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. / Doctor of Philosophy / Elastin like peptides (ELPs) are similar to the protein elastin in terms of amino acid sequence. They are used widely as thermoresponsive (change in properties at different temperatures) components in biomaterials due to their abnormally lower solubility at higher temperatures. While linear ELPs have been thoroughly investigated, few investigations in ELP dendrimers have been studied. Dendrimers are molecules that branch in a controlled way to achieve sphere-like structures with rich surface functionalities. We synthesized the ELP dendrimers by using lysine amino acids as branching units. A protein secondary structure change, typical of ELPs, was observed for all peptide dendrimers. Secondary structure transitions showed no dependence on the molecular branching/linear structures, but a higher transition temperature (T_t) was observed for dendritic peptides than for their linear control peptides with the same number of amino acids. Various techniques confirmed the existence of aggregates below the T_ts, which was never reported before. We further fabricated hydrogels that mimic the native extracellular matrix, by connecting hyaluronic acid (HA) with ELP dendrimers. Interestingly, most of the hydrogels studied retained transparent upon gelation, but after freeze-drying and addition of water remained opaque for days. This phenomenon was attributed to forming of small aggregates in the gelation step, which resulted in slow reswelling. We then prepared hydrogels with H₂S-releasing groups, which showed human neutrophil elastase-responsive H₂S-releasing properties with potential applications in treating chronic diseases with recurring inflammation. We then prepared a series of wedge-shaped triblock poly (ethylene glycol) (PEG)- ELP dendrimer-alkyl chain molecules. The triblock molecules were thermoresponsive and showed a change from small spheres to large aggregates. However, the hydrophilic shell limited the thermoresponsive assembly, leading to slow turbidity change in the LCST transition. We found evidence of coherent assembly of the PEG and ELP parts, probably due to the relatively short polymer chains. Utilizing the micelles with hydrophobic cavity, we were able to encapsulate hydrophobic drugs, with promising applications for localized drug release for cancer treatment.

Stimuli responsive coatings offer a versatile method by which to manipulate interfacial interactions of proteins in a desired way. However, there exists little guidance as to how the structure of a responsive polymer coating influences adsorption of proteins. In this dissertation, the adsorption behavior of immuglobulin G (IgG) on poly (N-isopropylacryamide) (PNIPAAm) hydrogel coatings was investigated as a function of film thickness. PNIPAAm exhibits a hydrophilic to hydrophobic transition above a critical temperature of ~32°C in aqueous solutions. In this research, through the use of quartz crystal microbalance with dissipation (QCM-D) it was observed that the adsorption was thickness dependent and became non-reversible as the temperature was decreased. Interestingly, QCM-D results also suggested a similar amount of protein adsorption on both hydrated and dehydrated PNIPAAm surfaces. A rigid film analysis using Sauerbrey equation revealed a multi-layer formation on the collapsed PNIPAAm coatings. Although it is allegedly reported that PNIPAAm favors adsorption above the critical temperature due to hydrophobic interactions, there have been several studies that reported adsorption of proteins below the critical temperature. To better understand the QCM-D results, hydrodynamic shear force assays in a spinning disk configuration were performed in order to quickly measure and quantify adhesion of polystyrene (PS) probe spheres (10μm) to the PNIPAAm coatings in both the solvated (hydrophilic) and collapsed (hydrophobic) state. The influence of polymer coating thickness, polymer chain cross-link density, microsphere concentration and adsorption time on the adhesion characteristics of the coatings was investigated in relation with volume phase transition of the polymer coatings. A series of experiments on quantification of the temperature dependent adhesion of proteins adsorbed on surface attached PNIPAAm coatings of thicknesses was performed as the surface chemistry was switched from hydrophilic to hydrophobic. First, adhesion of polystyrene (PS) microspheres on PNIPAAm coatings was quantified in order to have a guideline for temperature dependent adhesion performance of these coatings. PS particles were subjected to a range of detachment shear stresses through hydrodynamic flow in a spinning disk configuration. These experiments