Dimethylargenate is a stable species in the gas phase

The formation of dimethylargenate from silver diacetate has been explored using multistage mass spectrometry experiments in a quadrupole ion trap and DFT theory.

Side-chain radical losses from radical cations allows distinction of leucine and isoleucine residues in the isomeric peptides Gly-XXX-Arg

Sequencing of peptides via low-energy collision-induced dissociation of protonated peptides typically yields b(n) and y(n) sequence ions. The isomeric residues leucine and isoleucine rarely can be distinguished in these experiments since they give b(n) and y(n) sequence ions of the same m/z. Siu's pioneering work on electrospray ionization of copper complexes of peptides (Chu IK, Rodriquez CF, Lau TC, Hopkinson AC, Siu KWM. J. Phys. Chem. B 2000; 104: 3393) provides a way of forming radical cations of peptides in the gas phase. This method was used to generate M(+ small middle dot) ions of the two isomeric peptides Gly-Leu-Arg and Gly-Ile-Arg in order to compare their fragmentation reactions. Both radical cations fragment to give even electron y(2) and y(1) sequence ions as well as side-chain radical losses of CH(3) and CH(3)CH(2) for isoleucine and (CH(3))(2)CH for leucine. In contrast the [M + H](+) and [M + 2H](2+) ions do not allow distinction between the isomeric leucine and isoleucine peptides.

Fragmentation Reactions of All 64 Deprotonated Trinucleotide and 16 Mixed Base Tetranucleotide Anions via Tandem Mass Spectrometry in an Ion Trap

The gas-phase fragmentation reactions of the [M–H]– ions derived from all 64 trinucleotides and 16 isomeric tetranucleotides each containing adenine, thymine, guanine and cytosine were examined using multistage mass spectrometry (MS/MS and MS3) collision-induced dissociation in an ion trap mass spectrometer. The initial fragmentation pathway involved neutral base loss predominantly from the 5´ terminus, along with minor 3´ and internal base loss in the order A > T > G > C to form [M–H–BH]– ‘non-sequence’ ions. The resultant [M–H–BH]– ions often fragment via intramolecular proton transfer with concomitant cleavage of the 3´ C–O phosphodiester bond to yield an ion–molecule complex, which undergoes either direct dissociation or proton transfer to yield the w-type or (a–B)-type ‘sequence’ ions respectively.