title: Novel formation of [2M-H](+) species in positive electrospray mass spectra of indoles.
abstract: BACKGROUND
  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.
authors.affiliation: Chemical and Forensic Sciences, University of Bradford, Great Horton Road, Bradford, BD7 1DP, UK.
authors.forename: Amie
authors.initials: A
authors.lastname: Saidykhan
authors.forename: Stephen T
authors.initials: ST
authors.lastname: Ayrton
authors.forename: Richard T
authors.initials: RT
authors.lastname: Gallagher
authors.forename: William H C
authors.initials: WH
authors.lastname: Martin
authors.forename: Richard D
authors.initials: RD
authors.lastname: Bowen
mesh_terms: D019281:Dimerization; D007211:Indoles / Q000032:analysis* / Q000737:chemistry*; D009330:Nebulizers and Vaporizers; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*
chemical_list: D007211:Indoles