title: Role of 2-oxo and 2-thioxo modifications on the proton affinity of histidine and fragmentation reactions of protonated histidine.
abstract: A combination of electrospray ionisation (ESI), multistage and high-resolution mass spectrometry experiments was used to compare the gas-phase chemistry of the amino acids histidine (1), 2-oxo-histidine (2), and 2-thioxo-histidine (3). Collision-induced dissociation (CID) of all three different proton-bound heterodimers of these amino acids led to the relative gas-phase proton affinity order of: histidine >2-thioxo-histidine >2-oxo-histidine. Density functional theory (DFT) calculations confirm this order, with the lower proton affinities of the oxidised histidine derivatives arising from their ability to adopt the more stable keto/thioketo tautomeric forms. All protonated amino acids predominately fragment via the combined loss of H(2)O and CO to yield a(1) ions. Protonated 2 and 3 also undergo other small molecule losses including NH(3) and the imine HN=CHCO(2)H. The observed differences in the fragmentation pathways are rationalised through DFT calculations, which reveal that while modification of histidine via the introduction of the oxygen atom in 2 or the sulfur atom in 3 does not affect the barriers against the loss of H(2)O+CO, barriers against the losses of NH(3) and HN=CHCO(2)H are lowered relative to protonated histidine.
authors.affiliation: School of Chemistry, The University of Melbourne, Victoria 3010, Australia.
authors.forename: Adrian K Y
authors.initials: AK
authors.lastname: Lam
authors.forename: Craig A
authors.initials: CA
authors.lastname: Hutton
authors.forename: Richard A J
authors.initials: RA
authors.lastname: O'Hair
mesh_terms: D006639:Histidine / Q000031:analogs & derivatives / Q000737:chemistry; D007477:Ions / Q000737:chemistry; D007536:Isomerism; D008956:Models, Chemical; D011522:Protons; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*; D053719:Tandem Mass Spectrometry
chemical_list: D007477:Ions; D011522:Protons; D006639:Histidine