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. 2025 Apr 1;147(15):12627–12634. doi: 10.1021/jacs.5c00196

Serendipitous Discovery of Photolytic Thiosulfoxide Formation: Application for Visible-Light-Inducible Manipulation of Supersulfide Level in Biological Systems

Mitsuyasu Kawaguchi †,*, Katsutoshi Yoshino ‡, Tomoaki Ida §, Hibiki Moriyama †, Naoya Ieda †, Yuhei Ohta †, Shingo Kasamatsu ∥, Hideshi Ihara ∥, Hidehiko Nakagawa †,*
PMCID: PMC12007003  PMID: 40169142

Abstract

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Tools to enable spatiotemporally controlled upregulation of supersulfides, which are highly reactive, unstable sulfur species, are needed to study the pathophysiological roles of post-translational protein modification with catenated sulfur atoms. Here, we set out to design N,N-diethylaminocoumarin (DEAC)-based visible-light-responsive N-acetylcysteine persulfide donors (NAC-SS-DEAC), and serendipitously found that upon visible light irradiation, they donate a sulfane sulfur (S0) atom to nucleophiles, including thiols and cyanate. Light-assisted tautomerization of the disulfide moiety of NAC-SS-DEAC to transiently afford unstable thiosulfoxide plays a key role in the S0 donation. We show that this reaction can be utilized to achieve visible-light-inducible manipulation of supersulfide levels in living cells.

Introduction

Some thiol species, such as cysteine residues in proteins and glutathione in living organisms, are known to exist partly as hydropersulfides (RSSH), which can undergo S-sulfhydrylation by donating a sulfane sulfur (S0; a zerovalent sulfur atom).1−4 Cysteine persulfide (CysSSH) was thought to be produced via enzymatic reactions mediated by cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE),5 but it has recently been shown that S-sulfhydration can occur cotranslationally during protein synthesis, catalyzed by cysteinyl-tRNA synthetase (CARS2).6 It has also been established that the conformations and/or enzymatic activities of several proteins, including CaMKII, Keap1 and GAPDH, are regulated through S-sulfhydrylation.7−9 Because of their highly nucleophilic and electrophilic properties,10 RSSH and other supersulfides, including H2S, H2Sn (n > 2), and RSSnSR (n > 1), are highly reactive with reactive oxygen species (ROS),1,11,12 and thus serve as antioxidants.13,14 However, due to the unstable nature of supersulfides, it is difficult to study their biosynthetic pathways and physiological significance. To address these issues, various supersulfide donor compounds have been developed, which can be classified as spontaneous-release compounds or compounds responsive to stimuli,15,16 including enzymes,17,18 ROS,19,20 and light..21−23 Recently, Chakrapani and co-workers developed mitochondria-targeting 3-MST substrates and used them to regulate supersulfide levels in mitochondria.24 For our purpose, we focused on the irradiation-responsive CysSSH donor reported by Singh and Toscano,21−23 because such donors, which can release supersulfides under spatiotemporal control, are the most promising tools for elucidating sulfur biology. However, the use of ultraviolet (UV) light to release supersulfides is problematic when considering applications to cellular systems due to its phototoxicity.25 In this work, therefore, we designed an N,N-diethylaminocoumarin (DEAC)-based visible-light-responsive N-acetylcysteine persulfide (NAC-SSH) donor (NAC-SS-DEAC). Unexpectedly, however, this compound did not release NAC-SSH but instead generated interesting desulfurized photoproducts, which were identified by liquid chromatography-mass spectrometry (LC-MS) analysis. Detailed investigations revealed that NAC-SS-DEAC irradiation-dependently forms “thiosulfoxide” in aqueous solution and donates one sulfur atom (S0) to nucleophiles, including cyanate and biological thiols, such as GSH and GAPDH. We propose a mechanism of the photoreaction and describe its application for manipulating supersulfide levels in living cells by means of visible light irradiation.

