Poly(ethylene oxide) (PEO)-poly(propylene oxide) (PPO)-PEO triblock copolymers (so called Pluronic) generally show excellent biocompatibility and amphiphilic properties. A prominent feature of   Pluronic is that the individual block copolymer molecules are able to self-assemble into spherical micelle structures when the concentration is above the critical micelle concentration. Moreover the micelles physically cross-link to form hydrogels at greater concentrations. We have identified a peptide (DFNPYLGVTPVK) that specifically binds to PPO by affinity-based screening from a biologically constructed phage-displayed peptide library. In this study, the affinity-based functionalization of Pluronic assemblies with the peptide was investigated.

The PPO-binding peptide was chemically conjugated with N-(1-anilinonaphthyl-4)maleimide (ANM), which is well known as an environmentally responsive fluorescence molecule, to investigate interactions with Pluronic assemblies. Fluorescence intensity of the peptide-conjugated ANM drastically increased in the presence of Pluronic micelles (Fig.1, black line). On the other hand, the intensity of a fluorescence probe alone was slightly increased. Fluorescence intensities of the peptide-conjugated ANM with Pluronic with different chain length were also measured. The intensity of the peptide-conjugated ANM in the presence of Pluronic with short PPO segments was lower than that in the presence of Pluronic with longer PPO segments. Therefore, it was revealed that the affinity of the peptide for PPO, which is a component of the hydrophobic segment of Pluronic, was essential for the loading of the peptide into the micelles. Furthermore, the effect of the peptide affinity for the micelle on the hydrogelation behavior was analyzed. As a result, the gelation temperature was decreased by the addition of peptides, indicating that the specific binding of peptides is useful to control the physicochemical properties of Pluronic assemblies.