Aggregate formation and release behaviour of model substances with block co-polypeptide containing tryptophan

In this study, amphiphilic block co-polypeptide consisting of hydrophilic poly(N-hydroxyethyl l-glutamine) (PHEG) and hydrophobic poly(l-tryptophan) (poly(Trp)), PHEG-block-poly(Trp)-T, was prepared by aminolysis of poly(γ-benzyl l-glutamate)-block-poly(Trp) with 2-amino-1-ethanol and subsequent treatment with trifluoroacetic acid (TFA). By using the block co-polypeptide, aggregate formation and sustained-release behaviour of model substances were investigated. The block co-polypeptide formed aggregates in aqueous medium and showed the ability to uptake hydrophobic substances into their hydrophobic moiety. The block co-polypeptide exhibited pH-response, the critical aggregate concentration of PHEG-block-poly(Trp)-T and the sizes of the aggregates depended on pH and decreased at pH 2.0. Moreover, fluorescence studies indicated a loose aggregate structure at pH 2.0 and the release rate of the model substances from the polypeptide aggregates was higher at pH 2.0 than at pH 5.0. These results could be explained by dissociation of Trp residues in the hydrophobic cores of the aggregates.