Results and Discussion

Unexpected Photoreaction of NAC-SS-DEAC on Irradiation with Visible Light

To develop visible-light-controllable CysSSH donors, we focused on N,N-diethylaminocoumarin (DEAC), which has been widely used as a caging group to enable for spatiotemporally controlled delivery of various molecules, including inhibitors and gaseous molecules.26−28 It is generally believed that the pKa of the released molecule is important for efficient uncaging from DEAC-type donors,29 and since the theoretical pKa of CysSSH is calculated to be 4.3,30 we anticipated that CysSSH would be efficiently released. Accordingly, we designed and synthesized NAC-SS-DEAC and GSS-DEAC, without linker structures, to directly release N-acetylcysteine persulfide (NAC-SSH) and glutathione persulfide (GSSH), respectively, in response to visible-light irradiation (Schemes S1–S3).22

We first investigated their photophysical properties and found that both donors have an absorption maximum at around 400 nm (Figure S1 and Table S3), suggesting that light-emitting diode (LED) irradiation at 405 nm would be effective for photolysis. We next analyzed the photoproducts generated by photolysis of NAC-SS-DEAC in the presence of HPE-IAM, which can trap RSSH to form relatively stable alkylated adducts, such as NAC-SS-HPE-AM.31 If photolysis occurs as we expected, DEAC–OH and NAC-SS-HPE-AM should be formed through heterolytic cleavage of the C–S bond (Figure 1A). However, LC-MS analysis showed that little of either compound was formed and the unexpected photoproducts P1 and P2 were predominantly produced (Figures 1B,C and S2A). Time-course analysis of the photoproducts revealed that after photodegradation of NAC-SS-DEAC, P1 was first produced, followed by P2 (Figure 1D). Interestingly, since the mass numbers of both compounds were found to be 32 smaller than that of NAC-SS-DEAC, elimination of a sulfur atom seemed to have occurred during photolysis (Figure 1E–G). Finally, product analyses using HPLC, NMR and MS indicated that P1 is NAC-S-DEAC (Figures 1F, S2B and Scheme S4), which has a sulfide moiety instead of an original disulfide moiety, while P2 is the 3-position NAC-S-regioisomer of NAC-S-DEAC (Figures 1G, S3 and Scheme S5). We further confirmed that the photolysis of purified NAC-S-DEAC gave P2 by means of NMR and MS studies (Scheme S6). Furthermore, LC-MS analyses demonstrated that a similar photoirradiation-dependent desulfurization reaction proceeded in the case of GSS-DEAC (Figure S4), suggesting that these unexpected photoreactions may be a general phenomenon in DEAC-disulfide (DEAC-SS) motif-based compounds.

Figure 1.

Figure 1

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(A) Putative photodegradation scheme of NAC-SS-DEAC upon photoirradiation. Exact mass numbers of each compound are shown. (B) HPLC charts (absorbance at 405 nm) of NAC-SS-DEAC (100 μM) and HPE-IAM (1 mM) upon photoirradiation (405 nm LED, 50 mW/cm2) at the indicated times. (C) Single ion chromatograms (SIC) at 373 (m/z) of NAC-SS-DEAC upon photoirradiation at the indicated times together with the SIC of authentic NAC-SS-HPE-AM (50 μM). (D) Time-dependent change of the peak areas at 405 nm of NAC-SS-DEAC and photoproducts. Mass charts of (E) NAC-SS-DEAC (tR = 14.3 min), (F) P1 (tR = 13.4 min) and (G) P2 (tR = 13.8 min) and predicted structures of the photoproducts.

Fluorometric Analysis of the Sulfur Atom Released from NAC-SS-DEAC

Since the photolysis of NAC-SS-DEAC gave desulfurized products, we set out to identify how and in what form the desulfurized sulfur was transferred. We considered the following three possibilities; (i) hydrogen sulfide (H2S), (ii) sulfane sulfur (S0) and (iii) octasulfur (S8). We first checked H2S production by using a reported fluorescence probe, HSip-1,32 and found that H2S was not the dominant sulfur form: production of H2S was less than 1 μM from 100 μM NAC-SS-DEAC (i.e., less than 1%, Figures 2A and S5). We next examined S0 formation by using the S0-specific fluorescence probe, SSP4 and found that SSP4 fluorescence was significantly increased upon photoirradiation (Figure 2B),33 indicating that NAC-SS-DEAC donates S0 to the thiol of SSP4 upon visible-light irradiation, resulting in the generation of fluorescein. We excluded the possibility that NAC-SS-DEAC releases octasulfur on the basis of its poor water solubility (less than 6.1 nM in neutral water).34

Figure 2.

