title: On the origin of the non-haemic iron transferrin ESR signal: ESR investigations on histidine-iron-citric acid systems.
abstract: The nature of the high spin ferric iron complex located at g = 4.3 has been investigated by means of electron spin resonance spectroscopy and polarography. It could be shown that two complexes each exist in the acid and alkaline pH region, and that the iron is bound to two histidines, three citric acids, and probably to one bicarbonate. These results agree well with previous findings according to which the ligand field of iron should be composed mainly of oxygen and nitrogen atoms. Another low-field signal located at g = 3.6 appears in the pH range from 2 to 7 only and exhibits its maximum where the g = 4.3 signal has its minimum. Its exact nature is still unknown but it seems to represent some intermediate state of the ternary Fe3+-histidine-citric acid complex. When citric acid is used, the spin concentration seems to be always larger than in the case of ascorbic acid. Since the effect obtained with ascorbic acid and citric acid seems to be similar, it may be concluded that the biological function of both of the acids might be somehow related to each other.
mesh_terms: D001205:Ascorbic Acid / Q000032:analysis; D002951:Citrates / Q000032:analysis*; D019343:Citric Acid; D004578:Electron Spin Resonance Spectroscopy; D006639:Histidine / Q000032:analysis*; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000032:analysis*; D008024:Ligands; D008667:Metalloproteins / Q000032:analysis*; D019159:Nonheme Iron Proteins; D011048:Polarography; D014168:Transferrin / Q000032:analysis*
chemical_list: D002951:Citrates; D008024:Ligands; D008667:Metalloproteins; D019159:Nonheme Iron Proteins; D014168:Transferrin; D019343:Citric Acid; D006639:Histidine; D007501:Iron; D001205:Ascorbic Acid