Engineering Peptide Biomaterials With Tunable Mechanics and Proteolytic Stability

Stereocomplexation, or stereochemistry-directed interactions among complementary stereoregular macromolecules, is burgeoning as an increasingly impactful design tool, exerting exquisite control material mechanics, morphology, and lifetime. Though stereocomplexation is mostly studied in polymers, the vast compositional space and biological function of peptides offer compelling opportunities to design peptide stereocomplexes for modulating material properties. In this thesis, we leverage stereochemistry-directed interaction of peptides, or peptide stereocomplexes, to engineer tunable biomaterials for therapeutic applications. To build knowledge on peptide stereocomplexes, we first studied β-sheet pentapeptide KYFIL stereocomplexes (Chapter 2). Blending L- and D-KYFIL at 1:1 ratio triggers dual mechanical and morphological transformations from stiff fibrous hydrogels into less stiff networks of plates in phosphate buffered saline. Moreover, we found that stereocomplexation alters peptide crystallinity with L- and D-KYFIL to be amorphous whereas their blends are crystalline. Additionally, the blends shield L-KYFIL from proteolytic degradation, producing materials with comparable proteolytic stability to D-KYFIL. We next introduced peptide stereocomplexes as crosslinks for polymer hydrogels (Chapter 3). Attaching either L- or D- KYFIL to 4-arm PEG furnishes conjugates that are soluble in aqueous buffer, while 1:1 blends of these conjugates form hydrogels. Gelation correlated with β-sheet formation, underscoring the importance of peptide secondary structure in stereocomplex crosslinks. Moreover, we found the hydrogels to be dynamic, albeit with limited recovery of shear storage modulus under high strain, possibly due to the crystallinity of KYFIL stereocomplexes. This finding motivates intriguing future questions about how to retain the multifaceted benefits of stereocomplexation while increasing the dynamic character of these crosslinks. In addition, like unconjugated peptides, KYFIL the peptide stereocomplexed-crosslinks imbue proteolytic stability. To correlate peptide stereocomplex features with the properties of hydrogel, we then studied helical peptide stereocomplexes designed from a model polyalanine motif (Chapter 4). We varied peptide length, electrostatics, and hydrophobicity of the peptides and assessed gelation behavior of the resulting hydrogels with stereocomplexed crosslinks. We found hydrophobicity to be crucial for gel formation, as none of the L:D-conjugate blends gelled prior to introducing hydrophobic leucine residues. Stereocomplexed hydrogels with hydrophobic peptide crosslinks are stiffer than those formed from unblended L- or D-conjugates, indicating that stereocomplexation promotes gelation. Together, this thesis emphasizes the ability of peptide stereocomplexation as a compelling design consideration to orchestrate supramolecular assembly and tune application-critical properties of advanced biomaterials.