FIGURE 2.

The formation of covalent bonds between chitosan (CS), the physical mixture of gallic acid with chitosan (CS), and enzymatic grafted gallic acid with chitosan was observed using UV-vis spectra (A). Both CS and gallic acid showed absorption peaks between 250 and 350 nm. Meanwhile, in the spectra of gallic acid, GRFT CS exhibited an absorption peak at 262 nm shifted from 272 nm, compared to that of the gallic acid CS physical mixture attributed to the decrease in energy required for π-π* transition, due to covalent linkage of gallic acid and CS. FTIR spectra of chitosan (CS), physical mixture of gallic acid with chitosan (CS), and enzymatic grafted gallic acid with chitosan (B). The FTIR spectra indicated that between 4,000 and 2,500 cm⁻¹ and 1,000–400 cm⁻¹ for tested samples were identical. However, a slight shift was observed around 1,730 and 1,640 cm⁻¹, which indicated the presence of C=O stretching in esters and -C=O stretching of CS amide. Adapted with permission from Zheng et al. (2018) under CC BY version 4.0.