma-Aldrich (Shanghai, China). Lithium aluminum hydride (LiAlH4), N, N-diisopropylethylamine (DIPEA), mesyl chloride, sodium methanethiosulfonate, 5,5′-dithiobis(2-nitrobenzoic acid) (DTNB) were commercially available. All aqueous solutions were prepared using ultrapure water (18.2 MΩ·cm) from a Milli-Q automatic ultrapure water system. Stock solutions (1 mM) of TAM radicals as carboxylate sodium forms were prepared in phosphate buffer (PB, 20 mM, pH 7.4) and stored at -20 °C, whereas the solutions of thiols (10 mM) in PB were freshly prepared each day.

Synthesis of compound 2 To a solution of compound 1 (1 g, 0.8 mmol) in dry THF (10 mL) was slowly added LiAlH4 (1.2 mL, 1.2 mmol) in THF on an ice bath. Then, the ice bath was removed and the reaction mixture was stirred for 12 h under N2 atmosphere. The solvents were removed under vacuum and 10 mL of HCl (1 M) solution was added to quench the reaction. The resulting aqueous solution was extracted with ethyl acetate (3×10 mL). Then the combined organic layer was washed with saturated NaCl solution (3×10 mL), dried over anhydrous Na2SO4 and evaporated in vacuo. The resulting residue was purified by column chromatography on silica gel eluting with petroleum ether/CH2Cl2 = 2:1 to 1:5 to afford compound 2 as a light yellow solid (470 mg, 50%). In addition, ~ 400 mg of the compound 1 was recovered. 1H NMR (400 MHz, CDCl3): δ 6.69 (s, 1H), 4.70-4.61 (m, , 2H), 1.79 (m, 16H), 1.73 (m, 8H), 1.70-1.59 (m, 30H); 13C NMR (125 MHz, CDCl3): δ 165.4, 141.3, 141.2, 140.9, 140.4, 140.3, 140.0, 139.1, 138.4, 137.8, 137.6, 134.4, 134.3, 130.8, 130.4, 122.9, 122.8, 84.3, 84.2, 84.1, 65.8, 63.2, 62.4, 61.8, 60.9, 60.7, 34.8, 34.7, 33.9, 33.2, 31.9, 30.9, 30.0, 29.5, 29.2, 28.4, 27.8, 27.4.

Synthesis of compound 3 To a solution of 2 (200 mg, 0.18 mmol) and DIPEA (94 μL, 0.54 mmol) in dry CH2Cl2 (5 mL) was slowly added mesyl chloride (30 μL, 0.36 mmol) on an ice bath. Then, the ice bath was removed and the reaction mixture was continuously stirred overnight under N2 atmosphere. The reaction mixture was then acidified with HCl (1 M) solution and additional 10 mL of CH2Cl2 and 10 mL of water were added. The organic layer was separated and the resulting aqueous solution was further extracted with CH2Cl2 (2×10 mL). The combined organic layer was washed with saturated NaCl solution (3×10 mL), dried over anhydrous Na2SO4 and evaporated in vacuo. The resulting residue was purified by column chromatography on silica gel eluting with petroleum ether/CH2Cl2 = 10:1 to 4:1 to afford compound 3 as a light yellow solid (181 mg, 90%). 1H NMR (400 MHz, CDCl3): δ 6.69 (s, 1H), 4.72-4.55 (m, 2H), 1.79 (m, 16H), 1.73 (d, J = 2.7 Hz, 8H), 1.69-1.59 (m, 30H); 13C NMR (125 MHz, CDCl3): δ 165.4, 141.4, 141.3, 140.9, 140.7, 140.3, 140.1, 139.2, 139.1, 138.2, 137.5, 134.3, 134.0, 130.9, 128.1, 122.9, 122.8, 84.3, 84.2, 84.1, 63.6, 62.7, 62.0, 60.9, 60.7, 60.6, 46.8, 34.8, 34.1, 33.0, 31.9, 31.3, 29.8, 29.7, 29.1, 28.4, 27.6, 27.5, 27.3, 26.9.

