Formation of a₁ Ions Directly from Oxazolone b₂ Ions: an Energy-Resolved and Computational Study

Abstract

Abstract. It is well-known that oxazolone b 2 ions fragment extensively by elimination of CO to form a 2 ions, which often fragment further to form a 1 ions. Less well-known is that some oxazolone b 2 ions may fragment directly to form a 1 ions. The present study uses energy-resolved collision-induced dissociation experiments to explore the occurrence of the direct b 2 →a 1 fragmentation reaction. The experimental results show that the direct b 2 →a 1 reaction is generally observed when Gly is the C-terminal residue of the oxazolone. When the C-terminal residue is more complex, it is able to provide increased stability of the a 2 product in the b 2 →a 2 fragmentation pathway. Our computational studies of the relative critical reaction energies for the b 2 →a 2 reaction compared with those for the b 2 →a 1 reaction provide support that the critical reaction energies are similar for the two pathways when the C-terminal residue of the oxazolone is Gly. By contrast, when the nitrogen of the oxazolone ring in the b 2 ion does not bear a hydrogen, as in the Ala-Sar and Tyr-Sar (Sar=N-methylglycine) oxazolone b 2 ions, a 1 ions are not formed but rather neutral imine elimination from the N-terminus of the b 2 ion becomes a dominant fragmentation reaction. The M06-2X/6-31+G(d,p) density functional theory calculations are in general agreement with the experimental data for both types of reaction. In contrast, the B3LYP/6-31+G(d,p) model systematically underestimates the barriers of these S N 2-like b 2 →a 1 reaction. The difference between the two methods of barrier calculation are highly significant (PG0.001) for the b 2 →a 1 reaction, but only marginally significant (P=0.05) for the b 2 →a 2 reaction. The computations provide further evidence of the limitations of the B3LYP functional when describing S N 2-like reactions.

Water Loss from Protonated XxxSer and XxxThr Dipeptides Gives Oxazoline—Not Oxazolone—Product Ions

“…The m / z 116 product ion (neutral loss of 43, ethanimine, CH 3 CH=NH) suggests detachment of the ethylamine side-chain, 52 which is only plausible for the oxazolone and oxazoline structures. Neutral loss of the N-terminus as an alkylimine, rather than neutral loss of CO, has been shown to be diagnostic for b-type ions that are not oxazolones protonated on the oxazolone N. 36 , 60 Here, further evidence for a non-oxazolone structure is provided by the absence in the experimental spectrum of the main predicted IR band of oxazolone structure 2.4 near 1930 cm –1 (oxazolone ring C=O stretch). Again, this leaves the oxazoline structure as the only plausible product ion structure.…”

Interconversion between 4-Imidazolone Ions; Isomers of [b4]+ Derived from Protonated Tetraglycine

“…Loss of HN=CH 2 from the imidazolone (structure I) formed by loss of water from the first amide is easily rationalized in terms of C α −C bond cleavage adjacent to the fivemembered ring as it is analogous to the formation of an [a 1 ] + ion from a [b 2 ] + ion. 37 Loss of HN=CH 2 from the second residue requires rearrangement of structure II into I, thereby moving the second residue to the N-terminus. A possible pathway for rearrangement of II is given in Scheme 1.…”

Protonated α-N-Acetyl Galactose Glycopeptide Dissociation Chemistry

“…All three models consistently predict that the furanose-forming mechanism is the most likely (Table S2). Like the Kuo group, we find that the newer models, which contain dispersion terms and larger proportions of exact exchange, produce larger barriers for gas-phase glycan reactions. ,, Here, the key reactions are S N 2-like, a known weakness of the B3LYP model, which systematically underestimates S N 2 reaction barriers. − …”