Preparation of copolypeptide hydrogels having pH sensitive chromophores

We found that L-tryptophan (Trp) is colored and shows color change with pH variations by treatments with strong acids, such as trifluoroacetic acid (TFA), dichloroacetic acid (DCA), and nitric acid. Colored copolypeptide hydrogels containing Trp residues were prepared by immersing the hydrophilic membranes, which were obtained by aminolysis with 2-amino-1-ethanol in the presence of crosslinking agent on membranes of copoly(γ-benzyl L-glutamate-co-Trp), in strong acids with irradiating UV. The pH dependence of the degree of swelling (Qw) of colored hydrogels was investigated. The pH region where a change in color of hydrogels was observed agrees with the transition region of Qw values. These results indicate that the change in color of Trp is caused by the dissociation of indole ring.