| Humic acids (HAs) are environmental materials that have a vast array of physical and chemical properties. Their applications seem almost endless. The absolute structural identity of HA remains a mystery, and their origin is somewhat ambiguous as well. Some recent data supports the belief that the synthesis of this seemingly omnipresent biopolymer is the result of a carefully controlled biochemical processes and not the random breakdown of lignin or other plant matter, which is also proposed.; In the last ten years, there has been a focus on HAs implications on biological activity, human health and environmental catalysis. Some studies regard HAs as potential therapeutics whereas other reports portray HAs as carcinogenic and mutagenic. This research examines contradictory views pertaining to HAs efficacy by studying their interaction with iron and copper.; Iron and copper are essential redox active metals found in many proteins and enzymes. However, in their reduced state, they pose a potential threat to biomolecules because of their ability to generate free radicals in a mechanism that is ascribed to be analogous to the metal-catalyzed Haber-Weiss and Fenton reaction. HAs are viewed as excellent chelators and powerful reducing agents. These attributes can be deleterious and/or beneficial to biological systems when HAs are in the vicinity of iron and copper.; The free radical chemical reaction systems we studied were analyzed exclusively by electronic paramagnetic resonance (EPR) spectroscopy using the spin trapping technique. Iron- and copper-loaded HAs generated free radicals, however, the reaction was not nearly as efficient compared to when HA was used as a reductant towards these metals, whether they were “free” in solution or bound to a chelator. Several types of reactive oxygen species (ROS) were identified, e.g., the hydroxyl free radical (•OH), the superoxide radical (•O2−), singlet oxygen (1O2), and hypervalent/metal-peroxo complexes. In contrast, HAs are also shown to be effective free radical scavengers towards these highly reactive oxygen-containing species. The mechanism why HAs can act as antioxidant and/or a pro-oxidant is analyzed and discussed. |
