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. 1H NMR spectra confirmed that chitosan was chemically modified, although the degree of modification was low. © 1999 John Wiley & Sons, Inc. Biotechnol Bioeng 63: 154–165, 1999.

Enzyme Initiated Radical Polymerizations

“…For example, tyrosinases generate reactive quinones from phenols, which can react with chitosan [141,145,160,161]. Similarly, reactive quinones can be generated using laccases [140,146] of peroxidases [136,139].…”

Tyrosine‐based “Activatable Pro‐Tag”: Enzyme‐catalyzed protein capture and release

“…Scheme 1 shows that the tyrosine residues of the "pro-tag" are activated by 2 tyrosinase-catalyzed oxidation and converted to reactive o-quinones (Chen et al 2001;Chen et al 2003;Kumar et al 1999;Wu et al 2002a;Wu et al 2002b;Yi et al 2004;Yi et al 2003). The electrophilic o-quinones of the "pro-tag" can then undergo nonenzymatic reactions with the nucleophilic amino groups of chitosan to yield a proteinchitosan conjugate.…”

“…The electrophilic o-quinones of the "pro-tag" can then undergo nonenzymatic reactions with the nucleophilic amino groups of chitosan to yield a proteinchitosan conjugate. Chitosan's primary amines confer both nucleophilic and pHresponsive properties (Chen et al 2001;Chen et al 2003;Kumar et al 1999;Wu et al 2002a;Wu et al 2002b;Yi et al 2004;Yi et al 2003). At low pH, the amino groups are protonated, so that chitosan is a water-soluble cationic polyelectrolyte.…”

Approaches for Functional Modification or Cross‐Linking of Chitosan

“…Enzymes can selectively and specifically eliminate the hazards associated with chemical reagents. Also, they can modify the macromolecular structure, thereby enhancing the polymer function [119][120][121][122][123]. Enzymatic modification of chitosan results in derivatives with unique pH-sensitive, water-soluble, and adhesive properties.…”