On 2015 Nov 04, Milos Filipovic commented:

Dirty dancing of NO/RSNO and H2S

In this report by Cortese-Krott et al, 2015, the existence of SSNO− as a product of the anaerobic and aerobic reaction of H2S with NO or RSNOs, was claimed based on MS experiments. The authors failed to prove HSNO using a demo MS instrument, although HSNO was prepared by the acidification of nitrite, pulse radiolysis and trans-nitrosation and characterized by MS, IR (14N/15N labelling) and 15N-NMR (Filipovic at al, 2012), and even reported by Cortese-Krott et al, 2014. HSNO has also been recently detected by 15N NMR as a product of the reaction of PNP+SSNO- with sulphide (Wedmann et al, 2015), again going against the claim of Cortese-Krott et al, 2015 that “SSNO- mix” is stable in excess of sulphide.

The authors use LTQ OrbiTrap to concentrate reactive ions (as demonstrated by the absence of any MS signal in the first 1.5 min of continuous injection of the reaction mixture) all of which can intercombine in the ion trap (Figure 3C). They never show the control spectra, actual intensities of the signals, nor do they show the spectrum with broad m/z. It is puzzling that the signal of the reactant, SNAP, reaches its maximum almost at the same time as SSNO− and all other reaction products. While SNAP peak slowly decays in agreement with Uv/Vis experiments, the reaction products remain at maximum intensity (although most of them have questionable S/N ratio, Figure 3C). Contrary, HS2O3− starts to disappear, suggesting that it was present even before reaction started.

The authors refer to the formation of persulfide NONOate ([ONN(OH)S2]−). This species should have m/z 124.9479. While the Figure S6A in indeed has the title: “High resolution mass spectrum of [SS(NO)NOH] −“, the actual figure shows the spectrum of the species with m/z 141.9579, which authors assign to H4O2N3S2−. Strangely, in the same reaction mixture, identical mass peak is assigned to another species, HO5(14)N(15)NS−, (Figure 3B, right panel). 3 peaks are present in m/z ~143 in Figure S6A in. As this is the only MS spectrum shown in high resolution, one can calculate the mass of those unassigned peaks and observe that none of them show up in Figure 3B which is recorded under the same conditions. The isotopic pattern of SULFI/NO (Figure 3B (left panel)) is inconsistent with what should be expected for this species. In the Figure 3A (right panel) there is a huge unassigned background peak at m/z ~94.9252, which does not appear in the Figure 3A (left panel). It is unclear whether the reported peaks are smaller or bigger than actual background noise of the instrument.

In the unexplainable absence of 15N NMR and IR characterisation of SSNO−, which by authors’ claim is “stable” and “abundant”, and correct isotopic patterns for reported species, the authors should have performed experiments with pure 14N and 15N labelled SNAP to independently demonstrate the isotopic distribution of each species and their corresponding m/z shifts.

The authors use maXis Impact instrument to show that they cannot detect HSNO in the reaction of RSNO and H2S. The injection of buffer alone creates the signal intensities of ~1.5x107 (the upper detection limit of this instrument) that makes the background noise stronger than the actual signal of few milimolar GSNO (Figure S8C). The authors also send a message that due to the presence of DMSO and acetonitrile in the tubing no one should try to detect anything at m/z range 50-70. The authors could have cleaned the instrument instead, used new tubing for every measurement or used stainless steel tubing to solve this problem as it is done in the laboratories with MS experience. Furthermore, the ionization conditions which “break” DSMO (BDE ~ 50 kcal/mol, Blank et al, 1997) into CH3SO/CH3SO+ are inappropriate for RSNO/HSNO detection (BDE ~30 kcal/mol, Wedmann et al, 2015). Results look like as they were produced in a limited amount of time and on a very dirty demo instrument and should not have been used for publication.

To prove that 412 nm species is NO dependent the authors trap NO by cPTIO (Figure S4F), ignoring the fact that nitronyl nitroxides readily reacts with H2S and therefore no conclusion can be drawn from this experiment (Wedmann et al, 2013).

The authors also use water soluble triphenylphosphine (TXPTS) to trap nitroxyl from their “SSNO- mix” ignoring the fact that triphenylphosphines are good trapping agents for sulfane sulphur (by mixing PNP+SSNO- with triphenylphosphine Seel et al, formed SNO-), and that S-nitrosothiols react/decompose in the presence of triphenylphosphines in general and TXPTS in particular (Bechtold et al, 2010), so nothing can be concluded from those experiments either.

