## 2 Results and Discussion
### Selenium Speciation in Yeast Using the UHPLC-ESI-QqQ MS and UHPLC-ESI-Orbitrap MS Methods
and Supplementary Materials, Figure S8). The first compound is formed by intracellular regroupings of dimethylselenide or dimethylselenosulfenate, creating the C\({}_{2}\)H\({}_{5}\)Se cluster. A compound with a mass close to 181.0291 was identified in black unstard-_Brassica nigra_[28]. In the case of methylthiosenoglutathione (_m/z_ 402.0236), fragmentation of this selenium component (Figure 5) demonstrated the presence of the following ions: C\({}_{5}\)H\({}_{8}\)NO\({}_{3}\)\({}^{+}\), C\({}_{3}\)H\({}_{8}\)NSSe\({}^{+}\), C\({}_{6}\)H\({}_{10}\)NO\({}_{3}\)SSe\({}^{+}\), and C\({}_{6}\)H\({}_{13}\)NO\({}_{3}\)SSe\({}^{+}\). The presence of the first has been confirmed in the biomass of _S. cerevisiae_ CNCM I-3060 yeast [29]. It was the detected \(\gamma\)-Glu fragment included in the glutathione structure. Glutathione (GSH) is one of the most important molecules for cellular defense against chemically reactive toxic compounds or oxidative stress, which can be produced by high selenium concentrations occurring in culture media [30]. This protective function partially stems from its participation in conjugation reactions [31]. The three last compounds have not yet been presented. The conjugated 2,3-DHP-selenocysteine and glutathione form is one of the most common forms containing selenium, and occurs in almost all yeasts enriched with the element [32]. The transformation of the above compound via metabolic processes leads to the formation of 2,3-DHP-selenocysteine-cysteine in yeast biomass. Based on the analyses of mass spectrum fragmentation conducted (Figure 6), it was possible to identify individual compound ions characterized by molecular weights [M + H]\({}^{+}\) = from 83.9780 to 255.8982. The first compound corresponded to the \(\gamma\)-Glu fragment with a loss of formic acid [29; 33]. The last molecular weight presented corresponded to the occurrence of 2,3-dihydroxypropionyl-selenocysteine. The compound was identified in an extract from sunflower sprouts enriched with selenium [34]. In the cellular biomass of _C. utilis_ ATCC 9950 yeast, an Se-S conjugate of selenoglutathione-cysteine with _m/z_ 475.0405 mass was determined. Fragmentation of the spectrum of this compound led to the formation of seven ions (Figure 7). The ion presented with the chemical formula C\({}_{8}\)H\({}_{16}\)N\({}_{3}\)O\({}_{5}\)SSe\({}^{+}\) ([M + H]\({}^{+}\) = 345.9487) was responsible for the occurrence of two residues of \(\gamma\)-Glu [29]. Selenium compounds with _m/z_ 199.0382 and 230.9933 have not been previously reported. Figure 4: Fragmentation spectrum for ion with _m/z_ = 198.0032 and fragmentation pathway. Figure 5: Fragmentation spectrum for ion with \(m/z=402.0236\) and fragmentation pathway. Figure 6: Fragmentation spectrum for ion with \(m/z=376.9922\) and fragmentation pathway. In summary, the results obtained encourage further analyses on the identification of new selenium compounds in the biomass of various yeast species. Appropriate use of analytical methods and yeast culture conditions will allow for and enhance the understanding of the differences in metabolic processes of selenium transformation to its organic forms. Such an approach is associated with the pursuit of the discovery or creation of new products and will make it possible to obtain a dietary supplement enriched in a suitable form of the element. Future research should focus on an analysis of compounds, which will likely occur in commercially sold dietary supplements, and water-soluble, low-molecular selenium compounds. Biotechnological research has great potential for the discovery of new phenomena or a better understanding of those already studied and for further improvement of research. Thus, new, exciting possibilities in selenium metabolism may be identified.