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Novel formation of [2M–H] + species in positive electrospray mass spectra of indoles
Author(s) -
Saidykhan Amie,
Ayrton Stephen T.,
Gallagher Richard T.,
Martin William H. C.,
Bowen Richard D.
Publication year - 2014
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.6976
Subject(s) - chemistry , electrospray ionization , substituent , electrospray , mass spectrometry , mass spectrum , alkyl , covalent bond , ion , stereochemistry , medicinal chemistry , crystallography , organic chemistry , chromatography
RATIONALE When subjected to positive ion electrospray ionisation (ESI+) mass spectrometry (MS), indoles with a 3‐alkyl substituent show a propensity to form novel [2M–H] + 'covalently bound dimers'. This process, which appears to be initiated in the nebuliser of the instrument, is mechanistically interesting, analytically useful and potentially significant in organic synthesis. METHODS A selection of 2‐ and 3‐substituted indoles have been synthesised and analysed by ESI‐MS. The formation of the 'homo' and 'hetero' dimers of these compounds has been investigated using ESI+ mode. The mechanism of formation of the observed 'dimeric' species has been probed by synthesising authentic samples of the dimeric compounds. RESULTS 'Dimeric' species corresponding to [2M–H] + have been observed for all 3‐substituted indoles studied, but not for indoles substituted in just the 2‐position. By infusing equimolar mixtures of labelled and unlabelled indoles through the instrument, the expected approximately statistical mixture of homo‐ and heterodimeric species has been observed. Further experiments have established that this novel dimerisation occurs in the droplets formed in the nebuliser of the instrument. CONCLUSIONS It has been shown that 3‐substituted indoles form [2M–H] + dimers in high abundance in the spray obtained from the nebiliser of an ESI+ instrument. The mechanism for the dimerisation does not involve the known 2M dimeric species that is readily formed in the solution‐phase chemistry of indoles. Copyright © 2014 John Wiley & Sons, Ltd.