Sensitive β-substituted aliphatic Ile/Val-Xxx and aromatic Phe/Tyr/His-Xxx residues to form [a]+ ions in high energy CID of peptides

Multiple Hydrogen Loss from [M + H]+ and [a]+ ions of Peptides in MALDI In-Source Decay Using a Dinitro-Substituted Matrix

“…The presence of radicals makes cleavage at selective and specific bonds by radical-directed dissociation (RDD) possible. In recent mass spectrometry research, the formation and specific RDD reactions of radical ions have been of great interest in the field of peptides and protein analysis. − There are several methods for generating peptide radical ions, such as electron capture dissociation (ECD), electron-transfer dissociation (ETD), in-source decay (ISD), , and ultraviolet photodissociation (UVPD). − High-energy collision-induced dissociation (high-E CID) resulting in charge-remote fragmentation is also a form of radical ion formation. − …”

Matrix‐dependent a/x pair and overdegraded w/y/z ions generated by radical‐directed dissociation of peptide radical cations [M]+ in matrix‐assisted laser desorption/ionization in‐source decay

“…The [ d ] + ions are characteristically observed in high‐energy CID 22 and UVPD 5 of protonated peptides [M + H] + . The formation of [ d ] + ions in these methods can be explained by Norrish Type I homolytic cleavage at the Cα–C bond 5,22,23 . Energetic properties common to high‐energy CID and UVPD are electronic excitation at aromatic sidechains and backbone amide chromophores.…”

“…Energetic properties common to high‐energy CID and UVPD are electronic excitation at aromatic sidechains and backbone amide chromophores. High‐energy CID of peptides results in electronic excitation at local sites such as bulky side chains such as Ile, Val, Phe, Tyr, and His residues due to collisional interactions with target gas, leading to Cα–C bond homolysis and/or the loss of hydrogen as first events to form [ a ] + and [ d ] + ions 23 . In UVPD, the short wavelength UV photon absorption with the chromophores such as aromatic side chains and backbone amides directly results in electronic excitation, leading to the Cα–C bond homolysis and generation of radical fragments [ a + H] .+ and [ x + H] .+ (Figure 10).…”

“…The earliest event of MALDI is electronic excitation of excess amounts of matrix molecules via UV photon absorption, and immediately after that explosive evaporation occurs to form a dense‐gas like MALDI plume via relaxation processes from electronic to vibrational/rotational energy 24–28 . It is of importance to recognize that in MALDI frequent collisions between analyte and matrix molecules take place in the plume, 29 whereas electron and hydrogen atom emission from matrix molecules, proton transfer, and radical formation also simultaneously occur 23 . Such collisional interactions in the plume may result in vibrational/rotational excitation of analyte molecules via energy transfer from excited matrix molecules.…”

“…[24][25][26][27][28] It is of importance to recognize that in MALDI frequent collisions between analyte and matrix molecules take place in the plume, 29 whereas electron and hydrogen atom emission from matrix molecules, proton transfer, and radical formation also simultaneously occur. 23…”