Reactions of an aromatic σ,σ-biradical with amino acids and dipeptides in the gas phase

Abstract

Gas-phase reactivity of a positively charged aromatic ,-biradical (N-methyl-6,8-didehydroquinolinium) was examined toward six aliphatic amino acids and 15 dipeptides by using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR) and laser-induced acoustic desorption (LIAD). While previous studies have revealed that H-atom and NH 2 abstractions dominate the reactions of related monoradicals with aliphatic amino acids and small peptides, several additional, unprecedented reaction pathways were observed for the reactions of the biradical. For amino acids, these are 2H-atom abstraction, H 2 O abstraction, addition -CO 2 , addition -HCOOH, and formation of a stable adduct. The biradical reacts with aliphatic dipeptides similarly as with aliphatic amino acids, but undergoes also one additional reaction pathway, addition/C-terminal amino acid elimination (addition -CO -NHCHR C ). These reactions are initiated by H-atom abstraction by the biradical from the amino acid or peptide, or nucleophilic addition of an NH 2 or a HO group of the amino acid or peptide at the radical site at C-6 in the biradical. Reactions of the unquenched C-8 radical site then yield the products not observed for related monoradicals. The biradical reacts with aromatic dipeptides with an aromatic ring in N-terminus (i.e., Tyr-Leu, Phe-Val, and Phe-Pro) similarly as with aliphatic dipeptides. However, for those aromatic dipeptides that contain an aromatic ring in the C-terminus (i.e., Leu-Tyr and Ala-Phe), one additional pathway, addition/N-terminal amino acid elimination (addition -CO -NHCHR N ), was observed. This reaction is likely initiated by radical addition of the biradical at the aromatic ring in the C-terminus. Related monoradicals add to aromatic amino acids and small peptides, which is followed by C␣-C␤ bond cleavage, resulting in side-chain abstraction by the radical. For biradicals, with one unquenched radical site after the initial addition, the reaction ultimately results in the loss of the N-terminal amino acid. Similar to monoradicals, the C-S bond in amino acids and dipeptides was found to be especially susceptible to biradical attack. (J Am Soc Mass Spectrom 2010, 21, 1737-1752

Structure and Reactivity of theN-Acetyl-Cysteine Radical Cation and Anion: Does Radical Migration Occur?

“…While radical migration is often accompanied by significant energy barriers, CID conditions (even at low-energy CID in ion traps) provide enough energy to overcome those barriers as has been shown by us [32,37] and others [38]. Ion-molecule reactions in the gas phase provide much less energetic reaction conditions and can be used as a probe of the radical structure [27,28,39,40].…”

The formation of resonance-stabilized sulfur-based radical cations and their gas-phase reactivity

“…The variations in bond length could be explained by different stabilities, and thus different populations, of the two resonance structures shown in Eq. (11). Attempts to correlate reactivity with the C S bond length, spin density on the sulfur, and charge density were not successful.…”

“…(4)) leads to homolytic cleavage of the S-S bond initially resulting in the structure on the right in Eq. (11) with the charge on the protonated nitrogen atom and the radical site on the sulfur atom. The similarity between the chemistries displayed by these two types of radical ions can be explained by the identical resonance forms available for the systems which are shown below in Eq.…”

“…However, exciting findings have been reported in many other areas of gas-phase free radical chemistry, including contributions of Bierbaum's group to understanding of interstellar processes [4][5][6][7], as well as works of Kenttämaa and co-workers on chemistry of various phenyl radicals [8][9][10][11].…”

Gas-phase Reactivity of meta-Benzyne Analogs Toward Small Oligonucleotides of Differing Lengths

“…Previous studies have demonstrated that laser-induced acoustic desorption (LIAD) coupled with a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer provides a powerful tool to study the reactivity of highly reactive species (such as σ,σ-biradicals) toward thermally labile and nonvolatile biomolecules [22,23]. LIAD allows for desorption of thermally labile biomolecules, such as oligonucleotides, into the FT-ICR as intact neutral molecules with low internal and kinetic energies [24][25][26].…”