Elastin-Like Peptide Dendrimers: Design, Synthesis, and Applications

Elastin like peptides (ELPs)—derived from the protein elastin—are widely used as thermoresponsive components in biomaterials due to their LCST (lower critical solution temperature) behavior at a characteristic transition temperature (Tt). While linear ELPs have been well investigated, few reports focused on branched ELPs. Using lysine (Lys, with an additional side-chain amine) as branching units, ELP dendrimers were synthesized by solid-phase peptide synthesis (SPPS) with up to 155 amino acid residues. A secondary structure change with decreasing ratio of random coil and increasing ratio of β-turn upon heating, which is typical of linear ELPs, was confirmed by circular dichroism spectroscopy for all peptides. Conformational change did not show evident dependence on topology, while a higher Tt was observed for dendritic peptides than for their linear control peptides with the same number of GLPGL repeats. Variable-temperature small-angle X-ray scattering (SAXS) measurements showed a size increase and fractal dimension upon heating, even below the Tt. These results were further confirmed by cryogenic transmission electron microscopy (cryo-TEM), and micro differential scanning calorimetry (micro-DSC), revealing the presence of aggregates below the Tt. These results indicated the presence of a pre-coacervation step in the LCST phase transition of the ELP dendrimers.

We further prepared hydrogels by crosslinking hyaluronic acid (HA) with ELP dendrimers. We invesigated their physical properties with scanning electron microscopy (SEM), swelling tests, SAXS, and model drug loading/release experiments. Most of the HA_denELP hydrogels retained transparent upon gelation, but after lyophilization and reswelling remained opaque for days. This reswelling process was carefully investigated with time-course SAXS studies, and was attributed to forming pre-coacervates in the gelation step, which slowly reswelled during rehydration. We then prepared hydrogels with H2S-releasing aroylthiooxime (SATO) groups and showed human neutrophil elastase-responsive H2S-releasing properties with potential applications in treating chronic diseases with recurring inflammation.

Furthermore, we prepared a series of wedge-shaped triblock polyethylene glycol (PEG)-ELP dendrimer-C16 (palmitic acid) conjugate amphiphiles with adjustable Tts. Various techniques were used to investigate their hierarchical structures. The triblock PEG-peptide-C16 conjugate amphiphiles were thermoresponsive and showed a morphology change from small micelles to large aggregates. However, the hydrophilic shell and strong tendency for micelle formation limited the thermoresponsive assembly, leading to slow turbidity change in the LCST transition. The secondary structure was twisted from conventional β-sheet, and the thermoresponsive trend observed in typical ELP systems was not observed, either. Variable temperature NMR showed evidence for coherent dehydration of the PEG and ELP segments, probably due to the relatively short blocks. Utilizing the micelles with hydrophobic cavity, we were able to encapsulate hydrophobic drugs, with promising applications for localized drug release in hyperthermia.