The reactions of antioxidants with superoxide radical were studied by cyclic voltammetry (CV)—and hydrodynamic voltammetry at a rotating ring-disk electrode (RRDE). In both methods, the superoxide is generated in solution from dissolved oxygen and then measured after being allowed to react with the antioxidant being studied. Both methods detected and measured the radical scavenging but the RRDE was able to give detailed insight into the antioxidant behavior. Three flavonoids, chrysin, quercetin and eriodictyol, were studied, their scavenging activity of superoxide was assessed and the molecular structure of each flavonoid was related to its scavenging capability. From our improved and novel RRDE method, we determine the ability of these 3 antioxidants to react with superoxide radical in a more quantitative manner than the classical CV. Density Functional Theory (DFT) and single crystal X-ray diffraction data provide structural information that assists in clarifying the scavenging molecular mechanism. Hydroxyls associated with the A ring, as found in chrysin, scavenge superoxide in a different manner than those found in the B ring of flavonoids, as those in quercetin and eriodictyol.
Embelin, a plant natural product found in Lysimachia punctata (Primulaceae), and Embelia ribes Burm (Myrsinaceae) fruit, possesses interesting biological and pharmacological properties. It is a unique chemical species as it includes both quinone and hydroquinone functional groups plus a long hydrophobic tail. By using hydrodynamic voltammetry, which generates the superoxide radical in situ, we show an unusual scavenging capability by embelin. Embelin as a scavenger of superoxide is stronger than the common food additive antioxidant 2,6-bis(1,1-dimethylethyl)-4-20 methylphenol, (butylated hydroxytoluene, BHT). In fact, embelin is even able to completely abolish the superoxide radical in the voltaic cell. Computational results indicate that two different types of embelin scavenging actions may be involved, initially through π–π interaction and followed by proton capture in the cell. A related mechanism describes embelin’s ability to circumvent superoxide leaking by transforming the anion radical into molecular oxygen. In order to confirm its antioxidant properties, its biological activity was tested in a study carried out in THP-1 human leukemic monocytes and BV-2 mice microglia. A 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, proliferation curves and antioxidant activity by the use of a fluorescent probe showed good antioxidant properties at 24 h. This suggests that embelin’s long alkyl C10 tail may be useful for cell membrane insertion which stimulates the antioxidant defense system, and cytoprotection in microglia. In conclusion, embelin could be an interesting pharmacological tool able to decrease the damage associated with metabolic and neurodegenerative diseases.
The response of the solid-state Cu(II) ion-selective electrode in high chloride media was investigated and found to be Nernstian under normal laboratory and field measurement conditions. The previously reported "non-Nernstian" behavior has been identified as a combination of electrode fouling, due to specific storage and measurement conditions, and the action of stoichiometric levels of a Lewis base on the standard addition method of electrode calibration. Electrode calibration by external Cu(II) activity buffers in artificial seawater is described. The copper activity of the buffers is calculated from an ion-interaction model, and the electrode response in these buffers is shown to be consistent with that in chloride solutions over 10 orders of magnitude copper concentration and from pH 8 to 2.
Copper ion activity and concentrations of voltammetrically defined dissolved, surface bound, and tightly bound colloidal copper in samples from San Diego Bay were measured independently using potentiometric and stripping voltammetric methods, respectively. 'Free' copper, calculated from activity, was approximately three orders of magnitude less than the concentrations of the dissolved, surface bound and tightly bound copper fractions (indicating a high degree of complexation) and over-all, was well correlated to them, suggesting a common, heterogeneous, copper matrix, of probable organic nature, whose distribution is controlled by physical mixing. This finding supports the aggregate or 'onion' model of copper speciation in seawater. Within the bay, we also encountered environments in which copper activity was not directly correlated to concentration and we attribute this to local production of organic ligands by microalgae.
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