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On the aqueous phase chemistry of atmospheric-pressure plasma jets for biomedical applications
(2021)
Cold atmospheric-pressure plasmas are candidate biomedical tools proposed for various applications, such as biological decontamination, cancer regression, and promotion of wound healing. Plasmas, which are in the fourth state of matter, can be generated using inert gases (e.g., argon, helium, ambient air) and different source concepts. Together with the applied parameters, the source design defines the chemical-physical characteristics of the resulting plasma, leading in turn to variable biochemical effects on biological matter. The medical effectiveness of cold plasmas has been proven in vitro and in vivo, also in clinical trials for wound healing in patients using two certified plasmas sources, the kINPen MED and the PlasmaDerm. However, molecular mechanisms leading to those effects are unclear. In the same way, it must be studied if the modulation of plasma properties could improve the specificity of biological effects. These findings are needed to define the concept of plasma dose to be optimized in targeting peculiar pathologic conditions. The present thesis consisting of five peer-reviewed publications has investigated these aspects of plasma research.
In the gaseous phase of cold plasmas, various components with biological activity are produced, such as radiation (e.g., vacuum UV, UV) and reactive species (e.g., •O, 1O2, •OH, •NO, •NO2, O3). As most gaseous species are short-lived, liquid compartments surrounding cells and molecular structures could mediate their transformation and/or the production of other aqueous species. For this reason, plasma-induced aqueous chemistry has been mainly investigated in this thesis. The reaction pathways of reactive oxygen and nitrogen species in liquid were analyzed by monitoring the oxidative modifications induced on tyrosine and cysteine, which are biological structures essential in cellular protein functioning. Liquid chromatography and mass spectrometry-based strategies have been elaborated to elucidate structural changes and characterize the oxidative pattern occurring on the tracers after treatment with plasmas.
As a first result, it could be shown that the oxidative pattern induced on tyrosine or cysteine variated qualitatively and quantitatively with the applied conditions, reflecting the action of differently produced/deposited species in liquid. Biologically relevant structures were identified and in part quantified (e.g., cystine, sulfonic acid, sulfinic acid, S-sulfonate, S-nitrosocysteine, nitrotyrosine, nitrosotyrosine). By using isotopically labeled oxygen or nitrogen in the gas plasma, or labeled oxygen in the target liquid, the incorporation of gaseous or aqueous species in the tracer’s structures was monitored via mass spectrometry. With this strategy, the reaction mechanisms involving gaseous oxygen and nitrogen species at the liquid interface were clarified, as well as the de novo production of reactive species in liquid. Short-lived gaseous oxygen species such as atomic and singlet oxygen (•O, 1O2), predominantly formed in conditions with oxygen in the plasma gas, were able to modify the cysteine structures in highly oxidized derivatives, such as cysteine sulfonic acid. Due to their half-life, however, their activity occurred mainly at the interface. Vacuum UV radiation and •O also led to the formation in liquid of hydroxyl radicals (•OH) and hydrogen peroxide (H2O2), due to water photolysis and homolysis. Water-derived species were responsible for the formation of reversible modifications, such as cysteine S-sulfonate, cystine, and cystine sulfoxides. Nitrosative modifications (e.g., S-nitrosocysteine, nitrosotyrosine, nitrotyrosine) could be observed only in conditions with both nitrogen and oxygen in the plasma gas, and further optimization occurred in presence of water molecules in the gas. In this case, the formation and action of peroxynitrite (ONOO-) in generating nitrotyrosine was proven by using a scavenger molecule for ONOO-.
Finally, the cysteine product pattern was applied as a tool to characterize and compare the overall chemistry generated in liquid by different plasma sources and applied parameters. These findings aim to support and contribute to the definition of plasma dose for plasma medicine, through the standardization, control, tuning, and optimization of plasma parameters and plasma liquid chemistry. These results may be applied in the future to improve the specificity and selectivity of the biological effects generated by the described atmospheric-pressure plasma jets.