Controlling the Redox Catalytic Activity of a Cyclic Selenide Fused to 18-Crown-6 by the Conformational Transition Induced by Coordination to an Alkali Metal Ion.

trans-3,4-Dihydroxyselenolane (DHS), a water-soluble cyclic selenide, exhibits selenoenzyme-like unique redox activities through reversible oxidation to the corresponding selenoxide. Previously, we demonstrated that DHS can be applied as an antioxidant against lipid peroxidation and a radioprotector by means of adequate modifications of the two hydroxy (OH) groups. Herein, we synthesized new DHS derivatives with a crown-ether ring fused to the OH groups (DHS-crown-n (n = 4 to 7), 1-4) and investigated their behaviors of complex formation with various alkali metal salts. According to the X-ray structure analysis, it was found that the two oxygen atoms of DHS change the directions from diaxial to diequatorial by complexation. The similar conformational transition was also observed in solution NMR experiments. The 1H NMR titration in CD3OD further confirmed that DHS-crown-6 (3) forms stable 1:1 complexes with KI, RbCl and CsCl, while it forms a 2:1 complex with KBPh4. The results suggested that the 1:1 complex (3·MX) exchanges the metal ion with metal-free 3 through the formation of the 2:1 complex. The redox catalytic activity of 3 was evaluated using a selenoenzyme model reaction between H2O2 and dithiothreitol. The activity was significantly reduced in the presence of KCl due to the complex formation. Thus, the redox catalytic activity of DHS could be controlled by the conformational transition induced by coordination to an alkali metal ion.

A protein identification method for proteomics using amino acid composition analysis with IoT-based remote control.

In the present study, we developed a protein identification method using low-cost and easy-to-operate amino acid composition analysis. The identification program automatically compares the quantitative result for each amino acid concentration obtained from the amino acid analysis to the amino acid composition data retrieved from the UniProt protein database. We found that the accuracy of protein identification using amino acid composition analysis was comparable to that of mass spectrometry analysis. The method was able to distinguish and identify differences in amino acid substitutions of several residues between proteins with high sequence homology. The identification accuracy of proteins was also improved by correcting the concentrations in the program for Cys, Trp, and Ile residues, which cannot be quantified by general sample preparation for amino acid analysis. Moreover, the amino acid analyzer was remotely controlled in accordance with the growing demand for remote work. The measured amino acid data were automatically uploaded to the IoT portal within a few minutes of each measurement, allowing researchers to download data and analyze them using the identification program anywhere and at any time by connecting to a network. The results indicated that the present method is useful for protein identification.

Glutathione peroxidase-like functions of 1,2-diselenane-4,5-diol and its amphiphilic derivatives: Switchable catalytic cycles depending on peroxide substrates.

Amphiphilic derivatives of (±)-trans-1,2-diselenane-4,5-diol (DSTox) decorated with long alkyl chains or aromatic substituents via ester linkages were applied as glutathione peroxidase (GPx)-like catalysts. The reduction of H2O2 with the diselenide catalysts was accelerated through a GPx-like catalytic cycle, in which the diselenide (Se-Se) bond was reduced to the diselenolate form ([Se-,Se-]) by coexisting dithiothreitol, and the generated highly active [Se-,Se-] subsequently reduced H2O2 to H2O retrieving the original Se-Se form. In the lipid peroxidation of lecithin/cholesterol liposomes induced by 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH), on the other hand, the Se-Se form directly reduced lipid peroxide (LOOH) to the corresponding alcohol (LOH), inhibiting the radical chain reaction, to exert the antioxidative effect. Thus, the two GPx-like catalytic cycles can be switched depending on the peroxide substrates. Furthermore, hydrophilic compounds with no or short alkyl groups (C3) showed high antioxidative activities for the catalytic reduction of H2O2, while lipophilic compounds with long alkyl chains (C6-C14) or aromatic substituents were more effective antioxidants against lipid peroxidation. In addition, these compounds showed low cytotoxicity in cultured HeLa cells and exhibited sufficient anti-lipid peroxidative activities, suggesting their potentials as selenium-based antioxidative drugs.

Flexible Folding: Disulfide-Containing Peptides and Proteins Choose the Pathway Depending on the Environments.

