Formation of iminium ions by fragmentation of a2 ions

Tandem mass spectrometric experiments have been carried out on the protonated amides H-Gly-Ala-NH2, H-Ala-Gly-NH2, H-Ala-Val-NH2, H-Val-Ala-pNA, H-Leu-Phe-NH2, H-Phe-Leu-NH2, H-Phe-Tyr-NH2 and H-Tyr-Phe-NH2 with particular emphasis on the fragmentation of the isomeric a(2) ions derived therefrom. Primary fragmentation reactions of the protonated amides involve formation of the y(1)" and b(2) ions with further fragmentation of the b(2) ion to form the a(2) ion which fragments to form iminium ions. Collision-induced dissociation studies of the mass-selected a(2) ions were carried out. For the Gly-Ala, Ala-Gly and Val-Ala a(2) ions, weak signals were observed corresponding to loss of CO from the a(2) ion. With the exception of the Gly-Ala, Ala-Gly and Val-Ala a(2) ions, both possible iminium ions (a(1) and the internal iminium ion) are observed with the most abundant being that formed by proton attachment to the imine of higher proton affinity. The results provide strong support for the recently proposed (El Aribi et al. J. Am. Chem. Soc. 2003; 125: 9229) mechanism of fragmentation of a(2) ions which involves elimination of CO from the a(2) ion to form a proton-bound complex of two imines. Based on this mechanism ab initio calculations of the total energies of the a(2) ions and the transition states for fragmentation have been carried out giving the energy barrier for fragmentation of each a(2) ion. The experimental results are interpreted in terms of these energetics data, unimolecular rate constants calculated by using the RRKM theory, and the imine proton affinities. Copyright (C) 2004 John Wiley Sons, Ltd.