Synthesis of compound 4 To a solution of compound 3 (100 mg, 0.09 mmol) in dry DMF (2 mL) was added sodium methanethiosulfonate (24 mg, 0.18 mmol) at room temperature. The resulting reaction mixture was stirred overnight under N2 atmosphere. Then, 10 mL of CH2Cl2 and 20 mL of water were added. The organic layer was separated and the resulting aqueous solution was further extracted with CH2Cl2 (2×10 mL). The combined organic layer was washed with saturated NaCl solution (3×10 mL), dried over anhydrous Na2SO4 and evaporated in vacuo. The resulting residue was purified by column chromatography on silica gel eluting with petroleum ether/ CH2Cl2 = 8:1 to 2:1 to afford compound 4 as a light yellow solid (51 mg, 48%). 1H NMR (400 MHz, CDCl3): δ 6.71 (s, 1H), 4.47 (s, 2H), 3.35 (s, 3H), 1.83-1.75 (m, 16H), 1.75-1.69 (m, 8H), 1.69-1.58 (m, 30H); 13C NMR (125 MHz, CDCl3): δ 165.4, 141.5, 141.4, 141.3, 141.1, 141.0, 140.4, 140.2, 138.9, 138.8, 137.9, 137.3, 134.3, 133.9, 130.7, 125.1, 123.0, 122.9, 84.4, 84.3, 84.1, 64.1, 62.9, 62.3, 60.8, 60.7, 50.9, 42.1, 34.9, 34.8, 34.3, 32.8, 32.1, 31.6, 29.9, 29.3, 28.9, 28.4, 27.5, 27.3, 27.2, 26.9.

Synthesis of MTST Compound 4 (80 mg, 0.06 mmol) was treated with BF3-Et2O (71 μL, 0.48 mmol) in dry CH2Cl2 (2 mL) for 3 h under N2 atmosphere. Then SnCl2 (27 mg, 0.12 mmol) in dry THF (1 mL) was slowly added to the solution and the reaction mixture was continuously stirred for 10 min. Then, aqueous solution of NaH2PO4 (2 mL, 1 M) was added. The resulting aqueous solution was extracted with CH2Cl2 (3×10 mL). The combined organic layer was dried over anhydrous Na2SO4 and evaporated in vacuo. Then the residue was dissolved in phosphate buffer (10 mL, 0.2 M, pH 7.4) and purified by reversed phase C-18 column chromatography using water followed by 0-10% methanol in water as eluents to give the TAM radical MTST as a deep green solid (36 mg, 50%). Purity: 96% by HPLC (see the Supporting Information). HRMS (ESI, [C41H43O6S14]

•-, [M-H]-, m/z): 1079.8896 (measured) and 1079.9228 (calculated) (see the Supporting Information).

EPR experiments and spectral simulation EPR measurements were carried out on Bruker EMX-plus X-band spectrometer at room temperature. General instrumental settings were as follows: modulation frequency, 30-100 kHz; microwave power, 0.05-1mW; modulation amplitude, 0.03-0.08 G. Measurements were performed in 50 μL capillary tubes. In addition, EPR measurements under anaerobic conditions were carried out using a gas-permeable Teflon tube (i.d. = 0.8 mm). Briefly, the experimental solution was transferred to the tube which was then sealed at both ends. The sealed sample was placed inside a quartz EPR tube with open ends. Argon gas was bleeded into the EPR tube and then EPR spectrum was recorded after 5-min equilibrium. EPR spectral simulation was conducted by the home-made EPR simulation program (ROKI\EPR).[19a] For EPR spectrum containing

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multiple species with chemical exchange phenomenon, the simulation was operated through the newly developed program named as “EPRMULTI”. The details were well described in Supporting Information.

