Chemical modification of selenium-containing amino acids caused by non-thermal dielectric-barrier discharge atmospheric-pressure plasma.

Since non-thermal atmospheric-pressure ("cold") plasma sources, such as the dielectric-barrier discharge (DBD), have appeared to be remarkably active in wound healing medicine, the elucidation of cold plasma safety and possible secondary undesirable effects becomes of paramount importance. Selenium-containing amino acids, which are commonly incorporated in many enzymes, came in the spotlight for elucidating the plasma impact as easily oxidizable natural targets. The scope of this study was to analyse the impact of non-thermal plasma on selenium-containing amino acids. Moreover, this research examines the emerging role of metals in the context of oxidation potency of reactive species generated by plasma. The dielectric barrier discharge (DBD) was used to treat 4 mM solutions of Se-(methyl)seleno-l-cysteine (1), l-selenomethionine (2) and seleno-l-cystine (3) for varying treatment times to investigate possible degradation products. In this study we used two redox active iron complexes as well as a redox inert zinc complex in order to compare their capacity to affect chemical modifications caused by plasma. The solutions with selenium-containing amino acids after plasma treatment were analyzed by IR spectroscopy, electrospray ionization mass spectrometry (ESI-MS) and High Performance Liquid Chromatography (HPLC). Several oxidation products were observed as a consequence of plasma treatment, namely: Se-(methyl)seleno-l-cysteine (1) and l-selenomethionine (2) were oxidized to selenoxide and selendioxide derivatives, wheres the Se-Se dimer, seleno-l-cystine (3), was converted to Se-cysteine and seleninic acid. Additionally and to our surprise, redox active iron(ii) and iron(iii) complexes as well as the non-redox active zinc(ii) complex caused the same oxidation pattern when added to the plasma treatment mixtures. Finally, a comparison of the results from Se-containing amino acids with those of their S-containing counterparts revealed that Se-containing amino acids are less prone to cold plasma oxidation than S-containing molecules. By elucidating molecular details of plasma-biomolecule interactions herein we aim to contribute to a better understanding of the complex beneficial medical effects of cold plasma treatments.