In the last few decades, development of novel experimental techniques, such as new types of disulfide (SS)-forming reagents and genetic and chemical technologies for synthesizing designed artificial proteins, is opening a new realm of the oxidative folding study where peptides and proteins can be folded under physiologically more relevant conditions. In this review, after a brief overview of the historical and physicochemical background of oxidative protein folding study, recently revealed folding pathways of several representative peptides and proteins are summarized, including those having two, three, or four SS bonds in the native state, as well as those with odd Cys residues or consisting of two peptide chains. Comparison of the updated pathways with those reported in the early years has revealed the flexible nature of the protein folding pathways. The significantly different pathways characterized for hen-egg white lysozyme and bovine milk α-lactalbumin, which belong to the same protein superfamily, suggest that the information of protein folding pathways, not only the native folded structure, is encoded in the amino acid sequence. The application of the flexible pathways of peptides and proteins to the engineering of folded three-dimensional structures is an interesting and important issue in the new realm of the current oxidative protein folding study.

Molecular Interactions between Methylene Blue and Sodium Alginate Studied by Molecular Orbital Calculations.

A methylene blue (MB) indicator embedded in sodium alginate (SA) film was previously examined for detecting active oxygen species. In a previous study, spectrometry was used to identify and characterize the MB/SA complex. However, the decolorization mechanism was not fully assessed. In this study, our aim is to conduct computational calculations at the B3LYP/6-31G(d) level to clarify the exact types and positions of the interaction that cause the decolorization in MB. The results demonstrate that MB/SA interacts with carboxylates (-COO(superscript)-(superscript)) of SA and the N, C, and S atoms of MB, confirming previous experimental observations.

Modeling Thioredoxin Reductase-Like Activity with Cyclic Selenenyl Sulfides: Participation of an NH⋠⋠⋠Se Hydrogen Bond through Stabilization of the Mixed Se-S Intermediate.

At the redox-active center of thioredoxin reductase (TrxR), a selenenyl sulfide (Se-S) bond is formed between Cys497 and Sec498, which is activated into the thiolselenolate state ([SH,Se- ]) by reacting with a nearby dithiol motif ([SHCys59 ,SHCys64 ]) present in the other subunit. This process is achieved through two reversible steps: an attack of a cysteinyl thiol of Cys59 at the Se atom of the Se-S bond and a subsequent attack of a remaining thiol at the S atom of the generated mixed Se-S intermediate. However, it is not clear how the kinetically unfavorable second step progresses smoothly in the catalytic cycle. A model study that used synthetic selenenyl sulfides, which mimic the active site structure of human TrxR comprising Cys497, Sec498, and His472, suggested that His472 can play a key role by forming a hydrogen bond with the Se atom of the mixed Se-S intermediate to facilitate the second step. In addition, the selenenyl sulfides exhibited a defensive ability against H2 O2 -induced oxidative stress in cultured cells, which suggests the possibility for medicinal applications to control the redox balance in cells.

Protein Folding in the Presence of Water-Soluble Cyclic Diselenides with Novel Oxidoreductase and Isomerase Activities.

The protein disulfide isomerase (PDI) family, found in the endoplasmic reticulum (ER) of the eukaryotic cell, catalyzes the formation and cleavage of disulfide bonds and thereby helps in protein folding. A decrease in PDI activity under ER stress conditions leads to protein misfolding, which is responsible for the progression of various human diseases, such as Alzheimer's, Parkinson's, diabetes mellitus, and atherosclerosis. Here we report that water-soluble cyclic diselenides mimic the multifunctional activity of the PDI family by facilitating oxidative folding, disulfide formation/reduction, and repair of the scrambled disulfide bonds in misfolded proteins.

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 (1), by liquid-phase peptide synthesis (LPPS). Obtained 1 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 1 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 1 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 1 to repair misfolded proteins through reduction of the non-native SS bonds.

Development of SAAP3D force field and the application to replica-exchange Monte Carlo simulation for chignolin and C-peptide.

