1. Introduction

More common than you might think, flavonoids are present in human diet since they can be found in fruits, vegetables, seeds, grains, and beverages, such as wine and tea [1]. They are most famous to beautify fruits and vegetables with vivid colors, but flavonoids most powerful actions are still unknown for most of population, which is unaware the antioxidant [2, 3], anticarcinogenic [4, 5], antiinflammatory [6, 7], antiviral [8, 9], and antimicrobial [10, 11] effects provoked by these potent compounds. But, what are those compounds, after all?

Flavonoids, with over 4000 compounds identified until now [12], comprehend a wide group of molecules synthesized by plants as secondary metabolites responsible for ensuring vascular plant colonization and surviving on earth's environment [13]. These molecules play crucial roles to plants' life such as protection against insect attack and microbes invasion [14, 15] and by absorption of harmful ultraviolet radiation, attraction of insect pollinators by colorful anthocyanins synthesis [16], antioxidant action by inhibiting the generation of reactive oxygen species (ROS)

[17], involvement in pollen germination [13], involvement in biological communication in the rhizosphere [18] and action as regulators, involved in auxin transport and catabolism [19].

In general, flavonoids are polyphenolic compounds with the flavan nucleus as the structure skeleton, which consists of 15 carbon atoms arranged in 3 rings (C6-C3-C6), labeled by A, B, and C (Figure 1) [3].

Flavonoids are classified into classes such as flavonol, flavone, flavanone, flavanol, isoflavone, and anthocyanidin, just to mention a few [20]. The basic structure of each class is shown in Figure 2. The differences between flavonoid class structures are in the oxidation level and C-ring substitution pattern, while the difference among flavonoids belonging to the same class is in the pattern of A- and B-ring substitutions [3].

As reviewed by Aidyn Mouradov and German Spangenberg [13], flavonoid synthesis in plants occurs commonly from the aromatic amino acids phenylalanine and tyrosine. At the beginning of synthesis, the amino acid is converted into coumaroyl-CoA by a number of enzymatic reactions involving phenylalanine ammonia-lyase (PAL), cinnamate 4-hydroxylase (C4H) and 4-coumarate-CoA ligase (4CL). The naringenin-chalcone synthase (CHS) and chalcone isomerase (CHI)

Figure 1. Flavan nucleus, the structure skeleton of flavonoids.

Figure 2. Basic structures of different classes of flavonoids.

## Molecular Mechanism of Flavonoids Using Fluorescence Spectroscopy and Computational Tools DOI: http://dx.doi.org/10.5772/intechopen.84480

convert coumaroyl-CoA into naringenin; the pathway opens in different ways generating flavanones, dihydroflavonols, leucoanthocyanins, anthocyanidins, and flavan-3-ols by series of enzymatic reactions. These molecules can generate a couple of other compounds such as flavones and isoflavones and flavonols and anthocyanins. These flavonoids are generally found in plant epidermal cells, chloroplasts, vacuole, and nucleus [13].

Epidemiological studies reviewed by Romano and co-workers [21] have demonstrated an inverse relationship between dietary flavonoid intake and prevalence and risk of cardiovascular diseases and some types of cancer such as breast, colon, lung, prostate, and pancreas. Besides that, the authors emphasized the advances of the application of flavonoids in diseases related to the central nervous system, obesity, diabetes, inflammation, digestive system, and respiratory tract, as well as the effects of flavonoids in reproduction and the antimicrobial effects of these compounds.

Many efforts have been done to elucidate the mechanism of action of these compounds by using experimental approaches based on physical observable. Steady-state fluorescence spectroscopy is a technique widely used to follow the interaction of small molecules and biological targets. However, there are many others experimental techniques that have been used to describe biological systems, such as time-resolved fluorescence spectroscopy, nuclear magnetic resonance (NMR), isothermal titration calorimetry (ITC), circular dichroism (CD), and surface plasmon resonance (SPR).

Besides that, it has also used computational analysis in order to describe the interactions between flavonoids and molecular targets. Molecular docking and molecular dynamics are the most common computational techniques used to model biological systems.

In this way, this chapter comes not only as an attempt to gather some works dedicated to explain the molecular mechanisms of action of some flavonoids but also to introduce a brief theory of steady-state fluorescence spectroscopy and molecular docking.
