Picolinyl ester fragmentation mechanism studies with application to the identification of acylcarnitine acyl groups following transesterification

Abstract

Acyl picolinyl esters provide excellent data to identify the structures of acyl groups. However, the mechanisms for the formation of fragment ions from picolinyl esters are unsettled. Proposed structures for fragment ions have focused on long-chain groups and may not accommodate results from medium-and short-chain acyl groups. Using deuterium-labeled organic acids, we have investigated the mechanisms for the formation of fragment ions. Based on these studies, we propose a new mechanism that is consistent with the experimental data. We then tested the mechanisms by analyzing selected acylcarnitines. Transesterification of acylcarnitines was performed by reaction with 3-pyridylcarbinol and potassium tert-butoxide in dichloromethane to produce acyl picolinyl esters. The picolinyl esters were separated and detected by gas chromatography/electron ionization-mass spectrometry. Each mass spectrum contained a series of peaks with m/z differences of 12, 13, or 14 u depending on the acyl group's chemical structure. [4] in which the fragment ions of an acyl picolinyl ester result from the interaction of alkyl hydrogens with the ionized nitrogen on the pyridine ring. The mechanism invoking a McLafferty rearrangement, proposed by Harvey for the formation of the fragment ions from long-chain acyl groups, is not supported by data from Christie's laboratory [4, 5]. Furthermore, these mechanistic studies have not been conducted using either short-or medium-chain acyl groups.Acyl group identification is an important component in many analytical methods. Several reports detail procedures to identify the acyl groups of acylcarnitines.Lowes and Rose [6,7] reported that by converting acylcarnitines into acyloxylactones, partial, but not complete, structural information of the acyl group was obtained. Libert et al. [8 -10] used a procedure whereby acylcarnitines were extracted from urine and hydrolyzed, followed by conversion of the organic acids into acyl picolinyl esters. These acyl picolinyl esters were then characterized by GC/MS, which provided better structural data than in the case of acyloxylactones. However, the presence of organic acids in urine makes questionable the interpretation of the source of acyl groups as being exclusively from acylcarnitines. Although acyl picolinyl esters usually are produced from organic acids, transesterification has been reported to produce acyl picolinyl esters from lipids by direct reaction with 3-pyridylcarbinol, thus eliminating the need to hydrolyze the lipids first [11].In this paper, we propose mechanisms for the formation of the fragment ions of acyl picolinyl esters based on experiments using deuterium-labeled organic acids. We applied these results to the characterization of acyl groups of several acylcarnitine species following their transesterification to acyl picolinyl esters. The transesterification was performed by reacting acylcarnitines with 3-pyridylcarbinol and potassium tert-

cis-3,4-Methylene-heptanoylcarnitine: Characterization and verification of the C8:1 acylcarnitine in human urine

“…The method of Destaillats and Angers [18] was adapted for the transesterification of acylcarnitines [8]. Briefly, the residue containing acylcarnitines was reconstituted in 1.0 ml of anhydrous dichloromethane and reacted with a freshly prepared mixture of 100 l of 1.0 M potassium tert-butoxide in tetrahydrofuran and 200 l of 3-pyridylcarbinol.…”

“…We also have shown that the GC/MS spectra of the resulting acyl picolinyl esters can be used to deduce the chemical structure of corresponding acylcarnitines [8]. Meanwhile, the fragmentation mechanism of the 3-picolinyl 1570-0232/$ -see front matter © 2007 Elsevier B.V. All rights reserved.…”

“…(4) GC/EI-MS. The picolinyl esters of transesterified acylcarnitines, when subjected to EI type ionization, yield rich fragmentation patterns which allows for the detailed characterization of the acylcarnitine acyl group [8]. (5) Acylcarnitine analysis by HPLC/MS.…”

Assessment of the Ratio of Geometric Isomers of Dibenzalacetone Spectroscopically

“…Some fragments might be neutralized and thus not detected. The MS spectrum for DBA shows several signals that resulted from either cleaving or rearrangement in gas phase by mechanisms known as α-cleavage, β-cleavage, and Rearrangement migration shown [11][12][13][14] in schematic 3.…”

Analysis of Butyl Butyrate Mass Spectrum

“…The fragmentation mechanisms of all major peaks will comprehensively be analyzed (Yang, Minkler, Hoppel, & Tserng, 2006).…”