provide an indirect method to determine the force of adhesion since it is proportional to the hydrodynamic force. Model protein, IgG, was then linked to PS microspheres and the mean adhesion strength of the IgG coated PS microspheres were determined through the detachment shear stresses. The influence of PS deposition time, PS bead concentration, PNIPAAm coating thickness and PNIPAAm cross-link density on the adhesion strength were addressed. The results indicated that in the collapsed state, the adhesion of bare hydrophobic PS microspheres depends strongly on coating thickness. For hydrophilic charged PS microspheres the adhesion was always higher on the hydrated PNIPAAm surfaces and appeared not to be strongly affected by the increase in PNIPAAm coating thickness. The adhesion of IgG was higher on the collapsed PNIPAAm surfaces and the adhesion trend did not significantly change as the PNIPAAm film thickness was increased. For PNIPAAm coatings with the cross-link density reduced by factor of 10, the adhesion was again higher on the collapsed PNIPAAm surface and scaled linearly with thickness. Moreover, the influence of thickness became prominent at the higher thickness values (165 nm-185 nm). In addition, the adhesion of carboxylated microspheres on PNIPAAm did not reach equilibrium and increased linearly with microsphere deposition time. A study on the sensing characteristics of PNIPAAm coatings in response to heavy metal ions was also conducted in this dissertation. The temperature-dependent swelling behavior of poly(N-isopropylacrylamide) and tripeptide Gly-Gly-His/poly(NIPAAm) conjugate hydrogel coatings were investigated using a quartz crystal microbalance with dissipation (QCM-D) while in contact with NaCl, ZnCl2, NiCl2, and CuCl2 solutions. To fabricate the tripeptide conjugated gels, precursor gels of poly(NIPAAm-co-3-aminopropylmethacrylamide[3.5 mole%]) were synthesized via free radical polymerization. The metal binding tripeptide, Gly-Gly-His, was subsequently synthesized in the gel via a Merrifield solid phase peptide synthesis (SPPS) technique, in which the amino group of the copolymer gel provided a functional site to support peptide synthesis. It was found that the logarithm of the transition temperature of the tripeptide Gly-Gly-His/poly(NIPAAm) conjugate hydrogel was proportional to the ionic strength, showing two distinct regions at low and high ionic strengths for the divalent ions. In the low ionic strength regime, the salting out constants were 0.08 M-1, 0.07 M-1, and 0.06 M-1 for Cu2+, Ni2+, and Zn2+, respectively, which follows the known trend for binding of the ions to Gly-Gly-His. In the high ionic strength region, when the metal-ion binding sites in the tripeptide conjugate hydrogel were saturated, the salting out constants were similar to the salting out constants associated with pure poly(NIPAAm).

Controlling interactions in synthetic polymers as precisely as in proteins would have a strong impact on polymer science. Advanced structural and functional control can lead to rational design of, integrated nano- and microstructures. To achieve this, properties of monomer sequence defined oligopeptides were exploited. Through their incorporation as monodisperse segments into synthetic polymers we learned in recent four years how to program the structure formation of polymers, to adjust and exploit interactions in such polymers, to control inorganic-organic interfaces in fiber composites and induce structure in Biomacromolecules like DNA for biomedical applications. / Die Kontrolle von Wechselwirkungen in synthetischen Polymersystemen mit vergleichbarer Präzision wie in Polypeptiden und Proteinen hätte einen dramatischen Einfluss auf die Möglichkeiten in den Polymer- und Materialwissenschaften. Um dies zu erreichen, werden im Rahmen dieser Arbeit Eigenschaften von Oligopeptiden mit definierter Monomersequenz ausgenutzt. Die Integration dieser monodispersen Biosegmente in synthetische Polymere erlaubt z. B. den Aufbau von Peptid-block-Polymer Copolymeren. In solchen sogenannten Peptid-Polymer-Konjugaten sind die Funktionalitäten, die Sekundärwechselwirkungen und die biologische Aktivität des Peptidsegments präzise programmierbar. In den vergangen vier Jahren konnte demonstriert werden, wie in Biokonjugatsystemen die Mikrostrukturbildung gesteuert werden kann, wie definierte Wechselwirkungen in diesen Systemen programmiert und ausgenutzt werden können und wie Grenzflächen zwischen anorganischen und organischen Komponenten in Faserkompositmaterialien kontrolliert werden können. Desweiteren konnten Peptid-Polymer-Konjugate verwendet werden, um für biomedizinische Anwendungen DNS gezielt zu komprimieren oder Zelladhäsion auf Oberflächen zu steuern.