Collisionally activated dissociation (CAD) of the protonated polyalanines Ala-Ala,Ala-Ala-Ala, and Ala-Ala-Ala-Ala causes breakup of the peptide bonds leading to sequence-indicative fragment ions. The neutral molecules eliminated during these reactions are identified here using neutralization-reionization mass spectrometry (NRMS). N-terminal acylium ions (bn) arise after the C-terminus is lost as an intact amino acid or peptide; further loss of CO leads to immonium ions (an). Upon generation of C-terminal sequence ions (yn), a hydrogen atom attached to a nitrogen rearranges from the N-terminal to the C-terminal side yielding a protonated amino acid (y1) or peptide (y > or = 2) as the ionic fragment; the complementary neutral fragment is an aziridinone if the N-terminal amino acid is cleaved and a diketopiperazine if two N-terminal amino acid units are eliminated. Detection of neutral dissociation products can reveal valuable structure information, as demonstrated with the tetrapeptides Val-Gly-Ser-Glu and Val-Gly-Asp-Glu.
Benzo[a]pyrene 1 and dibenzo [a,e]pyrene 2 are monoprotonated with FS03H-S02ClF to give persistent arenium ions of protonation at C-6 (1H') and C-8 (2H+), respectively. The low temperature protonation of benzo[e]pyrene 3 under a variety of conditions produced a mixture of arenium ions of attack at C-1 (3aH') and C-3 (3bH').The charge distribution pattern in the resulting arenium ions (as probed by low temperature I3C and 'H NMR spectroscopy) remains very much analogous to those of alkyl(cycloalkyl)pyrenes, showing significant phenalenium ion character (predominant charge alternation at the periphery of the pyrene moiety).The arenium ion energies and charges were examined by semiempirical theory (AM 1). For 1, in agreement with experiment, protonation at C-6 is most favoured and the changes in charge distribution A Q [q,(ion)q,(neutral)] indicated a very distinct phenalenium unit. Attack at other sites is less favoured, with that at C-2 being least likely. The extent of positive charge delocalisation becomes more limited as the arenium ion energies increase.In accord with experiment, AM1 calculations indicate that for 2, the arenium ion of protonation at C-8 is the most stable followed by those of C-1 and C-3 which are 4.5 and 5.1 kcal mol-' less stable. Charge delocalisation away from the protonation site in these cations is predicted to be more extensive and involve a phenalenium moiety.For 3, in line with experiment, the arenium ion energies for attack at C-1 and C-3 are almost identical and most favoured.The results indicate that despite the pronounced effect that benzannellation has on carcinogenicity, the preferred site(s) of electrophilic attack are those which lead to the most highly delocalised carbocations. Among these, formation of a robust phenalenium ion moiety is always favoured. Possible implications of these findings in relation to carcinogenesis models mediated by arenium ions are discussed.The oxidation dications of 1-2 were also probed by the AM1 method.
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