## 3 Flavonoids
### Activity of Flavonoids towards ROS  
Flavonoids possess antioxidant, antimicrobial, anti-inflammatory, and many other properties (see above). They are widely used in medicine, in industry as dyes, tanning agents, etc. The antioxidant effect of flavonoids is to suppress the formation of ROS by chelation of microelements involved in the formation of free radicals, removal of ROS, and inhibition of enzymes that enhance the formation of free radicals. Flavonoids in plants also possess the ability to absorb UV-solar waves, thus inhibiting the over-production of ROS, and to quench ROS after their formation [228].  
The antioxidant activity of flavonoids depends on the location of functional groups in their structure. The antioxidant activity is mainly based on the presence of hydroxyl groups (OH), presumably in the B and C rings (Figure 3), while hydroxyl groups in the A ring seem to be less important [229; 230; 231; 232]. Flavonoids, similar to quinones, were shown to efficiently scavenge ROS such as O2*- and 1O2, with the constant rates in the latter case ranging from \(\sim\)10\({}^{6}\) M\({}^{-1}\) s\({}^{-1}\) to 10\({}^{8}\) M\({}^{-1}\) s\({}^{-1}\) in ethanol predominantly owing to physical quenching [233; 234]. The most abundant and largest subgroup of flavonoids in fruits and vegetables are flavones and flavan-3-oles. The latter, along with leucanthocyanins, are the most reduced flavonoids and, in addition, these compounds are most often present in a reduced form in plant cells. Their hydroxyl groups are responsible for biological activity, especially antioxidant activity. Studies on _A. thaliana_ mutants, which were genetically modified to overexpress a number of genes related to flavonoid synthesis and its regulatory transcription factors MYB12/PFG1 and MYB75/PAP1, showed increased resistance to drought [235]. Excessive accumulation of anthocyanins with strong antioxidant activity in vitro reduced the accumulation of ROS in vivo under drought conditions.  
Detailed analysis of the antioxidant properties of flavonoids was performed in [232] to understand which ring is the most important for antioxidant activity. In the study, various flavonoids--flavonols, flavanones, flavones, anthocyanins, hydroxycinnamates, and flavanols--were studied. These compounds have structural similarities but differ in the nature of their B and C rings. The focus was to understand their reactions with 2,2'-azinobis-(3-ethyl benzothiazoline 6-sulfonic acid) diammonium salt radical (ABTS*+). ABTS** is known to be a reactive compound, e.g., the rate constant of ABTS**+ with ascorbic acid at neutral pHs is 10\({}^{6}\) M\({}^{-1}\) s\({}^{-1}\)[236]. The study shows that the structure of the B ring, namely the position of hydroxyl group(s), is the main factor determining the antioxidant activity of flavonoids.  
The distinctive feature of flavonoids is their ability to function as antioxidants both in the aqueous phase and in lipophilic environments. Therefore, flavonoids neutralize O2*- in both water and membrane phases; in both phases, O2*- can be produced at a high rate. 1O2 is mainly generated within PS II (see above) or by any free chlorophyll molecule in chloroplasts to a very small extent. Flavonoids were found to be bound to the chloroplast envelope, so 1O2, if produced by the free chlorophylls in the vicinity to the envelope, should be accessible to these flavonoids [237].  
The phenolic hydroxyl groups in flavonoid structure are responsible not only for direct antioxidant activity of flavonoids but also for chelation of metals preventing their interaction with H2O2 and therefore the generation of highly reactive oxidizer, hydroxyl radical (OH*). Besides their direct antioxidant functions, flavonoids also act indirectly by inhibiting ROS-generating enzymes, such as mitochondrial succinoxidase, NADH oxidase, microsomal monooxygenase and others, as well as by upregulating and protecting antioxidant systems [238].