This article describes the fundamental cleavage reactions of (M-H)(-) anions of underivatized peptides that contain up to 25 amino acid residues. The experimental observations of these cleavages have been backed up by molecular modeling, generally at the AM1 level of theory. The basic cleavages are the ubiquitous alpha- and beta-backbone cleavage reactions, which provide information similar to that of the B and Y + 2 cleavages of MH(+) ions of peptides. The residues Asp and Asn also effect cleavages of the backbone (called delta- and gamma-cleavages), by reactions initiated from side chain enolate anions, causing elimination reactions that cleave the backbone between the Asp (Asn) N bond;C backbone bond. Glu and Gln also direct analogous delta- and gamma-cleavages of the backbone, but in this case the processes are initiated by attack of the side chain CO(2) (-) (CONH(-)) to form a lactone (lactam). Ser and Thr residues undergo characteristic fragmentations of the side chain. These processes, losses of CH(2)O (Ser) and MeCHO (Thr), convert these residues into Gly. In larger peptides, Ser and Thr can effect two backbone cleavage reactions, called gamma- and epsilon -processes. The C-terminal CO(2) (-) (or CONH(-)) forms a hydrogen bond with the side chain OH (of Ser or Thr), placing the C-terminal residue in a position where it may affect S(N) (2) attack at the electrophilic backbone CH of Ser, with concomitant cleavage of the backbone. All of the above negative ion cleavages require the peptide backbone to be conformationally flexible. However, there is a backbone cleavage that requires the peptide to have an alpha-helical conformation in order for the two reacting centers to approach. This cleavage is illustrated for the Glu 23-initiated backbone cleavage at Ile 21 for the (M-H)(-) anion of the antimicrobial peptide caerin 1.1.

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Ethylenedione C 2 O 2 is one of the elusive small molecules which have remained undetected even after numerous attempts with different experimental techniques. This is surprising, since theoretical studies predicted the triplet state of C 2 O 2 to be stable towards spinallowed dissociation and hence longlived. Here we report a comprehensive study of charged and neutral ethylenedione by means of charge reversal and neutralization ± reionization mass spectrometry. These experimental results, in conjunction with theoretical calculations, suggest that neutral ethylenedione is intrinsically short-lived rather than being elusive. Both the singlet and triplet states of C 2 O 2 are predicted to dissociate rapidly into two ground-state CO molecules, and for the triplet species, this dissociation involves facile curve-crossing to the singlet surface within a few nanoseconds.

Charge reversal (CR) and neutralization reionization (NR) experiments carried out on a 4-sector mass spectrometer demonstrate that isotopically labeled, linear C 4 anion rearranges upon collisional oxidation. The cations and neutrals formed in these experiments exhibit differing degrees of isotopic scrambling in their fragmentation patterns, indicative of (at least) partial isomerization of both states. Theoretical studies, employing the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory, favor conversion to the rhombic C 4 isomer on both cationic and neutral potential-energy surfaces with the rhombic structures predicted to be slightly more stable than the linear forms in each case. The combination of experiment with theory indicates that the elusive rhombic C 4 is formed as a cation and as a neutral following charge stripping of linear C 4 -.

Three different radical anions of the empirical formula C5H2 have been generated by negative ion chemical ionization mass spectrometry in the gas phase. The isomers C4CH2 •-, C2CHC2H•-, and HC5H•- have been synthesized by unequivocal routes and their connectivities confirmed by deuterium labeling, charge reversal, and neutralization reionization experiments. The results also provide evidence for the existence of neutrals C4CH2, C2CHC2H, and HC5H as stable species; this is the first reported observation of C2CHC2H. Ab initio calculations confirm these structures to be minima on the anion and neutral potential energy surfaces.