In conclusion, the data presented in this study ask for more critical and in-depth re-evaluation.

Cortese-Krott MM, et al. (2015) Key bioactive reaction products of the NO/H2S interaction are S/N-hybrid species, polysulfides, and nitroxyl. Proc Natl Acad Sci USA 112(34):E4651-60.

Filipovic MR, et al. (2012) Chemical characterization of the smallest S-nitrosothiol, HSNO; cellular cross-talk of H2S and S-nitrosothiols. J Am Chem Soc 134(29): 12016-27.

Cortese-Krott MM, et al. (2014) Nitrosopersulfide (SSNO(-)) accounts for sustained NO bioactivity of S-nitrosothiols following reaction with sulfide. Redox Biol 2:234-44.

Blank DA, North SW, Stranges D, Suits AG, Lee YT (1997) Unraveling the dissociation of dimethyl sulfoxide following absorption at 193 nm. J Chem Phys 106(2):539-550.

Wedmann R, et al. (2015) Does Perthionitrite (SSNO(-)) Account for Sustained Bioactivity of NO? A (Bio)chemical Characterization. Inorg Chem 54(19):9367-9380.

Wedmann R, et al. (2013) Working with “H2S”: facts and apparent artefacts. Nitric Oxide 41:85-96. Seel F, et al. (1985) PNP-Perthionitrit und PNP-Monothionitrit. Z Naturforsch 40b:1607–1617.

Bechtold E, et al. (2010) Water-soluble triarylphosphines as biomarkers for protein S-nitrosation. ACS Chemical Biology 5(4):405-414.

On 2015 Nov 04, Ivana Ivanovic-Burmazovic commented:

No evidence for SSNO- in “SSNO- mix”

The results and conclusions published by Cortese-Krott et al. 2015 (Proc Natl Acad Sci USA 2015, 112(34):E4651-60) in the manuscript entitled “Key bioactive reaction products of the NO/H2S interaction are S/N-hybrid species, polysulfides, and nitroxyl” urgently call for comments from the chemical point of view, because they include a number of chemical misconceptions.

The main conclusion of this work, as stated in its title, is that SSNO- (according to the authors a “S/N-hybrid species”) is one of three key bioactive reaction products of the reaction of H2S with NO or S-nitrosothiols (RSNOs). Based on authors’ claims, SSNO- can be obtained in high yields in aqueous solutions at pH 7.4 (even in the presence of oxygen) and it is stable for hours. However, everything known about the chemical/spectroscopic properties of SSNO- contradicts authors’ conclusions. We are afraid that the biological community will be confused by these results, and that researchers without inorganic chemistry background will use such undefined reaction mixtures of NO and H2S, as well as of RSNO and H2S, as a source of SSNO- that in reality is not present under given conditions at all. To help clarifying confusion in the field we bring important facts about SSNO- chemistry below.

In general, a history of the identification of S-S bond-containing compounds in solutions was rich in contradicting conclusions (Seel F, et al. 1977), and “some of those who dared to tackle this challenging task were victims of delusions because such species that were optically (or even by other methods) observed in non-aqueous solutions could not easily be established as defined substances” (Seel F and Wagner M, 1985). This is a translated quotation from Seel F, Wagner M 1985, who first synthesized SSNO- under exclusion of water and dioxygen (Seel F, et al. 1985). However, although citing work of Seel and Wagner, Cortese-Krott et al, 2015 do not mention that i) SSNO- solutions are sensitive to oxygen and water (Seel F and Wagner M 1985) and ii) even in very alkaline aqueous solutions only 10 % of SSNO- was obtained from NO and S2-, although even that was questionable, as SSNO- could not be confirmed by 15N-NMR in aqueous solutions (Seel F and Wagner M, 1988). This contradicts that SSNO- is stable for hours at pH = 7.4, especially in high concentrations in “SSNO--enriched mixtures” (“SSNO- mix”) (Cortese-Krott et al, 2015). Related to the claimed determination of the high SSNO- yield in “SSNO- mix”, in SI (page 9) Cortese-Krott et al, 2015, state: “The (theoretical) maximal yield of SSNO- under these conditions is 1 mM, corresponding to the concentration of added nitrosothiol (please refer to Fig. S9 for experimental determination of reaction yield).” However, Fig. S9 deals with MS of dimethylsulfoxide, and nowhere in SI the clamed experimental results confirming a high SSNO- yield could be found. Instead there is some confusing statement that their putative SSNO- contains two sulfur atoms based on an observation of “two times as much sulfide as sulfane sulfur” (Cortese-Krott MM, et al. 2015). (Even more general, since authors do not provide stoichiometry of the considered reactions, they cannot provide any quantification.)

