title: Formation of a(1) ions directly from oxazolone b(2) ions: an energy-resolved and computational study.
abstract: It is well-known that oxazolone b2 ions fragment extensively by elimination of CO to form a2 ions, which often fragment further to form a1 ions. Less well-known is that some oxazolone b2 ions may fragment directly to form a1 ions. The present study uses energy-resolved collision-induced dissociation experiments to explore the occurrence of the direct b2→a1 fragmentation reaction. The experimental results show that the direct b2→a1 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 a2 product in the b2→a2 fragmentation pathway. Our computational studies of the relative critical reaction energies for the b2→a2 reaction compared with those for the b2→a1 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 b2 ion does not bear a hydrogen, as in the Ala-Sar and Tyr-Sar (Sar = N-methylglycine) oxazolone b2 ions, a1 ions are not formed but rather neutral imine elimination from the N-terminus of the b2 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 SN2-like b2→a1 reaction. The difference between the two methods of barrier calculation are highly significant (P < 0.001) for the b2→a1 reaction, but only marginally significant (P = 0.05) for the b2→a2 reaction. The computations provide further evidence of the limitations of the B3LYP functional when describing SN2-like reactions.
authors: Department of Chemistry and Biochemistry, University of Missouri-St. Louis, St. Louis, MO, 63131, USA, bythellb@umsl.edu.
authors: Benjamin J
authors: BJ
authors: Bythell
authors: Alex G
authors: AG
authors: Harrison
mesh_terms: D019295:Computational Biology; D004151:Dipeptides / Q000032:analysis / Q000737:chemistry; D004735:Energy Transfer; D008958:Models, Molecular; D015394:Molecular Structure; D009842:Oligopeptides / Q000737:chemistry; D010081:Oxazolone / Q000031:analogs & derivatives* / Q000032:analysis / Q000737:chemistry; D055550:Protein Stability; D012521:Sarcosine / Q000032:analysis / Q000737:chemistry; D021241:Spectrometry, Mass, Electrospray Ionization; D013237:Stereoisomerism; D053719:Tandem Mass Spectrometry; D014835:Volatilization
chemical_list: D004151:Dipeptides; D009842:Oligopeptides; D010081:Oxazolone; D012521:Sarcosine