Figure 2

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(A) H2S detection using HSip-1 upon photoirradiation of NAC-SS-DEAC (50 mW/cm2, 10 min). (B) S0 detection using SSP4 with NAC-SS-DEAC and its analogues upon photoirradiation (100 mW/cm2, 10 min). (C) Thiocyanate detection of NAC-SS-DEAC or cysteine with NaCN and FeCl3 upon photoirradiation (50 mW/cm2, 10 min). Inset: calculated [−SCN] of each compound. N.D. = not detected. (D) Formation of hydropersulfide adducts (X-HPE-AM) with NAC-SS-DEAC, HPE-IAM and NAC or GSH upon photoirradiation (50 mW/cm2, 0, 0.5, 1, 3, 6 min). [X-HPE-AM] was calculated from a calibration curve of NAC-SS-HPE-AM and GSS-HPE-AM (Figures S8 and S9). (E) LC-MS analysis of human GAPDH S-sulfhydrylation at the Cys247 residue. The relative amounts of Cys247 forms, including Cys-SH, Cys-SSH and Cys-SSSH, were normalized by the amount of a reference peptide under each condition. The results are mean ± SD (n = 3). *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001, n.s. = not significant.

Confirmation of Visible-Light-Mediated Sulfane Sulfur (S0) Release from NAC-SS-DEAC

Recently, Dick’s group have shown that protein-SSH labeled by iodoacetamide (IAM), i.e., protein-SS-AM, spontaneously decomposes into protein-S-AM (Scheme 1A), meaning that light-independent desulfurization can occur.35 Importantly, their work suggested that protein-SS-AM spontaneously formed “thiosulfoxide” and its sulfur atom is easily trapped by nucleophiles due to the very weak nature of the thiosulfoxide bond.36,37 Based on these findings, we hypothesized that NAC-SS-DEAC photolysis proceeded similarly, i.e., we considered that NAC-SS-DEAC might photoirradiation-dependently generate “thiosulfoxide” (Scheme 1B), because NAC-SS-DEAC does not generate S0 in the dark (Figure 2B). To address this hypothesis, we investigated thiosulfoxide formation by trapping with (i) H2O in the presence of monobromobimane (mBB),38 (ii) cyanide,35 and (iii) biological thiols, such as NAC, glutathione (GSH) and human GAPDH. In the trapping assay of sulfenyl/sulfinyl with mBB, we could not directly detect the sulfenyl-mBB adduct, i.e., Bimane-SOH (Figure S6A), probably because of its highly unstable nature.38 However, sulfide and disulfide adducts with mBB were detected (Figure S6B,C), suggesting that the first formed sulfenyl/sulfinyl is degraded and converted to these products, but not via H2S formation (Figure 2A). Second, the result of absorption-based thiocyanate (−SCN) detection assay showed thiocyanate formation upon photoirradiation (Figure 2C), with 617 μM of thiocyanate formed from 1 mM NAC-SS-DEAC (Figure S7). Interestingly, cystine, which also has disulfide bond like NAC-SS-DAEC, did not generate thiocyanate even upon irradiation, suggesting that a disulfide motif is not sufficient for donating S0, and showing that NAC-SS-DEAC has a light-dependent rhodanese activity. To further support our hypothesis, we photoirradiated NAC-SS-DEAC in the presence of NAC or GSH, and found that HPE-IAM adducts of NAC-SSH (i.e., NAC-SS-HPE-AM) or GSSH (i.e., GSS-HPE-AM), respectively, were time-dependently generated, as determined by LC-MS analyses (Figures 2D, S8–S10, and Scheme S7), with 30–40 μM RSSH produced from 100 μM NAC-SS-DEAC. In addition, LC-MS product analyses of the photoreaction of GSH (20, 100, 500, and 1000 μM) with NAC-SS-DEAC (100 μM) for 5 min, followed by treatment with TME-IAM (1 mM),39 which is a more stable alkylating reagent than HPE-IAM, showed the photoirradiation-dependent formation of a series of supersulfides, including GSSH, GSSSH, GSSSG and GSSSSG (Figure S11). In this experiment, oxidation of GSH, i.e., GSSG generation, was observed, suggesting that photooxidation proceeded in the presence of coumarin dye, which is known to work as a photosensitizer.40,41 We speculated that this result was due to prolonged light exposure in the absence of TME-IAM. We last examined whether protein thiol can trap S0 from photoexcited NAC-SS-DEAC to induce S-sulfhydrylation. We used human glyceraldehyde-3-phosphate dehydrogenase (hGAPDH), which has three thiol groups (Cys152, Cys156, Cys247), as a model protein, and photoirradiated hGAPDH (∼4.4 μM) with NAC-SS-DEAC (100 μM) for 5 min, followed by treatment with TME-IAM (1 mM). In LC-MS/MS analyses after trypsin digestion of the protein samples, it was found that S-sulfhydrylation at Cys247 (i.e., formation of Cys-SSH and Cys-SSSH) was significantly increased in the “NAC-SS-DEAC with photoirradiation” condition compared with the other conditions (Figure 2E). These results strongly support our hypothesis that NAC-SS-DEAC can photoirradiation-dependently form thiosulfoxide and donate S0 to various nucleophiles, including protein thiols. However, in this experiment, photooxidation reaction of some peptides containing cysteine and methionine residues was observed in the presence of NAC-SS-DEAC, so we normalized the amount of the target peptide (VPTANVSVVDLTCR) by using a methionine-containing peptide (VVDLMAHMASK) as a reference peptide (Figures 2E, S12 and S13). LC-MS analyses showed that various oxidized forms of each amino acid were detected in peptides containing cysteine and methionine residues, including sulfinic acid, sulfonic acid, thiosulfonic acid, sulfoxide and sulfone (Figures S14–S17), and the generation of oxidized products led to a corresponding decrease of the native peptides (Figure S12). These results strongly justify the choice of normalizing the amount of the target peptide with methionine-containing peptides (Figures 2E and S13). The formation of sulfur oxides in peptides during photoirradiation is considered to be a limitation of coumarin-based caging groups.

