**1. Introduction**

Dietary polyphenols have interesting spectrum biological properties, including radical scavenging activity [1]. Anthocyanins are one of essential classes in the polyphenol family [2]. These are the plant pigments responsible for the bright redorange to blue-violet colors of many fruits and vegetables [3]. Since these are natural colored compounds, many pieces of literature come with their application as coloring or coloring material, especially in the food industry [4–9]. Naturally, these compounds found in glycoside form can collectively be called anthocyanins. At the same time, their aglycon forms are commonly called anthocyanidins [10]. The common aglycon forms are cyanidin, delphinidin, peonidin, petunidin, malvidin, and pelargonidin. The color pigments are most abundant in berries like black currants, elderberries, blueberries, strawberries, etc., red and purple grapes, red wine, sweet cherries, eggplants, black plums, and red cabbage, etc.

The basic structure of anthocyanins is the flavylium cationic ring in which oxygen carries a positive charge. The positively charged species exhibit several equilibrium structures due to different transformations like proton transfer, isomerization, and tautomerization under various pH conditions [11]. All molecules of a group of anthocyanins have their absorption range in the visible spectrum due to effective π conjugation within the molecule. These are the largest group of watersoluble pigments in the plant kingdom. Experimental and theoretical reports show an exponential increase in findings related to its properties, color, co-pigmentation, pH effect, antiradical properties, etc. [12–17].

The delphinidin (**1a**) and its four modifications (**1b, 1c, 1d & 1e**) are selected for the study. The structure of **1a** having three rings A, B, and C, where A & C are fused rings and B connected with A-C through a single bond. The structures of compounds under consideration are shown in **Figure 1**. The colored pigments with other colorless natural products are now proven to be very impressive due to their improved activity in the sense of color and property. Hence to enhance the property, the colorless, most important, small, widely studied polyphenol gallic acid is taken and coupled with **1a** through their 3 and 4' OH bonds and respectively formed delphinidin-3-O-gallate (**1c**) and delphinidin-4'-O-gallate (**1e**). Also, the results are compared with its glucose forms delphinidin-3-O-glucoside (**1b**) and delphinidin-4'-O-glucoside (**1d**).

Gallic acid (3,4,5- Trihydroxy benzoic acid) and its derivatives have been found in various natural sources like nuts, tea, grapes, gallnut, oak bark, etc. Biological studies show that gallic acid has variable effects, including antiviral, antioxidant, and anticancer activities [18–20]. Due to its potent antioxidant activity against free radicals is used in food, cosmetics, and pharmaceutical industries and as a source material for ink and paints [21]. GA possesses the most robust antiradical property than Trolox, and hence, in many cases, this molecule is widely used as a reference compound for antioxidant studies. Green tea contains the highest concentration of GA-based compounds responsible for the plant's antioxidant capacity [22, 23].

Once the compound possesses potent antioxidant activity and a specific range of absorbance in the visible spectrum can be effectively used for various purposes, especially in the food industry, they seek less hazardous, highly protective materials

**Figure 1.** *Structures of compound considered for the study.*

*DFT Study of Structure and Radical Scavenging Activity of Natural Pigment Delphinidin… DOI: http://dx.doi.org/10.5772/intechopen.98647*

for coloring purposes. Hence these are colored compounds, and this work concludes the antiradical property of delphinidin and its derivatives with the help of computational methods.
