Differentiation of heterocyclic regioisomers: a combined tandem mass spectrometry and computational study of N‐acridin‐4‐ylbenzylamide and N‐acridin‐2‐yl‐benzylamide

Abstract

Electrospray ionization (ESI) tandem mass spectrometry (MS/MS) has been used to differentiate two positional isomers of acridine derivatives, N-acridin-4-ylbenzylamide and N-acridin-2-ylbenzylamide. The study revealed that the isomeric ion structures produced by these heterocycles could be distinguished upon collision-induced dissociations (CID). In particular, the loss of a water molecule was shown to be a regiospecific reaction of the protonated N-acridin-4-ylbenzylamide, in which the location of the benzylamide substituent with respect to the acridinic nitrogen greatly assists proton migration by allowing the creation of intramolecular hydrogen bonds. To a lesser extent, the two isomers could also be distinguished by the difference in the abundance of the benzoyl cation in the MS/MS spectra of the [M+H]+ ions, as this ion is produced with a much higher rate from N-acridin-4-ylbenzylamide. Calculations based on quantum-mechanical models have been performed to evaluate the stability of the ion structures and to support mechanisms proposed for these two dissociation reactions.

Canα- andβ-alanine containing peptides Be distinguished based on the CID spectra of their protonated ions?

“…The resultant ion-molecule complex separates to give the imine (10). A very similar mechanism for water loss from protonated N-acridin-4-ylbenzylamide has been recently been reported based on DFT calculations [28].…”

Mass spectrometric‐based investigation of differentially protected azatryptophan derivatives using Orbitrap mass spectrometry: Differentiation of positional isomers under protonation and alkali‐cationization conditions

“…Besides, they help in obtaining abundant structural information that would be useful in the interpretation of structures of unknown compounds. [9][10][11][12][13][14][15][16][17][18] In 2004, Roche et al 9 reported a direct MS/MSbased method using gas-phase ion-molecule reactions with acetonitrile for the identification and differentiation of positional isomers of ortho, meta and para acyl and amidyl anilines or phenols or derivatives. Nagi Reddy et al 10 reported the differentiation of positional isomers of substituted aromatic sulfonic acids via ortho effects.…”

“…Nagi Reddy et al 10 reported the differentiation of positional isomers of substituted aromatic sulfonic acids via ortho effects. A series of aromatic aldehydes with electron-withdrawing groups and electron-donating groups were studied with methanol as the solvent by Wang et al 11 13 We have recently demonstrated the synthesis and characterization of a series of N,N 0 -substituted urea derivatives using ESI-MS/MS followed by differentiation of positional isomers. 19 In continuation of the development of mass spectrometric-based approaches for the identification and differentiation of isomeric compounds, here we report a detailed mass spectral study of a new series of positional isomers of 7-azatryptophan derivatives which were differentially protected with fluorenylmethyloxycarbonyl (Fmoc) for the amine, tert-butyl (tBu) for the carboxylic acid and tosyl (Ts) for the indole side chain (Scheme 1).…”

“…Over the past few decades, there has been growing interest in tandem mass spectrometric (MS/MS)‐based methods for the identification and differentiation of isomeric compounds, as such methods have several advantages such as rapid analysis, low detection limit and feasible transfer to chromatographic techniques. Besides, they help in obtaining abundant structural information that would be useful in the interpretation of structures of unknown compounds 9–18 . In 2004, Roche et al 9 reported a direct MS/MS‐based method using gas‐phase ion–molecule reactions with acetonitrile for the identification and differentiation of positional isomers of ortho , meta and para acyl and amidyl anilines or phenols or derivatives.…”

“…Recently, Butler and Cabrera 12 reported a study on a series of 11 pairs of positional isomers of mono‐substituted pyrazine N ‐oxides using ESI mass spectrometry and showed the differentiation of these regioisomeric N ‐oxides by comparison of the identity and relative intensities of the peaks originating from the adduct ions formed with metal ions. ESI‐MS/MS was used to differentiate two positional isomers of acridine derivatives and calculations based on quantum‐mechanical models were performed to evaluate the stability of the ion structures and to support the mechanisms proposed 13 …”