title: Extensive fragmentation of pheophytin-a by infrared multiphoton dissociation tandem mass spectrometry.
abstract: BACKGROUND
  In a previous tandem mass spectrometry (MS/MS) study on chlorophyll-a, infrared multiphoton dissociation (IRMPD) was demonstrated as a more effective fragmentation method than collision-induced dissociation (CID) and electron-induced dissociation (EID), where odd-electron product ions were observed ubiquitously in CID and IRMPD. To further understand the role of the macrocycle and the central Mg atom in the MS/MS process, the fragmentation behaviour of pheophytin-a, the Mg-free chlorophyll-a, was investigated.
  
  
  METHODS
  CID, IRMPD, and EID were applied to the singly protonated pheophytin-a using an ultra-high-resolution Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. The result is compared with the MS/MS study of chlorophyll-a.
  
  
  RESULTS
  For pheophytin-a, some different fragmentation patterns from chlorophyll-a were obtained by all three MS/MS methods, but IRMPD still appears the most efficient method of generating product ions. The detection of odd-electron fragments in the CID and IRMPD spectra of protonated pheophytin-a suggests that the macrocyclic structure effectively stabilizes radicals, and these radical ions seem to have a relatively higher abundance in the presence of the central Mg atom.
  
  
  CONCLUSIONS
  The strong absorption in the infrared region of pheophytin-a and secondary free radical rearrangement are proposed to explain the extensive frgmentation in IRMPD spectra. In addition, a comparison of the IRMPD spectra of chlorophyll-a and pheophytin-a shows that the macrocycle in the absence of the Mg atom is much more fragile.
authors.affiliation: Department of Chemistry, University of Warwick, Coventry, CV4 7AL, UK.
authors.forename: Juan
authors.initials: J
authors.lastname: Wei
authors.affiliation: Department of Chemistry, University of Warwick, Coventry, CV4 7AL, UK.
authors.forename: Peter B
authors.initials: PB
authors.lastname: O'Connor
mesh_terms: D007477:Ions / Q000032:analysis / Q000737:chemistry; D008958:Models, Molecular; D010674:Pheophytins / Q000032:analysis* / Q000737:chemistry*; D053719:Tandem Mass Spectrometry / Q000379:methods*
chemical_list: D007477:Ions; D010674:Pheophytins; C061694:pheophytin a