Relationship between the structure of Fe-Lignosulfonate complexes determined by FTIR spectroscopy and their reduction by the leaf Fe reductase

Fourier Transformed Infrared Spectroscopy (FTIR) was evaluated as an analytical technique for the estimation of the structural changes occurring in hardwood and softwood lignosulfonates (LS), when the concentration of functional groups capable to complex Fe was modified, as well as when Fe-LS complexes were formed. FTIR analysis revealed that the intensity of several bands varies after the formation of the Fe complexes, suggesting that structural changes occur in the molecule due to the addition of the metal. Modification of the concentration of functional groups is reflected in the spectra (especially at 2940, 1630 and 1230-1215 cm-1) indicating major stretching of CH, carbonyl, carboxyl, C-C, C-O and C=O groups in the modified LS. Hardwood and softwood LS present a similar pattern, irrespective of their different origins (eucalyptus and spruce, respectively) and characteristics. The ability of leaf discs of Fe deficient cucumber plants to reduce Lignosulfonate/Fe3+ complexes in light was studied. Reduction occurred for all the complexes, being more important in the complexes for which the bands corresponding to phenolic and carboxylic groups were more intense. It was concluded that FTIR spectroscopy can be a useful tool to elucidate the oxidation state of Fe and the molecular structure of Fe-LS complexes. It can also be used as an indicator of the effectiveness of LS complexes as leaf Fe reductase substrates. On the other hand, LS/Fe3+ complexes can supply Fe to leaf cells and constitute a promising Fe chlorosis corrector, but further research is necessary.