Scheme 1. (A) Dick’s Work Showing Spontaneous Thiosulfoxide Formation in Protein-SH.35 (B) Our Hypothesis: NAC-SS-DEAC Can Photoirradiation-Dependently Form Thiosulfoxide and Transfer S0 to Various Nucleophiles.

Scheme 1

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Sulfane Sulfur Release from NAC-SS-DEAC Analogues and Mechanism of the Photoreaction

To investigate the structure-efficiency relationship for S0 release, we designed and evaluated NAC-SS-DEAC analogues, as shown in Figure 3A (Schemes S8–S10). Dick’s report suggested that a β-carbonyl group, as in iodoacetamide, promotes thiosulfoxide formation, while a bulky N-alkylacetamide, such as N-tert-butylacetamide, suppresses S0 release.35 However, it remained unclear to what extent each moiety affected thiosulfoxide formation and/or S0 release by nucleophilic attack on thiosulfoxide. To clarify these points and gain insight into the mechanism of our photoreaction, we first measured the photophysical properties of the analogues and found that they showed similar spectra to NAC-SS-DEAC (Figure S18 and Table S3). Photodegradation experiments showed that most of the analogues were time-dependently degraded to afford two desulfurized products (Figures S19–S22), whose MS numbers were again 32 smaller than those of the donors, suggesting that all the analogues underwent similar photoreactions to NAC-SS-DEAC. In contrast, the degradation rate of each donor was slightly affected by β-carbonyl and/or bulky N-substituents (Figure 3B). In addition, it was found that all the donors released S0 from SSP4, though the amount of S0 generated was slightly affected by a bulky N-substituent (Figure 2B), as in tBuAA-SS-DEAC and NAC-SS-DEAC-Mito. To further examine the photolysis mechanism and kinetics, we first varied the light-intensity exposure of NAC-SS-DEAC in the presence of excess GSH (1 mM) and found that the photodegradation rate was light-intensity dependent (Figure 3C), suggesting that the thiosulfoxide formation is rate-limiting. We next performed photolysis experiments with different concentrations of GSH (0, 0.5, or 1 mM) (Figure 3D,E), and found that the photodegradation rate of AA-SS-DEAC was GSH concentration-dependent, whereas that of tBuAA-SS-DEAC was not, suggesting that the rate-limiting steps are different between AA-SS-DEAC and tBuAA-SS-DEAC. In these reaction mechanism analysis experiments using GSH (Figure 3C–E), we used HPLC to evaluate the peak areas of residual each donor at each irradiation time.

Figure 3.

