Thermal- and Light-driven Metathesis Reactions Between Different Diselenides

Photochemical Transformations of Peptides Containing the N-(2-Selenoethyl)glycine Moiety

The diselenide bond has attracted considerable attention due to its ability to undergo the metathesis reaction in response to visible light. In our previous study, we demonstrated visible-light-induced diselenide metathesis of selenocysteine-containing linear peptides, allowing for the convenient generation of peptide libraries. Here, we investigated the transformation of linear and cyclic peptides containing the N-(2-selenoethyl)glycine moiety. The linear peptides were highly susceptible to the metathesis reaction, whereas the cyclic systems gave only limited conversion yields of the metathesis product. In both cases, side reactions leading to the formation of mono-, di-, and polyselenides were observed upon prolonged irradiation. To confirm the radical mechanism of the reaction, the radical initiator 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (VA-044) was tested, and it was found to induce diselenide metathesis without photochemical activation. The data were interpreted in the light of quantum-chemical simulations based on density functional theory (DFT). The simulations were performed at the B3LYP-D3BJ/def2-TZVP level of theory using a continuum solvation model (IEF-PCM) and methanol as a solvent.

Selenium in Peptide Chemistry

In recent years, researchers have been exploring the potential of incorporating selenium into peptides, as this element possesses unique properties that can enhance the reactivity of these compounds. Selenium is a non-metallic element that has a similar electronic configuration to sulfur. However, due to its larger atomic size and lower electronegativity, it is more nucleophilic than sulfur. This property makes selenium more reactive toward electrophiles. One of the most significant differences between selenium and sulfur is the dissociation of the Se-H bond. The Se-H bond is more easily dissociated than the S-H bond, leading to higher acidity of selenocysteine (Sec) compared to cysteine (Cys). This difference in acidity can be exploited to selectively modify the reactivity of peptides containing Sec. Furthermore, Se-H bonds in selenium-containing peptides are more susceptible to oxidation than their sulfur analogs. This property can be used to selectively modify the peptides by introducing new functional groups, such as disulfide bonds, which are important for protein folding and stability. These unique properties of selenium-containing peptides have found numerous applications in the field of chemical biology. For instance, selenium-containing peptides have been used in native chemical ligation (NCL). In addition, the reactivity of Sec can be harnessed to create cyclic and stapled peptides. Other chemical modifications, such as oxidation, reduction, and photochemical reactions, have also been applied to selenium-containing peptides to create novel molecules with unique biological properties.

Thermally Driven Diselenide Metathesis: Polarization Process vs Radical Process

Diselenide, as a dynamic covalent bond, has been widely applied in functional materials due to its response to light, heat, sonication, pH, and other stimuli. Herein, a polarization-induced metathesis mechanism for diselenides under heating conditions in the dark is proposed. First, a radical trap experiment is used to prove that the exchange reaction of diselenides in the dark does not involve any radicals. Second, the dynamic exchange reaction of diselenides is found to be affected not only by the polarity of the solvent but also by the introduction of polar groups into the molecular skeleton. Furthermore, DFT calculations also support the notion that polarity has a large effect on the heterolytic rather than homolytic bond dissociation energies. The experimental results for allyl selenide small molecules, polymers, and polymer materials catalyzed by diselenide all support the polarization-induced metathesis mechanism. In short, we successfully enhanced the understanding of the mechanism for diselenide metathesis.