title: Fourier transform ion cyclotron resonance mass spectrometric analysis of the green fraction of soil humic acids.
abstract: BACKGROUND
  The green fraction of soil humic acids (HAs), designated Pg, contains 4,9-dihydroxyperylene-3,10-quinone (DHPQ) as a chromophore. Although various naturally occurring DHPQ derivatives are known to occur and Pg-like absorption is observed in the UV-visible spectra of HAs from various types of soils worldwide, the chemical structure of Pg is still unknown.
  
  
  METHODS
  For a better understanding of the chemical composition of Pg, the pre-isolated (crude Pg) and purified Pg (G2) samples and three soil HAs with different degrees of humification were analyzed using negative-mode electrospray ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) at 12 Tesla. The Pg samples were characterized using H/C-O/C ratios and based on the type and magnitude of the molecular formulae having DHPQ or related compounds as a potential structure.
  
  
  RESULTS
  The H/C-O/C diagram indicated that most of the lipids, proteins, other aliphatics, and lignin-like components were removed, while condensed aromatic components were concentrated during the purification process of crude Pg to G2 using Sephadex G-50. More than 27 molecular formulae resembling DHPQ, DHPQ substituted with varying numbers of carboxyl and/or hydroxyl groups, and phenyl-DHPQ derivatives were identified in both Pg samples. The peak magnitudes of these formulae in G2 (accounting for 40% of the total assigned magnitude) were greater than that in the crude Pg (21%). Most of the 27 molecular formulae were also detected in the three soil HAs, suggesting a common existence of Pg-related compounds in various soil HAs.
  
  
  CONCLUSIONS
  The important molecular formulae of Pg components were estimated utilizing FTICRMS. Copyright © 2013 John Wiley & Sons, Ltd.
authors.affiliation: Graduate School of Bioagricultural Sciences, Nagoya University, Chikusa, Nagoya, 464-8601, Japan; Department of Chemistry and Biochemistry, Old Dominion University, Norfolk, VA, 23529, USA.
authors.forename: Kosuke
authors.initials: K
authors.lastname: Ikeya
authors.forename: Rachel L
authors.initials: RL
authors.lastname: Sleighter
authors.forename: Patrick G
authors.initials: PG
authors.lastname: Hatcher
authors.forename: Akira
authors.initials: A
authors.lastname: Watanabe