Figure 3

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(A) Structures of NAC-SS-DEAC analogues. (B) Time-dependent degradation of NAC-SS-DEAC and its analogues upon photoirradiation (405 nm LED, 50 mW/cm2). (C) Light-intensity dependency (10 or 50 mW/cm2) for photodegradation of NAC-SS-DEAC (100 μM). [GSH]-dependency (0–1 mM) for photodegradation of (D) AA-SS-DEAC (100 μM) and (E) tBuAA-SS-DEAC (100 μM) upon photoirradiation (50 mW/cm2). The results are mean ± SD (n = 3).

Based on these observations, we propose the photoreaction mechanism shown in Scheme 2 and Figure S23. Thiosulfoxide formation is an equilibrium reaction and could occur spontaneously, and a β-carbonyl group critically promotes thiosulfoxide formation.35,36 However, our experimental results do not allow us to conclude definitively whether or not the β-carbonyl group has a role in promoting the thiosulfoxide formation in our photoreaction, because the irradiation energy can easily overcome the high energy barrier required for unstable thiosulfoxide generation. Referring to the reaction mechanism of the desulfurization of thiosulfoxide by triphenylphosphine in the Crick ligation reaction, we speculate that the sulfur adduct of the nucleophile is formed by the attack of the nucleophile on one of the sulfur atom of thiosulfoxide (S–S single bond is calculated to be a favored form between two resonance structures), which has a property as S0, via SN2 reaction mechanism.42 The unexpected attack of nucleophiles on thiosulfoxide is expected to be driven by the decomposition of unstable thiosulfoxide and disulfide, and the formation of more stable sulfide. As observed in the case of NAC-SS-DEAC, in general, light-dependent thiosulfoxide formation is a rate-limiting step and the generated thiosulfoxide reacts with nucleophiles as soon as it is formed. As for the slower reaction rate observed for AA-SS-DEAC and tBu-AA-SS-DEAC, compared with NAC-SS-DEAC (Figure 3B), we consider that the nucleophilic attack of nucleophiles is partially inhibited by the presence of anionic carboxylate through electrostatic repulsion in AA-SS-DEAC. In tBuAA-SS-DEAC, in contrast, the bulky N-substituent markedly inhibits the tautomerization step and partially inhibits the nucleophilic attack step through steric hindrance, leading to the differences of rate-limiting step and degradation rate between AA-SS-DEAC and tBuAA-SS-DEAC (Figure S23). Importantly, these results provide a clear answer to above question: the bulky N-substituent mainly suppresses the tautomerization step. This conclusion is also supported by our finding that in the case of tBuAA-SS-DEAC the total amount of S0 produced is almost unchanged, even though the photolysis reaction is slower than that of NAC-SS-DEAC (Figure 2B). We speculate that the difference of rate-limiting step arising from slight structural differences at the γ and δ positions of the disulfide bond means that the two reaction processes, i.e., tautomerization and nucleophilic attack, have comparable reaction rates and are too fast to permit the direct detection of thiosulfoxide formation by our analytical techniques. In other words, thiosulfoxide has an extremely short lifetime.37,42

Scheme 2. Proposed Photolysis Mechanism of NAC-SS-DEAC to Release Sulfane Sulfur and to Co-Generate NAC-S-DEAC and P2.

Scheme 2

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In Cellulo Supersulfide Manipulation with NAC-SS-DEAC

We next examined whether NAC-SS-DEAC could be used to manipulate intracellular supersulfide levels through GSH persulfidation, because GSH is a dominant biological thiol.43 We first evaluated the cytotoxicity of NAC-SS-DEAC and NAC-SS-DEAC-Mito, and found that they were not cytotoxic at concentrations up to 100 μM for 24 h incubation (Figure S24), though they showed weak cytotoxicity at >25 μM for 72 h exposure. Given that pretreatment of S0 donors for cellular delivery required 1 h incubation and that the photoproducts were less toxic (Figure S25), NAC-SS-DEAC’s cytotoxicity should be negligible in the present context. We next examined cell membrane permeability and localization in cells, and found that NAC-SS-DEAC and its analogues could permeate the cell membrane and were distributed throughout the cell (Figure S26), though NAC-SS-DEAC-Mito bearing a mitochondria-targeting motif showed only slight mitochondrial accumulation (Table S4).44 To clarify the reason for this, we incubated NAC-SS-DEAC-Mito with GSH (0–10 mM) for 24 h and found that NAC-SS-DEAC-Mito was converted to GSS-DEAC through a disulfide exchange reaction (Figure S27). However, we expected that NAC-SS-DEAC-Mito would still work as an S0 donor in cells because of the S0-releasing ability of GSS-DEAC.

