Correction of Freezing Artifacts in the Application of Electron Paramagnetic Resonance for Quantification of the Reduction of Ferric Hydroxamate by Chlorella vulgaris.

Electron paramagnetic resonance has been used to quantitate reduction of ferric ion bound to hydroxamate siderophores by iron-stressed fungi (DJ Ecker et al. 1982 J Biol Chem 257:8623-8626) and by iron-stressed Chlorella vulgaris (ATCC 11468) (FCT Allnutt, WD Bonner 1984 J Plant Nutr 7: 427-435). Application of this technique to quantitation of bound ferric ion has been questioned due to the possibility of freezing artifacts. Dipole-dipole interaction which resulted in broadening of the g = 4.3 signal was observed. Quantitation was affected when peak to trough amplitude was applied to quantitate the ferric ion but could be corrected by the application of internal standards and maintenance of constant cell content in the samples.