title: Electronic structure and gas-phase chemistry of protonated α- and β-quinonoid compounds: a mass spectrometry and computational study.
abstract: BACKGROUND
  The use of quinonoid compounds against tropical diseases and as antitumor agents has prompted the search for new naturally occurring and synthetic derivatives. Among these quinonoid compounds, lapachol and its isomers (α- and β-lapachone) serve as models for the synthesis of new compounds with biological activity, and the use of electrospray ionization tandem mass spectrometry (ESI-MS/MS) analysis as a tool to elucidate and characterize these products has furnished important information about these compounds.
  
  
  METHODS
  ESI-MS/MS analysis under collision-induced dissociation conditions was used to describe the fragmentation mechanisms for protonated 1,4-naphthoquinone, 1,2-naphthoquinone, α-lapachone, and β-lapachone. The B3LYP/6-31+G(d,p) model was used to obtain proton affinities, gas-phase basicities, and molecular electrostatic potential maps, thus indicating the probable protonation sites. Fragmentation pathways were suggested on the basis of the relative enthalpies of the product ions.
  
  
  RESULTS
  The ESI-MS signals of the cationized molecules of ortho quinonoid compounds were more intense than those of the protonated molecule. Formation of the major product ions with m/z 187 from protonated α- and β-lapachone has been attributed to a retro-Diels-Alder (RDA) reaction.
  
  
  CONCLUSIONS
  MS/MS studies on lapachol isomers (α- and β-lapachone) will facilitate the interpretation of the liquid chromatography (LC)-MS/MS analysis of new metabolites. MS/MS data on the 1,4-naphthoquinone, 1,2-naphthoquinone, α-lapachone and β-lapachone core will help characterize new derivatives from in vitro/in vivo metabolism studies in complex matrices. The product ions revealed the major fragmentation mechanisms and these ions will serve as diagnostic ions to identify each studied compound.
authors.affiliation: Núcleo de Pesquisa em Produtos Naturais e Sintéticos, Departamento de Física e Química, Faculdade de Ciências Farmacêuticas de Ribeirão Preto, Universidade de São Paulo, Brasil. vessecchi@bol.com.br
authors.forename: Ricardo
authors.initials: R
authors.lastname: Vessecchi
authors.forename: Flávio S
authors.initials: FS
authors.lastname: Emery
authors.forename: Norberto P
authors.initials: NP
authors.lastname: Lopes
authors.forename: Sérgio E
authors.initials: SE
authors.lastname: Galembeck
mesh_terms: D005740:Gases / Q000737:chemistry; D008958:Models, Molecular; D009285:Naphthoquinones / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*; D053719:Tandem Mass Spectrometry / Q000379:methods*
chemical_list: D005740:Gases; D009285:Naphthoquinones