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The selective C—C bond activation is a frontier research topic and of great significance in the field of chemistry, especially for biosciences. Due to the existence of many C—C bonds with similar activities in compounds, it is difficult to selectively activate one of the C—C bonds. In this paper, the newly synthesized peptide derivatives composed of unnatural amino acids were used as a subject to demonstrate how radical-mediated selective C—C bond activation and isomer differentiation work. TEMPO radical initiator was employed to introduce o-methylbenzoyl (Bz) radical into the peptide derivatives, and successfully prepared ［Bz-M+H］•+ radical ions in the gas phase. Through tandem mass spectrometry experiments, it had been found that ［Bz-M+H］•+ showed higher reactivity than the protonated peptide molecule ［M+H］+, giving a more diversified gas-phase dissociation reactions. The main fragmentation of ［M+H］+ was the cleavage of amide bond to give rise to ［y1+2H］+ (m/z 160.134 0, RA 100%), a1+ (m/z 86.097 2, RA 74%), and ［(M+H)-HCOOEt］+ (m/z 199.181 5, RA 52%). In contrast, the fragment ions of ［Bz-M+H］•+ included ［Bz•-a1］+ (m/z 202.97), ［Bz•-b1］+ (m/z 231.03), ［Bz•-c1+H］+ (m/z 133.95) and ［(Bz-M+H)-HCOOEt］+ (m/z 316.18, RA 100%). N-terminal fragments of ［Bz-M+H］•+ were observed with the radical part •CH2C6H4CO still attached to the fragmentation. To distinguish these ions from the normal fragments, the prefix Bz• was introduced, such as, ［Bz•-a1］+. More interestingly, ［(Bz-M+H)-HCOOEt］+ was the base peak of ［Bz-M+H］•+, which was produced by breaking the Cα—C adjacent to the ester group of peptide derivative. In contrast, the relative abundance of ［(M+H)-HCOOEt］+ from ［M+H］+ was only 50%. The formation mechanism of ［(M+H)-HCOOEt］+ had been experimentally confirmed to be completed by the two-step reaction of losing EtOH and CO successively. For isomers B1 and B2, their CID spectra of ［Bz-M+H］•+ had high similarity, but the abundance of fragment ion ［Bz•-a1］+ was obviously different (m/z 203, RA 40% for B1, RA 60% for B2). The fragment ions ［Bz•-a1］+ and ［(Bz-M+H)-HCOOEt］+ of ［Bz-M+H］•+ can be used as sensitive probes for isomer discrimination and selective C—C bond cleavage. The research provides new strategy to distinguish peptide isomers and to cleavage selective C—C bond with radical participation in mass spectrometer.

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[Display omitted] • The ternary g-C 3 N 4 /rGO/CdS heterojunction was successfully synthesized by a simple multi-step assembly strategy. • Intimate interfacial interaction reinforces the migration and separation of photoinduced carriers. • The introduction of electronic mediators inhibits the recombination of photogenerated carriers. • g-C 3 N 4 /rGO/CdS demonstrated excellent photocatalytic performance in cleaving the C—C bonds of lignin. Depolymerizing lignin into aromatic monomers with high yield and selectivity via photocatalysis remains a challenging task. Herein, we introduced reduced graphene oxide (rGO) nanosheets as solid electronic mediators and successfully synthesized g-C 3 N 4 /rGO/CdS ternary nanocomposites using a simple multi-step assembly strategy. The photocatalysts can efficiently break the C C bond of lignin under simulated sunlight conditions. The characterization results demonstrate that the loading of CdS effectively enhances the light absorption range and accelerates the transfer of photogenerated charge carriers. Simultaneously, the introduction of rGO inhibits the recombination of electron-hole pairs. When using the β-O-4 model substrate, a 95% conversion rate could be achieved with yields of 71% benzaldehyde and 69% phenyl formate, which are 3.09 times and 3.63 times higher than that of pure g-C 3 N 4 , respectively. Our research highlights the importance of energy band structure engineering and electronic mediator design in developing efficient photocatalysts for breaking C C bonds of lignin. [ABSTRACT FROM AUTHOR]

[Display omitted] • A nearly total conversion and > 98% selectivity of C-C bond cleavage is achieved. • Sufficient superoxide radicals are necessary for highly selective C-C bond cleavage under visible light. • Efficient photocatalytic C-C bonds breaking towards lignin β -O-4 model compounds. • Ru single atoms outperform the nanoparticles for photocatalytic C-C bond cleavage. • C β -H extraction by photogenerated hole is the critical step of the transformation. The selective cleavage of the C C bonds in lignin is highly pursued to obtain aromatic monomers from renewable resources but remains challenging due to their high dissociation energies and steric hindrance between bulky aromatic groups. Herein, a mesoporous carbon nitride-supported ruthenium single-atom photocatalyst (Ru 1 /MCN) is reported for efficient oxidative C α -C β bonds scissoring towards lignin β -O-4 model compounds with high activity and selectivity under visible–light irradiation and room temperature. With this strategy, a nearly total conversion of > 99% and > 98% selectivity of C C bond cleavage can be achieved upon 2–phenoxy–1–phenylethanol. In-depth investigations verify that the C β -H extraction by photogenerated holes is the critical step of the transformation, which directly generates the C β radical intermediates and thus promotes the selective C α -C β bonds breaking by coupling with superoxide radicals induced by photogenerated electrons. This work explores the potential of single-atom catalysts for photocatalytic production of aromatics from renewable biomass feedstocks. [ABSTRACT FROM AUTHOR]