Single amino acid potential (SAAP) would be a prominent factor to determine peptide conformations. To prove this hypothesis, we previously developed SAAP force field for molecular simulation of polypeptides. In this study, the force field was renovated to SAAP3D force field by applying more accurate three-dimensional main-chain parameters, instead of the original two-dimensional ones, for the amino acids having a long side-chain. To demonstrate effectiveness of the SAAP3D force field, replica-exchange Monte Carlo (REMC) simulation was performed for two benchmark short peptides, chignolin (H-GYDPETGTWG-OH) and C-peptide (CHO-AETAAAKFLRAHA-NH2). For chignolin, REMC/SAAP3D simulation correctly produced native ß-turn structures, whose minimal all-atom root-mean-square deviation value measured from the native NMR structure (except for H) was 1.2 Å, at 300 K in implicit water, along with misfolded ß-hairpin structures with unpacked aromatic side chains of Tyr2 and Trp9. Similar results were obtained for chignolin analog [G1Y,G10Y], which folded more tightly to the native ß-turn structure than chignolin did. For C-peptide, on the other hand, the α-helix content was larger than the ß content on average, suggesting a significant helix-forming propensity. When the imidazole side chain of His12 was protonated (i.e., [His12Hip]), the α content became larger. These observations as well as the representative structures obtained by clustering analysis were in reasonable agreement not only with the structures of C-peptide that were determined in this study by NMR in 30% CD3CD in H2O at 298 K but also with the experimental and theoretical behaviors having been reported for protonated C-peptide. Thus, accuracy of the SAAP force field was improved by applying three-dimensional main-chain parameters, supporting prominent importance of SAAP for peptide conformations.

Effects of Metal Ions, Temperature, and a Denaturant on the Oxidative Folding Pathways of Bovine α-Lactalbumin.

Bovine α-lactalbumin (αLA) has four disulfide (SS) bonds in the native form (N). On the oxidative folding pathways of this protein, two specific SS folding intermediates, i.e., (61-77, 73-91) and des[6-120], which have two and three native SS bonds, respectively, accumulate predominantly in the presence of Ca2+. In this study, we reinvestigated the pathways using a water-soluble cyclic selenoxide reagent, trans-3,4-dihydroxyselenolane oxide (DHSox), as a strong and quantitative oxidant to oxidize the fully reduced form (R). In the presence of ethylenediaminetetraacetic acid (EDTA) (under a metal-free condition), SS formation randomly proceeded, and N did not regenerate. On the other hand, two specific SS intermediates transiently generated in the presence of Ca2+. These intermediates could be assigned to (61-77, 73-91) and des[6-120] having two common SS bonds, i.e., Cys61-Cys77 and Cys73-Cys91, near the calcium binding pocket of the ß-sheet domain. Much faster folding to N was observed in the presence of Mn2+, whereas Na⁺, K⁺, Mg2+, and Zn2+ did not affect the pathways. The two key intermediates were susceptible to temperature and a denaturant. The oxidative folding pathways revealed were significantly different from those of hen egg white lysozyme, which has the same SS-bonding pattern as αLA, suggesting that the folding pathways of SS-containing proteins can alter depending on the amino acid sequence and other factors, even when the SS-bond topologies are similar to each other.

Glutathione Peroxidase-Like Activity of Amino-Substituted Water-Soluble Cyclic Selenides: A Shift of the Major Catalytic Cycle in Methanol.

We previously reported that water-soluble cyclic selenides can mimic the antioxidative function of glutathione peroxidase (GPx) in water through a simple catalytic cycle, in which the selenide (>Se) is oxidized by H2O2 to the selenoxide (>Se=O) and the selenoxide is reduced by a thiol back to the selenide. In methanol, however, the GPx-like activity could not be explained by this simple scenario. To look into the reasons for the unusual behaviors in methanol, monoamino-substituted cyclic selenides with a variable ring size were synthesized, and the intermediates of the catalytic cycle were characterized by means of 77Se-NMR and LC-MS spectroscopies. In water, it was confirmed that the selenide and the selenoxide mainly contribute to the antioxidative function, though a slight contribution from the dihydroxy selenane (>Se(OH)2) was also suggested. In methanol, on the other hand, other active species, such as hydroxyselenonium (>Se⁺-OH) and hydroxy perhydroxy selenane (>Se(OH)(OOH)), could be generated to build another catalytic cycle. This over-oxidation would be more feasible for amino-substituted cyclic selenides, probably because the ammonium (NH3⁺) group would transfer a proton to the selenoxide moiety to produce a hydroxyselenonium species in the absence of an additional proton source. Thus, a shift of the major catalytic cycle in methanol would make the GPx-like antioxidative function of selenides perplexing.

