Aggregates formation and pH response of mixed dodecyl-terminated copolypeptides containing tryptophan

In this study, two kinds of amphiphilic copolypeptide, C₁₂-poly(N-hydroxyethyl L-glutamine-co-L-tryptophan) (C₁₂-EGT-T) and C₁₂-poly(L-glutamic acid-co-L-tryptophan) (C₁₂-GAT-T) were prepared. By using the mixture of these amphiphilic copolypeptides, pH response of aggregate formation, critical aggregate concentration, the sizes of the aggregates and sustained-release behavior of model substances were investigated. The mixture of the amphiphilic copolypeptides formed aggregates in aqueous medium and showed the ability to uptake model substances such as pyrene and 5-fluorouracil into their hydrophobic moiety. Moreover, pH response was observed in the sustained-release behavior of model substances and the pH region where these properties changed was from pH 7.4 to pH 4.5 in the mixture of C₁₂-EGT-T/C₁₂-GAT-T at 50/50 (mol/mol). These results could be explained by dissociation of trifluoroacetic-acid-treated tryptophan residues in the hydrophilic moieties and destabilization of the aggregates by electrostatic repulsion.