In agreement with the original work of Seel and Wagner, we demonstrated SSNO- inherent instability by preparing pure crystals of PNP+SSNO- and characterizing its properties by 15N-NMR, IR, EPR, MS, X-ray analysis, electrochemical and computational methods (Wedmann R, et al. 2015). For example, when ca. 10% water was added to an acetone solution of a pure SSNO- salt (Wedmann R, et al. 2015), it decomposed within ca. 100 s. Cortese-Krott et al. report that SSNO- does not react with thiols, H2S and cyanide (Cortese-Krott MM, et al. 2015). However, solutions of a pure SSNO- salt, which Cortese-Krott et al. never used, quickly decompose in the presence of thiols, H2S (Wedmann R, et al. 2015) and cyanide. These authors state that SSNO- is resistant to the biological reductants (Cortese-Krott MM, et al. 2015). However, SSNO- is reduced at a physiological potential of −0.2 V vs. NHE (Wedmann R, et al. 2015). Being unstable at pH = 7.4, in the presence of thiols and biological reducing agents, SSNO- cannot exist under physiological conditions in any relevant concentration. They also report that HSSNO is more stable than HSNO, because HSSNO supposedly has increased electron density on the proximal sulfur (which is a statement for which they do not provide any experimental support) and therefore does not easily react with HS- and positive metal centers (which is contradictio in adjecto) (Cortese-Krott MM, et al. 2015). The facts are quite different: i) the proximal-S has a +0.24 charge (Wedmann R, et al. 2015), ii) the S-N bonds in HSSNO and SSNO- (calculated BDE 16.0 and 22.1 kcal/mol, respectively; B3LYP/aug-cc-pv5z, in the presence of solvent/water) are weaker than those in HSNO and SNO− (BDE 27.74 and 36.21 kcal/mol, respectively), which makes (H)SSNO more prone to homolysis than (H)SNO, and iii) SSNO- reacts with metal centers (as evidenced by the reaction with [Fe3+(TPP)]) (Wedmann R, et al. 2015). Cortese-Krott et all. quote that (H)SNO is (only) stable at 12 K (Cortese-Krott MM, et al. 2015), but the PNP+SNO- crystals have been isolated at room temperature (Seel F, et al. 1985). Furthermore, Cortese-Krott et al. have previously observed alone that (H)SNO forms at room temperature from a 1:1 mixture of RSNO and sulfide in water (pH = 7.4) at even higher yield than their “SSNO-“ (Cortese-Krott MM, et al. 2014).

Thus, it is highly problematic to make further conclusions about the physiological effects and reactivity of the product mixtures with undefined chemical composition. To obtain valid (bio)chemical conclusions, use of pure compounds instead of undefined reaction mixtures is recommended. We are willing to provide pure SSNO- and SS15NO- salts to interested researchers.

References:

Cortese-Krott MM, et al. (2015) Key bioactive reaction products of the NO/H2S interaction are S/N-hybrid species, polysulfides, and nitroxyl. Proc Natl Acad Sci USA 112(34):E4651-60.

Seel F, Guttler, H-J, Simon G, Wieckowski A (1977) Colored sulfur species in EPD-solvents. Pure Appl. Chem. 49:45-54.

Seel F, Wagner M (1985) The reaction of polysulfides with nitrogen monoxide in non-acqueous solvents - nitrosodisulfides. Z Naturforsch 40b:762–764, and refernces therein.

Seel F, et al. (1985) PNP-Perthionitrit und PNP-Monothionitrit. Z Naturforsch 40b:1607–1617.

Seel F, Wagner M (1988) Reaction of sulfides with nitrogen monoxide in aqueous solution. Z Anorg Allg Chem 558(3):189–192.

Wedmann R, et al. (2015) Does Perthionitrite (SSNO(-)) Account for Sustained Bioactivity of NO? A (Bio)chemical Characterization. Inorg Chem 54(19):9367-9380.

Cortese-Krott MM, et al. (2014) Nitrosopersulfide (SSNO(-)) accounts for sustained NO bioactivity of S-nitrosothiols following reaction with sulfide. Redox Biol 2:234-44.

