Are disulfide bonds resilient to double ionization? Insights from coincidence spectroscopy and ab initio calculations.

Disulfide bonds (-S-S-) are commonly present in biomolecules and have also been detected in astrophysical environments. In this work, the stability of the disulfide bond towards double ionization is investigated using quantum chemical calculations and photoelectron photoion photoion coincidence (PEPIPICO) spectroscopy measurements on the prototype dimethyl disulfide (CH3SSCH3, DMDS) molecule. The experiments were performed using high energy synchrotron radiation photons before (2465.0 eV) and at (2470.9 eV) the first sigma resonance around the S 1s edge. We applied the multivariate normal distribution analysis to identify the most plausible ionic fragmentation mechanisms from the doubly ionized DMDS. By mapping the minimum energy structures on the dicationic C2H6S2 2+ potential energy surface, we show that disulfide bonds are only present in high-lying isomers, in contrast to their analogous neutral systems. Our results also indicate that the number of fragment ions containing a disulfide bond for both photon energies is negligible. Taken together, our results reveal that the disulfide bond is severely damaged as a consequence of sulfur core-shell ionization processes, due to the lowering of its thermodynamic stability in multiply-charged systems.

Application of a multivariate normal distribution methodology to the dissociation of doubly ionized molecules: The DMDS (CH3 -SS-CH3 ) case.

RATIONALE: The ion-ion-coincidence mass spectroscopy technique brings useful information about the fragmentation dynamics of doubly and multiply charged ionic species. We advocate the use of a matrix-parameter methodology in order to represent and interpret the entire ion-ion spectra associated with the ionic dissociation of doubly charged molecules. This method makes it possible, among other things, to infer fragmentation processes and to extract information about overlapped ion-ion coincidences. This important piece of information is difficult to obtain from other previously described methodologies. METHODS: A Wiley-McLaren time-of-flight mass spectrometer was used to discriminate the positively charged fragment ions resulting from the sample ionization by a pulsed 800 eV electron beam. We exemplify the application of this methodology by analyzing the fragmentation and ionic dissociation of the dimethyl disulfide (DMDS) molecule as induced by fast electrons. The doubly charged dissociation was analyzed using the Multivariate Normal Distribution. RESULTS: The ion-ion spectrum of the DMDS molecule was obtained at an incident electron energy of 800 eV and was matrix represented using the Multivariate Distribution theory. The proposed methodology allows us to distinguish information among [CHn SHn ]+ /[CH3 ]+ (n = 1-3) fragment ions in the ion-ion coincidence spectra using ion-ion coincidence data. Using the momenta balance methodology for the inferred parameters, a secondary decay mechanism is proposed for the [CHS]+ ion formation. As an additional check on the methodology, previously published data on the SiF4 molecule was re-analyzed with the present methodology and the results were shown to be statistically equivalent. CONCLUSIONS: The use of a Multivariate Normal Distribution allows for the representation of the whole ion-ion mass spectrum of doubly or multiply ionized molecules as a combination of parameters and the extraction of information among overlapped data. We have successfully applied this methodology to the analysis of the fragmentation of the DMDS molecule. Copyright © 2015 John Wiley & Sons, Ltd.