Enzymatic grafting of a natural product onto chitosan to confer water solubility under basic conditions

Abstract

Chitosan is a natural biopolymer whose rich amine functionality confers water solubility at low pH. At higher pH's (greater than 6.5), the amines are deprotonated and chitosan is insoluble. To attain water solubility under basic conditions we enzymatically grafted the hydrophilic compound chlorogenic acid onto chitosan. Despite its name, chlorogenic acid is a nonchlorinated phenolic natural product that has carboxylic acid and hydroxyl functionality. The enzyme in this study was tyrosinase, which converts a wide range of phenolic substrates into electrophilic o-quinones. The o-quinones are freely diffusible and can undergo reaction with the nucleophilic amino groups of chitosan. Using slightly acidic conditions (pH = 6), it was possible to modify chitosan under homogeneous conditions. When the amount of chlorogenic acid used in the modification reaction exceeded 30% relative to chitosan's amino groups, the modified chitosan was observed to be soluble under both acidic and basic conditions, and to have a pH window of insolubility at near neutral pH. 1 H NMR spectra confirmed that chitosan was chemically modified, although the degree of modification was low.

Artificial Polymeric Flavonoids: Synthesis and Applications

“…For example, the enzymatic treatment of chitosan in the presence of chlorogenic acid produced the modified chitosan, soluble under both acidic and basic conditions. [57] The PPO catalyzed reaction of chitosan and 3,4-dihydroxyphenylethylamine (dopamine) provided water resistant adhesive properties to chitosan. [58] A chitosan derivative modified with hydroxy or dihydroxybenzaldehyde was crosslinked by PPO to give stable and self-sustaining gels.…”

Chitosan derivatives obtained by chemical modifications for biomedical and environmental applications

“…A potential environmental benefit for using enzymes is that their selectivity may be exploited to eliminate the need for waste full protection and deprotection steps. Finally, enzymes specificity may offer the potential for precisely modifying macromolecular structure to better control polymer function [45,46]. For instance, enzymatic modification can yield chitosan derivatives with unique pH-sensitive water solubility and adhesive properties.…”

“…Kumar et al [45] reported that enzymatic grafting of phenolic compounds onto chitosan confer water solubility under basic conditions (Fig. 3).…”

Water Purification through Bioconversion of Phenol Compounds by Tyrosinase and Chemical Adsorption by Chitosan Beads

“…In summary, the results in Figures 1 and 2 provide UV-visible evidence for the covalent modification of chitosan by p-cresol and tyrosinase. The differences between chitosan and cellulose and the observed absorption peak at 460 nm for the modified chitosan are consistent with a covalent modification through amino groups of chitosan previously reported (19)(20)(21)(22)(23).…”