On 2015 Nov 04, Ivana Ivanovic-Burmazovic commented:

No evidence for SSNO- in “SSNO- mix”

The results and conclusions published by Cortese-Krott et al. 2015 (Proc Natl Acad Sci USA 2015, 112(34):E4651-60) in the manuscript entitled “Key bioactive reaction products of the NO/H2S interaction are S/N-hybrid species, polysulfides, and nitroxyl” urgently call for comments from the chemical point of view, because they include a number of chemical misconceptions.

The main conclusion of this work, as stated in its title, is that SSNO- (according to the authors a “S/N-hybrid species”) is one of three key bioactive reaction products of the reaction of H2S with NO or S-nitrosothiols (RSNOs). Based on authors’ claims, SSNO- can be obtained in high yields in aqueous solutions at pH 7.4 (even in the presence of oxygen) and it is stable for hours. However, everything known about the chemical/spectroscopic properties of SSNO- contradicts authors’ conclusions. We are afraid that the biological community will be confused by these results, and that researchers without inorganic chemistry background will use such undefined reaction mixtures of NO and H2S, as well as of RSNO and H2S, as a source of SSNO- that in reality is not present under given conditions at all. To help clarifying confusion in the field we bring important facts about SSNO- chemistry below.

In general, a history of the identification of S-S bond-containing compounds in solutions was rich in contradicting conclusions (Seel F, et al. 1977), and “some of those who dared to tackle this challenging task were victims of delusions because such species that were optically (or even by other methods) observed in non-aqueous solutions could not easily be established as defined substances” (Seel F and Wagner M, 1985). This is a translated quotation from Seel F, Wagner M 1985, who first synthesized SSNO- under exclusion of water and dioxygen (Seel F, et al. 1985). However, although citing work of Seel and Wagner, Cortese-Krott et al, 2015 do not mention that i) SSNO- solutions are sensitive to oxygen and water (Seel F and Wagner M 1985) and ii) even in very alkaline aqueous solutions only 10 % of SSNO- was obtained from NO and S2-, although even that was questionable, as SSNO- could not be confirmed by 15N-NMR in aqueous solutions (Seel F and Wagner M, 1988). This contradicts that SSNO- is stable for hours at pH = 7.4, especially in high concentrations in “SSNO--enriched mixtures” (“SSNO- mix”) (Cortese-Krott et al, 2015). Related to the claimed determination of the high SSNO- yield in “SSNO- mix”, in SI (page 9) Cortese-Krott et al, 2015, state: “The (theoretical) maximal yield of SSNO- under these conditions is 1 mM, corresponding to the concentration of added nitrosothiol (please refer to Fig. S9 for experimental determination of reaction yield).” However, Fig. S9 deals with MS of dimethylsulfoxide, and nowhere in SI the clamed experimental results confirming a high SSNO- yield could be found. Instead there is some confusing statement that their putative SSNO- contains two sulfur atoms based on an observation of “two times as much sulfide as sulfane sulfur” (Cortese-Krott MM, et al. 2015). (Even more general, since authors do not provide stoichiometry of the considered reactions, they cannot provide any quantification.)

In agreement with the original work of Seel and Wagner, we demonstrated SSNO- inherent instability by preparing pure crystals of PNP+SSNO- and characterizing its properties by 15N-NMR, IR, EPR, MS, X-ray analysis, electrochemical and computational methods (Wedmann R, et al. 2015). For example, when ca. 10% water was added to an acetone solution of a pure SSNO- salt (Wedmann R, et al. 2015), it decomposed within ca. 100 s. Cortese-Krott et al. report that SSNO- does not react with thiols, H2S and cyanide (Cortese-Krott MM, et al. 2015). However, solutions of a pure SSNO- salt, which Cortese-Krott et al. never used, quickly decompose in the presence of thiols, H2S (Wedmann R, et al. 2015) and cyanide. These authors state that SSNO- is resistant to the biological reductants (Cortese-Krott MM, et al. 2015). However, SSNO- is reduced at a physiological potential of −0.2 V vs. NHE (Wedmann R, et al. 2015). Being unstable at pH = 7.4, in the presence of thiols and biological reducing agents, SSNO- cannot exist under physiological conditions in any relevant concentration. They also report that HSSNO is more stable than HSNO, because HSSNO supposedly has increased electron density on the proximal sulfur (which is a statement for which they do not provide any experimental support) and therefore does not easily react with HS- and positive metal centers (which is contradictio in adjecto) (Cortese-Krott MM, et al. 2015). The facts are quite different: i) the proximal-S has a +0.24 charge (Wedmann R, et al. 2015), ii) the S-N bonds in HSSNO and SSNO- (calculated BDE 16.0 and 22.1 kcal/mol, respectively; B3LYP/aug-cc-pv5z, in the presence of solvent/water) are weaker than those in HSNO and SNO− (BDE 27.74 and 36.21 kcal/mol, respectively), which makes (H)SSNO more prone to homolysis than (H)SNO, and iii) SSNO- reacts with metal centers (as evidenced by the reaction with [Fe3+(TPP)]) (Wedmann R, et al. 2015). Cortese-Krott et all. quote that (H)SNO is (only) stable at 12 K (Cortese-Krott MM, et al. 2015), but the PNP+SNO- crystals have been isolated at room temperature (Seel F, et al. 1985). Furthermore, Cortese-Krott et al. have previously observed alone that (H)SNO forms at room temperature from a 1:1 mixture of RSNO and sulfide in water (pH = 7.4) at even higher yield than their “SSNO-“ (Cortese-Krott MM, et al. 2014).

