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

The formation and radical-directed dissociation of multiple hydrogen-abstracted peptide cations [M + H - mH]· ⁺ has been reported using MALDI-ISD with dinitro-substituted matrices. The MALDI-ISD of synthetic peptides using 3,5-dinitrosalicylic acid (3,5-DNSA) and 3,4-dinitrobenzoic acid (3,4-DNBA) as matrices resulted in multiple hydrogen abstraction from the analyte [M + H] ⁺ and fragment [ a ] ⁺ ions, i.e., [M + H - m H] ⁺ and [ a - m H] ⁺ ( m = 1-8). All of the ISD spectra showed unusually intense [ a ] ⁺ ions originating from cleavage at the Cα-C bond of the Leu-Xxx residues when peptides without Phe/Tyr/His/Cys residues were used. The intensity of the [ a _n ] ⁺ series ions generated using 3,5-DNSA and 3,4-DNBA rapidly decreased with increasing residue number n , suggesting cleavage at multiradical sites of [M + H - m H] ^•+ . It was suggested that multiple hydrogen abstraction from protonated peptides [M + H] ⁺ mainly takes place from the backbone amide nitrogen.