In this computational work (MP2/aug-cc-pVTZ) we investigated the features of the outer electronic shells of R-SeH, R-Se(−) and R-SeOH fragments (R = CH ₃), which can be considered as simplified models for the forms of the active centres of glutathione peroxidases GPx along their catalytic pathway (reduction of peroxides). It is shown that the preferential direction of a nucleophilic attack on the R-Se(−) fragment by a peroxide molecule is determined by the presence of the electron-depleted region of the selenium atom in front of the C-Se bond and nucleophilic attack can be facilitated by the solvation of R-Se(−) by water molecules. Such solvation does not block the direction of potential nucleophilic attack and also leads to the increase of the maximal value of the molecular electrostatic potential on the selenium atom. It was shown that the ⁷⁷Se NMR chemical shift is sensitive both to the oxidation state and the hydration state of the selenium-containing fragment.