Specific Cα-C Bond Cleavage of β-Carbon-Centered Radical Peptides Produced by Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry.

Radical-driven dissociation (RDD) of hydrogen-deficient peptide ions [M - H + H] has been examined using matrix-assisted laser dissociation/ionization in-source decay mass spectrometry (MALDI-ISD MS) with the hydrogen-abstracting matrices 4-nitro-1-naphthol (4,1-NNL) and 5-nitrosalicylic acid (5-NSA). The preferential fragment ions observed in the ISD spectra include N-terminal [a] + ions and C-terminal [x], [y + 2], and [w] ions which imply that β-carbon (Cβ)-centered radical peptide ions [M - Hβ + H] are predominantly produced in MALDI conditions. RDD reactions from the peptide ions [M - Hβ + H] successfully explains the fact that both [a] and [x] ions arising from cleavage at the Cα-C bond of the backbone of Gly-Xxx residues are missing from the ISD spectra. Furthermore, the formation of [a] ions originating from the cleavage of Cα-C bond of deuterated Ala(d3)-Xxx residues indicates that the [a] ions are produced from the peptide ions [M - Hβ + H] generated by deuteron-abstraction from Ala(d3) residues. It is suggested that from the standpoint of hydrogen abstraction via direct interactions between the nitro group of matrix and hydrogen of peptides, the generation of the peptide radical ions [M - Hβ + H] is more favorable than that of the α-carbon (Cα)-centered radical ions [M - Hα + H] and the amide nitrogen-centered radical ions [M - H + H], while ab initio calculations indicate that the formation of [M - Hα + H] is energetically most favorable. Graphical Abstract ᅟ.