Marked difference in fragmentation between collision-induced excitation and chemi-excitation of keto esters produced from dioxetanes bearing a 4-(benzothiazol-2-yl)-3-hydroxyphenyl moiety in negative-mode matrix-assisted laser desorption/ionization time-of-flight tandem mass spectrometry

摘要

A dioxetane bearing a hydroxyphenyl group produces an unstable oxidoaryl anion by deprotonation which rapidly decomposes with accompanying emission of light effectively by the intramolecular charge-transfer-induced decomposition (CTID) mechanism. Although several mechanisms have been proposed to explain chemi-excitation in CTID, strong experimental evidence is still lacking. In the course of our investigation to clarify the chemi-excitation process, negative-mode matrix-assisted laser desorption/ionization time-of-flight collision-induced dissociation tandem mass spectrometry (MALDI-TOF-CID-MS/MS) was used to investigate the decomposition of bicyclic dioxetanes bearing a 4-(benzothiazol-2-yl)-3-hydroxyphenyl moiety 1 and their related keto esters 2 in a gas phase. Dioxetanes 1 decomposed to give 2 in an electronically excited state, which underwent α-cleavage of a ketone moiety, while authentic 2 (ground state) hardly showed fragmentation in MS1. On the other hand, 2 displayed fragment ions in CID-MS/MS, though the fragmentation pattern was significantly different between 2a (R?=?tert-butyl) and 2b–2d (R?=?isopropyl, ethyl and methyl, respectively): 2a exhibited mainly α-cleavage of a ketone moiety, while 2b–2d showed β-cleavage of the aromatic ester moiety. The marked difference in fragmentation between 2 (electronically excited state) produced directly from 1 and authentic 2 under CID was most likely due to the difference in the excitation processes: chemi-excitation of 2 by CTID of 1 versus vibrational excitation of 2 induced by collision in MS/MS.