Preparation of Selenoinsulin as a Long-Lasting Insulin Analogue.

Synthetic insulin analogues with a long lifetime are current drug targets for the therapy of diabetic patients. The replacement of the interchain disulfide with a diselenide bridge, which is more resistant to reduction and internal bond rotation, can enhance the lifetime of insulin in the presence of the insulin-degrading enzyme (IDE) without impairing the hormonal function. The [C7UA ,C7UB ] variant of bovine pancreatic insulin (BPIns) was successfully prepared by using two selenocysteine peptides (i.e., the C7U analogues of A- and B-chains, respectively). In a buffer solution at pH 10 they spontaneously assembled under thermodynamic control to the correct insulin fold. The selenoinsulin (Se-Ins) exhibited a bioactivity comparable to that of BPIns. Interestingly, degradation of Se-Ins with IDE was significantly decelerated (τ1/2 ≈8 h vs. ≈1 h for BPIns). The lifetime enhancement could be due to both the intrinsic stability of the diselenide bond and local conformational changes induced by the substitution.

dl-trans-3,4-Dihydroxy-1-selenolane (DHSred) heals indomethacin-mediated gastric ulcer in mice by modulating arginine metabolism.

BACKGROUND: The importance of the arginine metabolism in gastric ulcer-healing is given relatively less attention. Hence the role of controlling this pathway by dl-trans-3,4-dihydroxy-1-selenolane (DHSred) and omeprazole against indomethacin-induced stomach ulceration in mouse was investigated. METHODS: Swiss albino mice were ulcerated with indomethacin followed by treatment with the test samples, and the activities of myeloperoxidase (MPO), total nitric oxide synthase (NOS) and arginase, the expressions of inducible nitric oxide synthase (iNOS) and endothelial nitric oxide synthase (eNOS), and the pro-/anti-inflammatory cytokine levels were assayed. NOS-inhibitors were also used to establish the biochemical mechanism. RESULTS: Indomethacin induced maximum ulceration in mice on the 3rd day, associated with reduced arginase activity, eNOS expression, along with increased MPO and total NOS activities, nitric oxide (NO) generation, iNOS expression, and pro-/anti-inflammatory (Th1/Th2) cytokine ratio. Treatment with DHSred (2.5mgkg(-1)×3days) restored the cytokine balance to shift the iNOS/NO axis to the arginase/polyamine axis as revealed from the increased arginase activity and eNOS expression, and reduced iNOS expression, total NOS activity and NO level. CONCLUSIONS: The ulcer-healing property of DHSred, but not of omeprazole was due to a favorable pro-/anti-inflammatory cytokine ratio that shifted the arginine metabolism to the polyamine pathway and increased the eNOS/iNOS ratio. The healing action of omeprazole was not significantly associated with these parameters. GENERAL SIGNIFICANCE: The shift in the ariginine-metabolism from the iNOS/NO axis to the arginase/polyamine axis is guided by Th1/Th2 cytokines ratio and plays an important role in gastric ulcer-healing. The favourable effects of the non-toxic and water-soluble compound, DHSred on these pathways and other COX-dependent and antioxidative parameters suggested it to be a promising anti-ulcer formulation for further studies.

Fatty acid conjugates of water-soluble (±)-trans-Selenolane-3,4-diol: effects of alkyl chain length on the antioxidant capacity.

Fatty acid monoesters of the title compound (DHS(red) ), of variable carbon chain length (propionate, laurate, myristate, palmitate, and stearate), were synthesized, and their antioxidant capacities were evaluated by means of a lipid peroxidation assay with lecithin/cholesterol liposomes. The selenides with long alkyl chains exhibited significant antioxidant activity (IC50 =9-34 µM) against accumulation of lipid hydroperoxide. Incorporation of the myristate into the liposome was ≈50 % by EPMA analysis. Intermediacy of the selenoxide was examined by NMR. In addition, enhancement of interfacial redox catalytic activity was observed for the myristate, but not for PhSeSePh and edaravone, in a PhCl/H2 O biphasic peroxidation assay. These results suggested that a combination of a hydrophilic selenide moiety as a redox center with a long alkyl chain is an effective approach to selenium antioxidants with interfacial glutathione-peroxidase-like (GPx-like) activity. The activity can be controlled by the alkyl chain length.

