Electrospray-ionization mass spectrometry .3. Acid-catalyzed isomerization of N,N'-bis [(E)-3-(4-hydroxyphenyl)prop-2-enoyl]spermidines by the zip reaction

The electrospray tandem mass spectra (ESI-MS/MS) of the three N,N'-bis[(E)-3-(4-hydroxyphenyl)prop-2-enoyl]spermidines 1-3 displayed the same fragment-ion signals. These isomers could not be differentiated by ESI-MS/MS, since their fragmentation patterns are similar. (E,E)-N-(3-[(15)N]Aminopropyl)-3,3'-bis(4-hydroxyphenyl)-N,N'-(butane-1,4-diyl)bis[prop-2-enamide]([N-15(1)]-1) was synthesized in order to get further information about the fragmentation mechanisms. The comparison of the ESI-MS/MS of 1 and [N-15(1)]-1 revealed a transamidation, the Zip reaction, under mass-spectral conditions of the [1 + H](+) ions. Because of this reaction, the three isomers 1-3 could not be distinguished.