Dissociation of [b5]+ ions containing an α-methyltryptophan and four alanine residues; losses of CO2 and the oxazolone ring

Abstract The collision-induced dissociation spectra of isomeric [b 5 ] + ions consisting of an α-methyltryptophan and four alanine residues are very similar, indicating that there is a substantial amount of isomerization prior to fragmentation. The major dissociation products are the combination of the [b 4' ] + ion composed of four alanine residues and the complementary [a 1 ] + ion, the imine derived from α-methyltryptophan. These products indicate that the ion undergoing dissociation has the structure [AAAAW α-Me oxa  + H] + . At higher collision energies the ion produced by loss of CO 2 is the most abundant and isotopic labeling established that one of the oxygen atoms lost is from the alanine residue located at the N-terminal side of the α-methyltryptophan. Density functional calculations at the B3LYP/6–31++G(d,p) level show the barrier to loss of CO 2 from [AAAAW α-Me oxa  + H] + to be 44.2 kcal mol −1 , only 8.9 kcal mol −1 higher than the energy required to generate the [a 1 ] + ion. Another higher energy dissociation pathway has the concomitant losses of water and an oxazolone that originate from the alanine residues on either side of the α-methyltryptophan of [AAAW α-Me A oxa  + H] + . The effect of the α-methyl substituent on the tryptophan is assessed by comparison with the CID spectra of [b 5 ] + ions containing normal tryptophan residue.