Fragmentation of deprotonated diacylhydrazine derivatives in electrospray ionization tandem mass spectrometry: generation of acid anions via intramolecular rearrangement.

Kezhi Jiang,Jianmei Wang,Mingrong Qian,Hu Zhang,Fei Li

doi:10.1371/journal.pone.0063097

Abstract

The gas-phase fragmentation pathways of deprotonated diacylhydrazine derivatives (R1(C = O)-N(t-Bu)NH(C = O)R2, Compounds 1–6) were investigated by the combination of electrospray ionization tandem mass spectrometry (ESI-MS/MS) and theoretical calculations. Upon collisional activation, the deprotonated molecular ions [M – H]− dissociate in two reaction channels, both of which involve intramolecular rearrangement. The main product ion is confirmed to be an anionic acid species, [R1-CO2]−, generated through intramolecular rearrangement of [M – H]− initiated by the nucleophilic attack of the amide O6 on the carbonyl C2 (Path-1). The minor fragment channel (Path-2) involves methylpropene elimination of the precursor ion, followed by a similar nucleophilic displacement reaction to produce another acid anion [R2-CO2]−. Density functional theory calculations at the B3LYP/6-31+G(d,p) level indicate that Path-1 is more favorable than Path-2 for dissociation of the deprotonated halofenozide.

Highlights

Derivatives of diacylhydrazine (Figure 1) were widely-used as insect growth regulators against Lepidoptera pests in agriculture, which induce premature molting and cause insect death by mimicking their hormones [1,2,3,4,5]
Methods have been developed to determine and quantify these diacylhydrazine residues in fruits and vegetables, of which liquid chromatography-electrospray tandem mass spectrometry (HPLCESI-MS/MS) is the most powerful and widely-used analytical tool [6,7,8,9,10,11,12,13]. These compounds, other than halofenozide, were mainly investigated by the positive ion ESI tandem mass spectrometry, and the results showed that the major dissociation reactions of protonated diacylhydrazines observed in MS/MS were the cleavage of the amide bond and the loss of methylpropene from the tert-butyl group via proton transfer [6,7,8,9,10,12]
A few recent studies of diacylhydrazines were reported to be detected in the negative ion mode [11,13]

Summary

Introduction

Derivatives of diacylhydrazine (Figure 1) were widely-used as insect growth regulators against Lepidoptera pests in agriculture, which induce premature molting and cause insect death by mimicking their hormones [1,2,3,4,5]. Methods have been developed to determine and quantify these diacylhydrazine residues in fruits and vegetables, of which liquid chromatography-electrospray tandem mass spectrometry (HPLCESI-MS/MS) is the most powerful and widely-used analytical tool [6,7,8,9,10,11,12,13] These compounds, other than halofenozide, were mainly investigated by the positive ion ESI tandem mass spectrometry, and the results showed that the major dissociation reactions of protonated diacylhydrazines observed in MS/MS were the cleavage of the amide bond and the loss of methylpropene from the tert-butyl group via proton transfer [6,7,8,9,10,12]. The purpose of the present work is to perform a detailed mechanistic investigation of the fragmentation pathways of the deprotonated diacylhydrazines in the negative ion mode

Objectives

Methods

Results

Conclusion