The fragmentation of protonated tyrosine and iodotyrosines: The effect of substituents on the losses of NH3 and of H2O and CO

The gas-phase dissociation chemistries of protonated 3-iodo-l-tyrosine, 3,5-diiodo-l-tyrosine and 3,3′,5,5′-tetraiodo-thyronine (thyroxine) have been examined using a combination of tandem mass spectrometry and density functional theory (DFT) calculations. It was found that, at low collision energy, all protonated tyrosines exhibit common fragmentation pathways, including the competitive eliminations of NH3 and the concomitant loss of H2O and CO, but there are significant differences in relative abundances, depending on the combined electron-donating abilities of the substituents in the phenyl ring. The ions initially formed by loss of NH3 are phenonium ions, but subsequent fragmentation is most easily understood in terms of the isomeric benzyl cation structures. These [M + H − NH3]+ ions fragment at relatively low collision energies, mainly by loss of ketene; by contrast, the [M + H − H2O − CO]+ ions are more stable towards dissociation. At higher collision energies, losses of one, two and even three iodine atoms were observed. DFT calculations (at the B3LYP/DZVP level of theory) were performed on protonated 3-iodotyrosine to compare the reaction profiles for the fragmentation mechanisms. The iodo-substituent in the 3-position is weakly electron-withdrawing and this results in a barrier (27.5 kcal/mol at 0 K) that is slightly higher than that for protonated tyrosine (26.8 kcal/mol). The phenoxy group PhO– is a weaker electron-donor than HO– and protonated 3,5-diiodo-4-phenoxytyrosine has an even higher barrier (31.1 kcal/mol) to NH3 loss than protonated 3,5-diiodotyrosine (28.8 kcal/mol). Linear free energy plots for ΔH0°‡ and ΔG298°‡ against σ+ for the four protonated tyrosine derivatives show good correlations. More importantly, as the products of the dissociation are higher in energy than the transition states to their formation, the plots of ΔH0° and ΔG298° for the overall reaction for NH3 loss also correlate very well with σ+ (correlation coefficients of 0.99 and 0.98, respectively). The positive slopes of these Hammett plots show that the barriers to the loss of NH3 by the neighboring-group mechanism are increased by the presence of electron-withdrawing groups in the phenyl rings.