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.

Mimicking the lipid peroxidation inhibitory activity of phospholipid hydroperoxide glutathione peroxidase (GPx4) by using fatty acid conjugates of a water-soluble selenolane.

A series of fatty acid conjugates of trans-3,4-dihydroxy-1-selenolane (DHS) were synthesized by reacting DHS with appropriate acid chlorides. The obtained monoesters were evaluated for their antioxidant capacities by the lipid peroxidation assay using a lecithin/cholesterol liposome as a model system. The observed antioxidant capacities against accumulation of the lipid hydroperoxide (LOOH) increased with increasing the alkyl chain length and became saturated for dodecanoic acid (C12) or higher fatty acid monoesters, for which the capacities were much greater than those of DHS, its tridecanoic acid (C13) diester, and PhSeSePh. On the other hand, the bacteriostatic activity of myristic acid (C14) monoester, evaluated through the colony formation assay using Bacillus subtilis, indicated that it has higher affinity to bacterial cell membranes than parent DHS. Since DHS-fatty acid conjugates would inhibit lipid peroxidation through glutathione peroxidase (GPx)-like 2e- mechanism, higher fatty acid monoesters of DHS can mimic the function of GPx4, which interacts with LOOH to reduce it to harmless alcohol (LOH). Importance of the balance between hydrophilicity and lipophilicity for the design of effective GPx4 mimics was suggested.

Stable selones in glutathione-peroxidase-like catalytic cycle of selenonicotinamide derivative.

Selenonicotinamide, 2,2'-diselenobis[3-amidopyridine] (NictSeSeNict) exhibits glutathione-peroxidase (GPx)-like activity, catalyzing the reduction of hydrogen peroxide (H2O2) by glutathione (GSH). Estimated reactivity parameters for the reaction of selenium species, according to the Dalziel kinetic model, towards GSH (ϕGSH) and H2O2 (ϕH2O2), indicated that the rate constant for the reaction of NictSeSeNict with GSH is higher as compared to that with H2O2, indicating that the activity is initiated by reduction. (77)Se NMR spectroscopy, HPLC analysis, mass spectrometry (MS) and absorption spectroscopy were employed to understand the nature of selenium intermediates responsible for the activity. The (77)Se NMR resonance at 525 ppm due to NictSeSeNict disappeared in the presence of GSH with the initial appearance of signals at δ 364 and 600 ppm, assigned to selone (NictC=Se) and selenenyl sulfide (NictSeSG), respectively. Reaction of H2O2 with NictSeSeNict produced a mixture of selenenic acid (NictSeOH) and seleninic acid (NictSeO2H) with (77)Se NMR resonances appearing at 1069 and 1165 ppm, respectively. Addition of three equivalents of GSH to this mixture produced a characteristic (77)Se NMR signal of NictSeSG. HPLC analysis of the product formed by the reaction of NictSeSeNict with GSH confirmed the formation of NictC=Se absorbing at 375 nm. Stopped-flow kinetic studies with global analysis revealed a bimolecular rate constant of 4.8 ± 0.5 × 10(3) M(-1) s(-1) and 1.7 ± 0.6 × 10(2) M(-1) s(-1) for the formation of NictC=Se produced in two consecutive reactions of NictSeSeNict and NictSeSG with GSH, respectively. Similarly the rate constant for the reaction of NictC=Se with H2O2 was estimated to be 18 ± 1.8 M(-1) s(-1). These studies clearly indicated that the GPx activity of NictSeSeNict is initiated by reduction to form NictSeSG and a stable selone, which is responsible for its efficient GPx activity.

Seleno-relaxin analogues: effect of internal and external diselenide bonds on the foldability and a fibrosis-related factor of endometriotic stromal cells.

