EPR in Chemistry

Detection and study of the active site of Cu,Zn-SOD
Many enzyme reactions involve one-electron oxidation steps with formation of paramagnetic transient state of the enzyme detectable by EPR. The paramagnetic center where the unpaired electron is located, is usually centered at a transition metal (metalloproteins) or is an amino acid derived radical. Detection and identification of the paramagnetic centers is important to understand the function of the enzymes. For example, in the native SOD1 enzyme, the active site contains one Cu(II) ion that gives a very characteristic EPR spectrum.

Kinetics analysis of vitamin C antioxidant ability
Many chemical reactions involve the transfer of one electron. Each electron transfer results in an unpaired electron creating paramagnetic free radicals. EPR is the ideal spectroscopic technique to measure these species as well as to monitor the time behavior of their creation and disappearance. EPR solely has the ability to detect free radicals unambiguously. For example, antioxidants such as vitamin C are important in neutralizing dangerous free radicals in living things and kinetics indicates their effectiveness.

Light degradation of hops in beer
The majority of photochemical reactions take place through free radical formation as intermediates. For example, hops used in the brewing process contain a mixture of active components that include humulones, cohumulones, adhumulones, beta acids and essential oils. Some forms of these components are photo-active. Light exposure of beer leads to the formation of free radicals that combines with sulfur compounds and gives unpleasant flavor and odor to the beer.

Hydroxyl radical generation through photocatalytic reaction of TiO2
The modern chemical industry relies heavily on homogeneous and heterogeneous catalysts. Understanding the operational mode, or reactivity, of these catalysts is crucial for improved developments and enhanced performance. Where paramagnetic centers are involved, ranging from transition metal ions to defects and radicals, EPR spectroscopy is without doubt the technique of choice. For example, photocatalytic oxidation of organic pollutants is frequently carried out using semiconducting polycrystalline powders such as TiO2. A hydroxyl radical is easily formed by light irradiation of TiO2 and detected by EPR using spin traps.

EPR electrochemistry study of ruthenium complexes
Electrochemical generation method combined with EPR has been used to identify and investigate free radicals derived from both organic and inorganic compounds. Inorganic dyes can be used to improve the efficiency of solar cells. In order to optimize the ligands, one must understand the electronic structure of the dye. Here the electrochemistry and EPR combined with DFT calculations and UV/Vis spectroscopy show the unpaired electron is delocalized between the metal and the ligand.

Enzymatic activity of SOD protein studied via Cu(II) reduction
Enzymes in the human body regulate oxidation-reduction reactions. These complex proteins, of which several hundred are known, act as catalysts, speeding up chemical processes in the body. Oxidation-reduction reactions also take place in the metabolism of food for energy, with substances in the food broken down into components the body can use. For example, the dismutase activity of Cu,Zn-SOD protein involves reduction of Cu(II)-SOD to Cu(I)-SOD:

Detection of ascorbate radical upon oxidation of vitamin C
The delicate balance between the advantageous and detrimental effects of free radicals is one of the important aspects of human (patho)physiology. Imbalanced generation of toxic radicals is highly correlated with the pathogenesis of many diseases which require the application of selected antioxidants to regain the homeostasis. EPR is used to determine the oxidative status of biological systems using endogenous long-lived free radicals (ascorbyl radical, tocopheroxyl radical, melanin) as markers.