We last applied NAC-SS-DEAC to HEK293T cells and photoirradiated them. Examination of the intracellular supersulfide level with SSP4 under a fluorescence microscope33 revealed intracellular fluorescence enhancement under the “NAC-SS-DEAC with photoirradiation” condition, while no significant fluorescence was observed under other conditions (Figure 4A). A similar result was observed in HeLa cells (Figure S28). Next, we investigated whether NAC-SS-DEAC could irradiation-dependently stimulate nitric oxide (NO) production through intracellular supersulfide upregulation, since Fukuto and co-workers have recently shown that RSSH react with S-nitrosothiols, such as S-nitrosoglutathione (GSNO), and release NO.45 Using SNAP as an NO donor, DAR-4M-AM as an NO fluorescence probe and NAC-SS-DEAC, we measured the production of NO under photoirradiation with a fluorescence microscope. NO production was observed under the “NAC-SS-DEAC with photoirradiation” condition, while negligible fluorescence was observed under other conditions (Figure 4B). These in cellulo results clearly demonstrated that NAC-SS-DEAC can be used to manipulate supersulfide levels in complex biological systems in response to visible-light irradiation.

Figure 4.

Figure 4

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(A) Intracellular supersulfide imaging in HEK293T cells. The cells were treated with NAC-SS-DEAC (10 μM) for 1 h, then washed and photoirradiated (50 mW/cm2, 10 min). The cells were treated with SSP4 (10 μM) in serum-free DMEM containing CTAB (100 μM) for 15 min, then confocal fluorescence images were acquired. (B) Intracellular nitric oxide (NO) imaging in HeLa cells. The cells were treated with SNAP (250 μM) for 30 min, then washed and treated with NAC-SS-DEAC (100 μM) and DAR-4M-AM (10 μM) for 1 h. After washing with DPBS, the cells were photoirradiated (50 mW/cm2, 10 min) and further incubated for 30 min, then confocal fluorescence images were acquired. The fluorescence intensities of cells treated with DMSO or NAC-SS-DEAC with or without photoirradiation were compared. The results are mean ± SD (indicated cell number).

Conclusions

We have developed NAC-SS-DEAC and its analogues as visible-light-controllable S0 donors, which can photoirradiation-dependently manipulate the levels of supersulfides and NO in living cells. Photoirradiation-dependent thiosulfoxide formation appears to play a key role in the S0 donation.35−37 The disulfide to thiosulfoxide tautomerization of the DEAC-SS motif leading to S0 release is highly tolerant to structural expansion, i.e., addition of NAC or GSH structures in NAC-SS-DEAC or GSS-DEAC does not affect the S0-donating ability. This strategy is expected to provide new tools for investigating supersulfide biology via site-specific S-sulfhydrylation in target proteins and organelles,24 because reagents such as Na2S2, currently the most widely used reagent in sulfur research, induce exhaustive S-sulfhydration of various proteins. This feature of our S0 donor is expected to greatly contribute to a detailed understanding of the physiological significance of S-sulfhydrylation of specific proteins in complexed biological systems.

Acknowledgments

The authors thank all the members of H.N.’s laboratory for fruitful discussions. This work was supported in part by JSPS KAKENHI Grant Number JP 21H00290, 22K06505 (M.K.) and 19H03354, 19KK0197, 21H05259, 23H02612 (H.N.), as well as by the Hori Sciences and Arts Foundation, and Mochida Memorial Foundation for Medical and Pharmaceutical Research (M.K.). The authors are grateful for the assistance of the Research Equipment Sharing Center at the Nagoya City University.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.5c00196.

  • Experimental procedures; MRM parameters; mass chart of DEAC-OH ; HPLC charts; synthetic procedures; and characterization data and spectra (PDF)

Author Present Address

# Graduate School of Pharmaceutical Sciences, Hokkaido University, N12 W6, Kita-ku, Sapporo, 060-0812, Japan

The authors declare no competing financial interest.

Supplementary Material

ja5c00196_si_001.pdf (5.3MB, pdf)

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