Rapid Electrochemical Deprotection of the Iso­nicotinyl­oxycarbonyl Group from Carbonates and Thiocarbonates in a Microfluidic Reactor

Electro­reductive deprotection of the iso­nicotinyl­oxy­carbonyl (<i>i</i>Noc) group from hydroxy, thiol, and amino groups was carried out in an electro­chemical micro­reactor. The small distance of the platinum electrodes in the micro­reactor enables a rapid electro­chemical redox reaction without added electrolytes. As a result, the electro­chemical deprotection of <i>O</i>- and <i>S</i>-<i>i</i>Noc aromatic substrates was achieved in short reaction times (<2 min), while <i>N</i>-<i>i</i>Noc and non­aromatic substrates did not react under the same reaction conditions. This method enables a rapid and site-selective deprotection of <i>O</i>- or <i>S</i>-<i>i</i>Noc groups without removal of <i>N</i>-<i>i</i>Noc moieties

Selenoglutathione Diselenide: Unique Redox Reactions in the GPx-Like Catalytic Cycle and Repairing of Disulfide Bonds in Scrambled Protein

Selenoglutathione (GSeH) is a selenium analogue of naturally abundant glutathione (GSH). In this study, this water-soluble small tripeptide was synthesized in a high yield (up to 98%) as an oxidized diselenide form, i.e., GSeSeG (<b>1</b>), by liquid-phase peptide synthesis (LPPS). Obtained <b>1</b> was applied to the investigation of the glutathione peroxidase (GPx)-like catalytic cycle. The important intermediates, i.e., GSe^– and GSeSG, besides GSeO₂H were characterized by ⁷⁷Se NMR spectroscopy. Thiol exchange of GSeSG with various thiols, such as cysteine and dithiothreitol, was found to promote the conversion to GSe^– significantly. In addition, disproportionation of GSeSR to <b>1</b> and RSSR, which would be initiated by heterolytic cleavage of the Se–S bond and catalyzed by the generated selenolate, was observed. On the basis of these redox behaviors, it was proposed that the heterolytic cleavage of the Se–S bond can be facilitated by the interaction between the Se atom and an amino or aromatic group, which is present at the GPx active site. On the other hand, when a catalytic amount of <b>1</b> was reacted with scrambled 4S species of RNase A in the presence of NADPH and glutathione reductase, native protein was efficiently regenerated, suggesting a potential use of <b>1</b> to repair misfolded proteins through reduction of the non-native SS bonds

Effects of Ring Size and Polar Functional Groups on the Glutathione Peroxidase-Like Antioxidant Activity of Water-Soluble Cyclic Selenides

To elucidate the effects of ring structure and a substituent on the glutathione peroxidase- (GPx-) like antioxidant activities of aliphatic selenides, series of water-soluble cyclic selenides with variable ring size and polar functional groups were synthesized, and their antioxidant activities were evaluated by NADPH-coupled assay using H₂O₂ and glutathione (GSH) in water and also by NMR spectroscopy using H₂O₂ and dithiothreitol (DTT^red) in methanol. Strong correlations were found among the GPx-like activity in water, the second-order rate constants for the oxidation of the selenides, and the HOMO energy levels calculated in water. The results support the conclusion that the oxidation process is the rate-determining step of the catalytic cycle. On the other hand, such correlations were not obtained for the activity observed in methanol. The optimal ring size was determined to be five. The type of substituent (NH₂ < OH < CO₂H) and the number can also control the activity, whereas the stereoconfiguration has only marginal effects on the activity in water. In methanol, however, the activity rank could not be explained by the simple scenarios applicable in water

Model Study Using Designed Selenopeptides on the Importance of the Catalytic Triad for the Antioxidative Functions of Glutathione Peroxidase

Although the catalytic triad of glutathione peroxidase (GPx) has been well recognized, there was little evidence for the relevance of the interactions among the triad amino acid residues, i.e., selenocysteine (U), glutamine (Q), and tryptophan (W), to the GPx antioxidative functions. Using a designed selenopeptide having an amino acid sequence of GQAUAWG, we demonstrate here that U, Q, and W present at the active site can interact with each other to exert the enzymatic activity. The amino acid sequence was chosen on the basis of the Monte Carlo molecular simulation for various selenopeptides in polarizable continuous water using the SAAP force field (SAAP-MC). Measurement of the GPx-like activity for the selenopeptide obtained by solid-phase peptide synthesis revealed that the antioxidant activity is cooperatively enhanced by the presence of Q and W proximate to U, although the activity was low compared to selenocystine (U₂). The effect of Q on the activity was more important than that of W. In addition, the fluorescence spectrometry suggested a close contact between U and W. These experimental observations were supported by SAAP-MC simulation as well as by ab initio calculation. The latter further suggested that the interaction mode among the triad changes depending on the intermediate states