Spin concentration measurements of high-spin (g′ = 4.3) rhombic iron(III) ions in biological samples: theory and application

• Published in: Journal of biological inorganic chemistry 2008-01, Vol.13 (1), p.15-24
• Main Authors: Bou-Abdallah, Fadi, Chasteen, N. Dennis
• Format: Article
• Language: English
• Subjects: Animals; Biochemistry; Biomedical and Life Sciences; Electron Spin Resonance Spectroscopy; Ferric Compounds - chemistry; Hydrogen-Ion Concentration; Life Sciences; Microbiology; Reference Standards; Spin Labels; Techniques
• ISSN: 0949-8257; 1432-1327
• DOI: 10.1007/s00775-007-0304-0

Electron paramagnetic resonance (EPR) signals at g ′ = 4.3 are commonly encountered in biological samples owing to mononuclear high-spin ( S  = 5/2) Fe 3+ ions in sites of low symmetry. The present study was undertaken to develop the experimental method and a suitable g ′ = 4.3 intensity standard and for accurately quantifying the amount of Fe 3+ responsible for such signals. By following the work of Aasa and Vänngård ( J. Magn. Reson. 19:308–315, 1975 ), we present equations relating the EPR intensity of S  = 5/2 ions to the intensities of S  = 1/2 standards more commonly employed in EPR spectrometry. Of the chelates tested, Fe 3+ –EDTA (1:3 ratio) in 1:3 glycerol/water (v/v), pH 2, was found to be an excellent standard for frozen-solution S  = 5/2 samples at 77 K. The spin concentrations of Cu 2+ –EDTA and aqua VO 2+ , both S  = 1/2 ions, and of Fe 3+ –transferrin, an S  = 5/2 ion, were measured against this standard and found to agree within 2.2% of their known metal ion concentrations. Relative standard deviations of ±3.6, ±5.3 and ±2.9% in spin concentration were obtained for the three samples, respectively. The spin concentration determined for Fe 3+ –desferrioxamine of known Fe 3+ concentration was anomalously low suggesting the presence of EPR-silent multimeric iron species in solution.