title: Synchrotron vacuum ultraviolet (VUV) photo-induced fragmentation of cyclic dipeptides radical cations.
abstract: Cyclic dipeptides, due to their chemical properties and various bioactivities, are very attractive for medicinal chemistry. Fragmentations of three simple cyclic dipeptides including cyclo(Gly-Gly), cyclo(Ala-Ala) and cyclo(Gly-Val) in the gas-phase are determined with synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry (VUV PIMS) and theoretical calculations. Cyclo(Gly-Gly) and cyclo(Ala-Ala) show the similar fragmentation pathways. The primary decomposition reactions of cyclo(Gly-Gly) and cyclo(Ala-Ala) radical cations are found to be HNCO loss and CO elimination. The appearance energies (AEs) of fragment ions [CH2NHCOCH2]+• and [CH3CHNHCOCHCH3]+• are measured to be 10.21 and 9.66±0.05 eV, respectively, which are formed from cyclo(Gly-Gly) and cyclo(Ala-Ala) radical cations with HNCO elimination. Due to the stabilization of the radical cation of cyclo(Gly-Val) with isopropyl side group, the dominant fragment ion m/z 114 assigned as [C4H6N2O2]+• is produced by γ-H migration and i cleavage to lose propylene. The ionization energies (IEs) of three cyclic dipeptides decrease in the order cyclo(Gly-Gly) (9.33±0.05 eV)>cyclo(Ala-Ala) (9.21±0.05 eV)>cyclo(Gly-Val) (9.09±0.05 eV) from measurements of photoionization efficiency spectra. It implies that IEs of cyclic dipeptides are affected by substituent groups and symmetrical characterization of molecular structures. These observations of the chemical properties of cyclic dipeptides radical ion (M+•) may be important for understanding gas-phase molecular reactivity of 2,5-diketopiperazines and guiding diketopiperazine-based drug design.
authors.affiliation: National Synchrotron Radiation Laboratory, University of Science and Technology of China, 230029, Hefei, Anhui, People's Republic of China. zly0605@ustc.edu.cn
authors.forename: Liyun
authors.initials: L
authors.lastname: Zhang
authors.forename: Liangyuan
authors.initials: L
authors.lastname: Jia
authors.forename: Lidong
authors.initials: L
authors.lastname: Zhang
authors.forename: Huijun
authors.initials: H
authors.lastname: Guo
authors.forename: Zhongyue
authors.initials: Z
authors.lastname: Zhou
authors.forename: Junjie
authors.initials: J
authors.lastname: Weng
authors.forename: Fei
authors.initials: F
authors.lastname: Qi
mesh_terms: D002412:Cations / Q000737:chemistry; D054659:Diketopiperazines; D004151:Dipeptides / Q000737:chemistry*; D013058:Mass Spectrometry; D015394:Molecular Structure; D010456:Peptides, Cyclic / Q000737:chemistry*; D010879:Piperazines / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet; D017356:Synchrotrons; D014466:Ultraviolet Rays
chemical_list: D002412:Cations; D054659:Diketopiperazines; D004151:Dipeptides; D010456:Peptides, Cyclic; D010879:Piperazines; C010939:2,5-dioxopiperazine; C053149:cyclo(alanylalanyl)