**4. Investigating the basics of** *Camellia sinensis* **antioxidant thermodynamics and kinetics**

Many authors reported that *C. sinensis* antioxidant power is remarkable due to the capacity of its constituents to promptly reduce standard free radicals such as 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) [18, 24, 30, 31]. Although results are often noteworthy, colorimetric tests tend to be biased due to the strong color of *C. sinensis* extracts, which may lead to imprecise results. In this regard, many authors pursued the exploration of the redox behavior of this plant using electrochemical methods, which may provide less color-biased results due to their unique dependence on electron transfer [17, 32, 33].

The electroanalytical investigation of plantstuff is a growing field on science due to its promising perspectives regarding the quality control, authenticity, and physicochemical characterization of plant secondary metabolites [32–34]. Nonetheless, most voltammetric assays can be applied to plants without strenuous pretreatment of the vegetal sample [32]. However, given that plant secondary metabolites of phenolic origin such as those of *C. sinensis* showcase electrochemical processes which are mainly controlled by mass transfer in the bulk solution [17], as well as proneness of oxidation products to undergo adsorption on electrode surface, a careful electrode surface renewal protocol needs to be adopted.

Regarding the basic background on voltammetric studies, these tests involve the interpolation of two functions, namely, electric potential *versus* time (*E* × *t*) and

#### *Thermodynamics and Kinetics of* Camellia sinensis *Extracts and Constituents: An Untamed… DOI: http://dx.doi.org/10.5772/intechopen.92813*

electric current *versus* time (*I* × *t*). When graphically displayed, a voltammogram is the plot of electric current *versus* the applied electric potential following a specific signal pattern during a defined time interval (*I* × *E*). Therefore, any change in electric current which is non-capacitive by nature can be attributed to redox processes taking place in the electrochemical cell [35–37].

During voltammetric investigation, the scanning of electric potential toward positive values, aka anodic scan, leads to the visualization of oxidative processes, while the reverse scan, aka cathodic scan, leads to the visualization of reduction processes [35, 37]. Taking these concepts into account, the redox profiling and electrochemical characterization of both isolated plant constituents and vegetal extracts can be elucidated by varying the kind of scan which is being performed [10, 17, 33, 34]. **Figure 3** showcases an example of a cyclic voltammogram presenting a response which could be attributed to a reversible redox reaction, while **Figure 4** depicts an overview of the main mechanisms which are involved in the electrooxidation of *C. sinensis* constituents [7, 27, 38–40].

Literature reports that *C. sinensis* extracts showcase anodic peaks at electric potentials bellow 0.5 V when analyzed under voltammetry [17, 18, 30, 31, 41], which is nonetheless a remarkable feature. Considering that most of the endogenous antioxidant arsenal operates at electric potentials close to this value, the reductive power of *C. sinensis* constituents is noteworthy, since they could undergo oxidation thereby stabilizing ROS or restituting endogenous antioxidants [8, 10, 27, 42].

Notwithstanding, many authors showcased evidence of redox reversibility in the processes which take place at 0.5 V in *C. sinensis* extracts, which suggests that the antioxidant compounds could undergo followed redox reactions to promote the reduction of ROS in biological systems [32, 33]. When compared to voltammetric profiles of isolated compounds, *C. sinensis* voltammograms evidence the richness of electroactive compounds which are present in this plant, which further corroborates to the appeal of this plant in the development of therapeutic and nutraceutical products to balance oxidative stress in biological systems.

#### **Figure 3.**

*Example of a cyclic voltammogram presenting a response which could be attributed to a reversible reaction.*

#### **Figure 4.**

*Main mechanisms involved in the electrooxidation of phenolic compounds which are occurrent in* C. sinensis*. Note that catechol oxidation is reversible, while phenol undergoes irreversible oxidation, leading to both 1,2 and 1,4 catechol and an electro-polymerized product.*