Human relaxin-2 (H2 relaxin) is a peptide hormone of about 6 kDa, first identified as a reproductive hormone involved in vasoregulation during pregnancy. It has recently attracted strong interest because of its diverse functions, including anti-inflammatory, anti-fibrotic, and vasodilatory, and has been suggested as a potential peptide-based drug candidate for a variety of diseases. Mature H2 relaxin is constituted by the A- and B-chains stabilized by two interchain disulfide (SS) bridges and one intrachain SS linkage. In this study, seleno-relaxins, SeRlx-α and SeRlx-ß, which are [C11UA,C11UB] and [C10UA,C15UA] variants of H2 relaxin, respectively, were synthesized via a one-pot oxidative chain assembly (folding) from the component A- and B-chains. The substitution of SS bonds in a protein with their analogue, diselenide (SeSe) bonds, has been shown to alter the physical, chemical, and physiological properties of the protein. The surface SeSe bond (U11A-U11B) enhanced the yield of chain assembly while the internal SeSe bond (U10A-U15A) improved the reaction rate of the folding, indicating that these bridges play a major role in controlling the thermodynamics and kinetics, respectively, of the folding mechanism. Furthermore, SeRlx-α and SeRlx-ß effectively reduced the expression of a tissue fibrosis-related factor in human endometriotic stromal cells. Thus, the findings of this study indicate that the S-to-Se substitution strategy not only enhances the foldability of relaxin, but also provides new guidance for the development of novel relaxin formulations for endometriosis treatment.

Radical cations of aromatic selenium compounds: role of Se···X nonbonding interactions.

Selenium centered radical cations in aliphatic selenium compounds are stabilized by formation of two-center-three electron (2c-3e) hemi bonds either with nearby heteroatoms forming monomer radicals or with selenium atoms of the parent molecules forming dimer radicals. Such radicals in aromatic selenium compounds would generally be stabilized as monomers by the delocalization of the spin density along the aromatic ring. To test the assumption if aromatic selenides having Se···X nonbonding interactions can show different types of radical cations, we have performed pulse radiolysis studies of three structurally related aromatic selenium compounds and the results have been substantiated with cyclic voltammetry and quantum chemical calculations. The three aromatic selenium compounds have functional groups like -CH2N(CH3)2 (1), -CH2OH (2), and -CH3 (3) at ortho position to the -SeCH3 moiety. The energy of Se···X nonbonding interactions (E(nb)) for these compounds is in the order 1 (Se···N) > 2 (Se···O) > 3 (Se···H). Radical cations, 1(•+), 2(•+) and 3(•+) were produced by the one-electron oxidation of 1, 2 and 3 by radiolytically generated (•)OH and Br2(•-) radicals. Results on transient spectra, lifetime, and secondary reactions of 1(•+), 2(•+), and 3(•+) indicated that 1(•+) shows a significantly different absorption spectrum, longer lifetime, and less oxidizing power compared to those of 2(•+) or 3(•+). Quantum chemical calculations suggested that 1(•+) is stabilized by the formation of a 2c-3e bond between Se and N atoms, whereas 2(•+) and 3(•+) acquire stability through the delocalization of the spin density on the aromatic ring. These results provide evidence for the first time that stronger nonbonding interactions between Se···N in the ground state, facilitate the formation of stabilized radical cations, which can significantly influence the redox chemistry and the biological activity of aromatic selenium compounds.

Reinvestigation of the oxidative folding pathways of hen egg white lysozyme: switching of the major pathways by temperature control.

It has been well established that in the oxidative folding of hen egg white lysozyme (HEL), which has four SS linkages in the native state (N), three des intermediates, i.e., des[76-94], des[64-80], and des [6-127], are populated at 20 °C and N is dominantly formed by the oxidation of des[64-80] and des[6-127]. To elucidate the temperature effects, the oxidative folding pathways of HEL were reinvestigated at 5-45 °C in the presence of 2 M urea at pH 8.0 by using a selenoxide reagent, DHSox. When reduced HEL was reacted with 1-4 equivalents of DHSox, 1S, 2S, 3S, and 4S intermediate ensembles with 1-4 SS linkages, respectively, were produced within 1 min. After the oxidation, 3S was slowly converted to the des intermediates with formation of the native structures through SS rearrangement. At 5 °C, des[76-94] was populated in the largest amount, but the oxidation to N was slower than that of des[64-80] and des[6-127]. At 35 °C, on the other hand, des[64-80] and des[6-127] were no longer stable, and only des[76-94] was populated. The results suggested that the major folding pathways of HEL can be switched from one to the other by temperature control.