Thus, it is highly problematic to make further conclusions about the physiological effects and reactivity of the product mixtures with undefined chemical composition. To obtain valid (bio)chemical conclusions, use of pure compounds instead of undefined reaction mixtures is recommended. We are willing to provide pure SSNO- and SS15NO- salts to interested researchers.

References:

Cortese-Krott MM, et al. (2015) Key bioactive reaction products of the NO/H2S interaction are S/N-hybrid species, polysulfides, and nitroxyl. Proc Natl Acad Sci USA 112(34):E4651-60.

Seel F, Guttler, H-J, Simon G, Wieckowski A (1977) Colored sulfur species in EPD-solvents. Pure Appl. Chem. 49:45-54.

Seel F, Wagner M (1985) The reaction of polysulfides with nitrogen monoxide in non-acqueous solvents - nitrosodisulfides. Z Naturforsch 40b:762–764, and refernces therein.

Seel F, et al. (1985) PNP-Perthionitrit und PNP-Monothionitrit. Z Naturforsch 40b:1607–1617.

Seel F, Wagner M (1988) Reaction of sulfides with nitrogen monoxide in aqueous solution. Z Anorg Allg Chem 558(3):189–192.

Wedmann R, et al. (2015) Does Perthionitrite (SSNO(-)) Account for Sustained Bioactivity of NO? A (Bio)chemical Characterization. Inorg Chem 54(19):9367-9380.

Cortese-Krott MM, et al. (2014) Nitrosopersulfide (SSNO(-)) accounts for sustained NO bioactivity of S-nitrosothiols following reaction with sulfide. Redox Biol 2:234-44.

On 2015 Nov 04, Milos Filipovic commented:

Dirty dancing of NO/RSNO and H2S

In this report by Cortese-Krott et al, 2015, the existence of SSNO− as a product of the anaerobic and aerobic reaction of H2S with NO or RSNOs, was claimed based on MS experiments. The authors failed to prove HSNO using a demo MS instrument, although HSNO was prepared by the acidification of nitrite, pulse radiolysis and trans-nitrosation and characterized by MS, IR (14N/15N labelling) and 15N-NMR (Filipovic at al, 2012), and even reported by Cortese-Krott et al, 2014. HSNO has also been recently detected by 15N NMR as a product of the reaction of PNP+SSNO- with sulphide (Wedmann et al, 2015), again going against the claim of Cortese-Krott et al, 2015 that “SSNO- mix” is stable in excess of sulphide.

The authors use LTQ OrbiTrap to concentrate reactive ions (as demonstrated by the absence of any MS signal in the first 1.5 min of continuous injection of the reaction mixture) all of which can intercombine in the ion trap (Figure 3C). They never show the control spectra, actual intensities of the signals, nor do they show the spectrum with broad m/z. It is puzzling that the signal of the reactant, SNAP, reaches its maximum almost at the same time as SSNO− and all other reaction products. While SNAP peak slowly decays in agreement with Uv/Vis experiments, the reaction products remain at maximum intensity (although most of them have questionable S/N ratio, Figure 3C). Contrary, HS2O3− starts to disappear, suggesting that it was present even before reaction started.

