Background: Treatment with antioxidants might theoretically act to prevent the propagation of tissue damage. Enrichment in selenium, the essential microelement, enhances the antioxidant, reducing and free radical scavenging activity of extracts from several mushroom mycelia. The selenium uptake by the mushroom cultures from different systematic groups results in the sulphur atom substitution by selenium in L-methionine to form selenomethionine.
Objective: Is to elucidate the theoretical prerequisites for preferential S atom substitution by Se in L-methionine, but not in L-cysteine - another sulphur-containing amino acid. Therewith the reactivity of these α-amino acids is discussed not only with respect to the well-studied inorganic compounds of selenium, but also for the potentially feasible interaction of amino acids with 1,5-diphenyl-3-selenopentanedione-1,5 (I). This is related to the directive regulation of fungal developmental parameters.
Method: At the B3LYP/6-311++G(3df,3pd) level of theory with the implementation of NBO analysis and QTAIM, we have performed comparative quantum chemical research in the electronic structure and reactivity of L-cysteine and L-methionine at the initial steps of their interaction with electrophilic reagents. The selected QSAR properties of L-cysteine and L-methionine have been discussed.
Results: From the viewpoint of the analysis of natural charge distribution, natural Wiberg bond order, second order perturbation energy, topological electron density characteristics, electron density in highest occupied molecular orbital, dipole moment, molecular size, polarizability of the molecules, and lipophilicity, a theoretical explanation has been presented for the preferable substitution reaction substrate, L-methionine compared with L-cysteine, on sulphur atom replacement by selenium.