High-performance liquid chromatography (HPLC) analysis Stock solutions of MTST, GSH and Cys in distilled water were freshly prepared. To plot the peak area of the thiol conjugate on HPLC chromatogram as a function of concentration, aqueous solutions of thiol (GSH or Cys) at various concentrations (10, 20, 50, 80 and 100 μM) were mixed with the solution of MTST (100 μM). After 2 min, the reaction mixture (20 μL) was injected into the HPLC system (Agilent 1100 spectrometer) on a C18 Tosoh Bioscience ODS-80Tm column (250×4.6 mm, 5 μm). The peak area of the thiol conjugate T-GSH or T-Cys on the HPLC chromatograms was plotted versus the concentration of thiol (μM). Each experiment was repeated three times. The standard curve of MTST was obtained by plotting the peak areas obtained from MTST as a function of its concentrations. The standard curves of MTST and two thiol conjugates were determined as follows: A = 14.47C - 6.55 (MTST, R2 = 0.997); A = 14.40C - 7.74 (T-GSH, R2 = 0.991); A = 14.44C - 10.97 (T-Cys, R2 = 0.993), where A is the peak area and C the concentration. Using these linear equations, the concentrations of MTST and two thiol conjugates in the samples can be easily determined by HPLC.

Measurement of GSH/Cys efflux from HepG2 cells by the thiol-trapping method HepG2 cells were cultured in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum (FBS) and 1% penicillin/streptomycin at 37 ºC in a humidified atmosphere of 5% CO2 and 95% air. HepG2 cells were used between passages 5 and 7. Stock solutions of BSO (10 mM) and PDTC (10 mM) was prepared in doubly distilled water and diluted 100-fold using fresh medium to initiate the treatment. Firstly, cells were incubated with BSO (200 µM) or PDTC (200 µM) for 1 h. Then, the culture medium was removed and the cells were immediately washed twice with cold phosphate buffered saline. Subsequently, the resulting cells were recultured in fresh medium containing 1% penicillin/streptomycin but without FBS at 37 ºC. To completely trap the thiols in the medium, excess of MTST has to be used. Thus, the classical DTNB method was firstly used to measure the total concentrations of the thiols at various time points.[30] The maximal total concentration of the thiols during the cell cultures was determined to be approximately 100 µM. Then, at different time points (0, 1, 3, 5, 8, 10 and 12 h), 40 µL of the culture medium was taken and mixed with the solution of MTST (10 µL, 1 mM) in PB. The resulting solution was transferred to a glass capillary tube and EPR or HPLC analyses were carried out under aerobic conditions. Control experiments were performed adding phosphate buffered saline alone. Total protein amounts in the cells were determined by the BCA protein assay according to the commercial kit instruction. Concentrations of the biothiols were determined by both EPR and HPLC methods and expressed as nmol biothiols per mg protein.

Acknowledgements

This work was partially supported by the National Natural Science Foundation of China (Nos. 21572161, 31500684, 31971174, 21871210 and 21603163), Tianjin Municipal 13th five-year plan (Tianjin Medical University Talent Project), The Open Project program of Hefei National Laboratory for Physical Sciences at the Microscale and Hungarian National Research, Development and Innovation Office (NKFTH) Grant Nr. K119442.

Keywords: Electron paramagnetic resonance (EPR) • trityl

radical • biothiol • thiol trapping • discriminative detection

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The concept of EPR-“thiol-trapping” which allows for simultaneous quantitation of multiple biothiols in a single analysis was proposed and verified. Using this novel technique, efflux of GSH and Cys from HepG2 cells was monitored. This work represents the first example for discriminative detection and quantitation of thiols by EPR.

Xiaoli Tan, Kaiyun Ji, Xing Wang, Ru Yao, Guifang Han, Frederick A. Villamena, Jay L. Zweier, Yuguang Song,* Antal Rockenbauer,* and Yangping Liu*

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Discriminative Detection of Biothiols through Thiol-Trapping Technique using Methanethiosulfonate Trityl Probe and Electron Paramagnetic Resonance Spectroscopy

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