多肽衍生物中自由基介导的选择性C-C键断裂及异构体区分

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 C_α-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.