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 [b5]+ 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 [b4']+ ion composed of four alanine residues and the complementary [a1]+ ion, the imine derived from α-methyltryptophan. These products indicate that the ion undergoing dissociation has the structure [AAAAWα-Meoxa+H]+. At higher collision energies the ion produced by loss of CO2 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 CO2 from [AAAAWα-Meoxa+H]+ to be 44.2kcalmol−1, only 8.9kcalmol−1 higher than the energy required to generate the [a1]+ 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 [AAAWα-Me Aoxa+H]+. The effect of the α-methyl substituent on the tryptophan is assessed by comparison with the CID spectra of [b5]+ ions containing normal tryptophan residue.