**8. Biological activity of Schiff bases**

Schiff bases and metal complexes are vital compounds for biological systems because they have anticancer [67, 68], antibacterial [69–74], antifungal [75, 76], and antiviral [77] properties.

**Figure 17.** *N*d*N*d*O type Schiff base metal complex.*

**Figure 18.** *O*d*N*d*N*d*O type Schiff base metal complex.*

**Figure 19.** *N*d*N*d*N*d*N type Schiff base metal complex.*

The most important biological activity of Schiff bases is the role they play in amino acid biosynthesis, one of the basic processes of life. Schiff bases are important intermediates formed in the biosynthesis of α-amino acids (RCHNH2COOH) used in the synthesis of proteins in organisms. If there is not enough essential amino acid in the food, the organism converts an excess amino acid to the amino acid it needs by transamination reaction in some cases. In this process, the amino group of the excess amino acid is transferred to the keto acid *via* a series of Schiff bases (**Figure 20**) [78].

In addition, Schiff bases have been proven to be present in a variety of natural, semisynthetic, and synthetic compounds and are essential for their biological activity (**Figure 21**) [26].