The authors refer to the formation of persulfide NONOate ([ONN(OH)S2]−). This species should have m/z 124.9479. While the Figure S6A in indeed has the title: “High resolution mass spectrum of [SS(NO)NOH] −“, the actual figure shows the spectrum of the species with m/z 141.9579, which authors assign to H4O2N3S2−. Strangely, in the same reaction mixture, identical mass peak is assigned to another species, HO5(14)N(15)NS−, (Figure 3B, right panel). 3 peaks are present in m/z ~143 in Figure S6A in. As this is the only MS spectrum shown in high resolution, one can calculate the mass of those unassigned peaks and observe that none of them show up in Figure 3B which is recorded under the same conditions. The isotopic pattern of SULFI/NO (Figure 3B (left panel)) is inconsistent with what should be expected for this species. In the Figure 3A (right panel) there is a huge unassigned background peak at m/z ~94.9252, which does not appear in the Figure 3A (left panel). It is unclear whether the reported peaks are smaller or bigger than actual background noise of the instrument.

In the unexplainable absence of 15N NMR and IR characterisation of SSNO−, which by authors’ claim is “stable” and “abundant”, and correct isotopic patterns for reported species, the authors should have performed experiments with pure 14N and 15N labelled SNAP to independently demonstrate the isotopic distribution of each species and their corresponding m/z shifts.

The authors use maXis Impact instrument to show that they cannot detect HSNO in the reaction of RSNO and H2S. The injection of buffer alone creates the signal intensities of ~1.5x107 (the upper detection limit of this instrument) that makes the background noise stronger than the actual signal of few milimolar GSNO (Figure S8C). The authors also send a message that due to the presence of DMSO and acetonitrile in the tubing no one should try to detect anything at m/z range 50-70. The authors could have cleaned the instrument instead, used new tubing for every measurement or used stainless steel tubing to solve this problem as it is done in the laboratories with MS experience. Furthermore, the ionization conditions which “break” DSMO (BDE ~ 50 kcal/mol, Blank et al, 1997) into CH3SO/CH3SO+ are inappropriate for RSNO/HSNO detection (BDE ~30 kcal/mol, Wedmann et al, 2015). Results look like as they were produced in a limited amount of time and on a very dirty demo instrument and should not have been used for publication.

To prove that 412 nm species is NO dependent the authors trap NO by cPTIO (Figure S4F), ignoring the fact that nitronyl nitroxides readily reacts with H2S and therefore no conclusion can be drawn from this experiment (Wedmann et al, 2013).

The authors also use water soluble triphenylphosphine (TXPTS) to trap nitroxyl from their “SSNO- mix” ignoring the fact that triphenylphosphines are good trapping agents for sulfane sulphur (by mixing PNP+SSNO- with triphenylphosphine Seel et al, formed SNO-), and that S-nitrosothiols react/decompose in the presence of triphenylphosphines in general and TXPTS in particular (Bechtold et al, 2010), so nothing can be concluded from those experiments either.

In conclusion, the data presented in this study ask for more critical and in-depth re-evaluation.

Cortese-Krott MM, et al. (2015) Key bioactive reaction products of the NO/H2S interaction are S/N-hybrid species, polysulfides, and nitroxyl. Proc Natl Acad Sci USA 112(34):E4651-60.

Filipovic MR, et al. (2012) Chemical characterization of the smallest S-nitrosothiol, HSNO; cellular cross-talk of H2S and S-nitrosothiols. J Am Chem Soc 134(29): 12016-27.

Cortese-Krott MM, et al. (2014) Nitrosopersulfide (SSNO(-)) accounts for sustained NO bioactivity of S-nitrosothiols following reaction with sulfide. Redox Biol 2:234-44.

Blank DA, North SW, Stranges D, Suits AG, Lee YT (1997) Unraveling the dissociation of dimethyl sulfoxide following absorption at 193 nm. J Chem Phys 106(2):539-550.

Wedmann R, et al. (2015) Does Perthionitrite (SSNO(-)) Account for Sustained Bioactivity of NO? A (Bio)chemical Characterization. Inorg Chem 54(19):9367-9380.

Wedmann R, et al. (2013) Working with “H2S”: facts and apparent artefacts. Nitric Oxide 41:85-96. Seel F, et al. (1985) PNP-Perthionitrit und PNP-Monothionitrit. Z Naturforsch 40b:1607–1617.

Bechtold E, et al. (2010) Water-soluble triarylphosphines as biomarkers for protein S-nitrosation. ACS Chemical Biology 5(4):405-414.