Research

Inspired by the high functionality of small peptides, particularly within polymeric materials, and the impact of stereochemistry on material properties, we are developing peptide-polymer composites to interact productively with biological systems and respond to environmental cues. Our main research directions include:

Delivering Therapeutic Peptides:

From anti-microbial to anti-cancer, from wound-healing to diabetes-treating, therapeutic peptides are quite versatile, but share a common limitation hampering their clinical implementation: these therapeutics get cleared out of the body quickly, i.e. have short circulatory half-lives. Our lab explores two branching techniques to protect these therapeutics: complexation and conjugation. Complexation relies on a designing a polymer with suitable hydrophobicity and electrostatics to attract the peptide and form a complex. If peptides aren’t readily amenable to complex (typically if they have a low net charge), we reversibly modify them to tailor them to complex. Conjugation involves a chemical bond between the polymer and the peptide – tunable enough to form different architectures such as stars or combs. Traditionally, inert polymers are used for conjugation, but we are also exploring ones that improve the therapeutic effect.

1. Degradable Poly(ß-amino ester)s or PBAEs

Degradable polymers, such as PBAEs, are promising carriers for therapeutic peptides – protecting them through complexation and releasing them upon degradation. PBAEs are easy to synthesize and functionalize, and we can equip the traditionally cationic carriers with anionic groups. The anionic groups drive complexation with cationic peptides and the esters can be hydrolytically degraded. Degradation can be slowed by decreasing pH, increasing hydrophobicity, or post-synthesis modification, but these changes often affect complexation as well. Thus, we are interested in exploring factors such as molecular weight and charge ratio to see if we can find enough knobs to control both complexation and degradation for controlled drug delivery.

Relevant Publications:

(1)   Kuenen, M. K.; Cuomo, A. M.; Gray, V. P.; Letteri, R. A. Net Anionic Poly(β-Aminoester)s: Synthesis, pH-Dependent Behavior, and Complexation with Cationic Cargo. Polym. Chem. 2023, 14 (4), 421– 423. https://doi.org/10.1039/d2py01319c.

(2)   Kuenen, M. K.; Mullin, J. A.; Letteri, R. A. Buffering Effects on the Solution Behavior and Hydrolytic Degradation of Poly(Β‐amino Ester)s. Journal of Polymer Science 2021, 59 (19), 2212–2221. https://doi.org/10.1002/pol.20210377.

(3)   Kuenen, M. K.; Reilly, K. S.; Letteri, R. A. Elucidating the Effect of Amine Charge State on Poly(β- Amino Ester) Degradation Using Permanently Charged Analogs. ACS Macro Letters 2023, 12 (10), 1416–1422. https://doi.org/10.1021/acsmacrolett.3c00440.

2. Presenting Antimicrobial Peptides

Antimicrobial peptides (AMPs) are known for disrupting bacterial cell membranes and killing bacteria. Despite their potential, AMPs are often limited by their small size, making them prone to rapid degradation within the body. Our group explores the potential of enhancing AMPs’ half-life and activity by attaching them to different charge-neutral hydrophilic polymers. We are specifically exploring the structure-performance and hydrophilicity-performance relationships of these polymer-AMP attachments. Our research has demonstrated that conjugating AMPs to hydrophilic poly (ethylene glycol) (PEG) polymers significantly reduces their degradation rates and improves their antimicrobial activity, offering a promising pathway for stabilizing AMPs and enhancing their therapeutic potential.

Relevant Publications:

(1) Cui, Z.; Crawford, M. A.; Rumble, B. A.; Krogh, M. M.; Hughes, M. A.; Letteri, R. A. Antimicrobial Peptide–Poly(Ethylene Glycol) Conjugates: Connecting Molecular Architecture, Solution Properties, and Functional Performance. ACS Polym. Au 2024, 4 (1), 

45–55. https://doi.org/10.1021/acspolymersau.3c00026.


(2) Cui, Z.; Luo, Q.; Bannon, M. S.; Gray, V. P.; Bloom, T. G.; Clore, M. F.; Hughes, M. A.; Crawford, M. A.; Letteri, R. A. Molecular Engineering of Antimicrobial Peptide (AMP)– Polymer Conjugates. Biomater. Sci. 2021, 9 (15), 5069–5091. https://doi.org/10.1039/D1BM00423A.


(3)   Crawford, M. A.; Ward, A. E.; Gray, V.; Bailer, P.; Fisher, D. J.; Kubicka, E.; Cui, Z.; Luo, Q.; Gray, M. C.; Criss, A. K.; Lum, L. G.; Tamm, L. K.; Letteri, R. A.; Hughes, M. A. Disparate Regions of the Human Chemokine CXCL10 Exhibit Broad-Spectrum Antimicrobial Activity against Biodefense and Antibiotic-Resistant Bacterial Pathogens. ACS Infect. Dis. 2023, 9 (1), 122–139. https://doi.org/10.1021/acsinfecdis.2c00456.

3. Reversible Modification of Therapeutic Peptides for Complexation

Reversible ways to change the net charge of therapeutic peptides are required to realize both incorporation into and eventual release from drug delivery vehicles (protective packaging of the therapeutic). Prior work from our lab established a way to reversibly modify multiple carboxylic acids on α-carboxyl terminus 11 (𝛂CT11, RPRPDDLEI), a cardioprotective and wound-healing peptide can increase its cell membrane permeability, using esterification. As esterification increases the net positive charge of 𝛂CT11, we envisioned that it could also enable incorporation into a delivery vehicle via electrostatically driven particle formation with an anionic material. We aim to leverage esterification to enable both the incorporation and subsequent release of a peptide with low net charge, would otherwise be unable to form particles with an anionic material, unless a permanent modification hindering release was to be made. This technology can be extended to other similar therapeutic peptides accelerating the clinical implementation of this class of unique and promising therapeutics.

Relevant Publications:

(1)   Bannon, M. S.; Ellena, J. F.; Gourishankar, A. S.; Marsh, S. R.; Trevisan-Silva, D.; Sherman, N. E.; Jourdan, L. J.; Gourdie, R. G.; Letteri, R. A. Multi-Site Esterification: A Tunable, Reversible Strategy to Tailor Therapeutic Peptides for Delivery. Mol. Syst. Des. Eng. 2024, 10.1039.D4ME00072B.


Stereochemistry-Directed Assembly of Peptides:

Peptides are biologically relevant molecules that have unique structure-property relationships which often dictate their therapeutic function. Despite their structural diversity and extraordinary functions, these naturally occurring biological molecules often suffer from poor enzymatic stability. To increase the enzymatic stability, we are leveraging stereochemistry-directed assembly of peptides, or stereocomplexation, which simply means the assembly of one or more left- and right-handed molecules. Interestingly, when these left- and right-handed molecules (often denoted as L- and D-) come together, they change the bulk material properties and protect peptides from enzymatic degradation. Our goal is to broaden the peptide stereocomplex design toolkit by determining how peptide molecular features (e.g. length, charge, hydrophobicity) translate to bulk properties of biomaterials (e.g. mechanics, proteolytic stability, binding strength). With this design knowledge in hand, we envision stereocomplexation of peptides to find applications in regenerative medicine, drug delivery, and protein separations.

1. Peptide stereocomplexation orchestrates supramolecular assembly of hydrogel biomaterials1

Since peptide stereocomplexation can be used to tailor biomaterial properties, we wanted to understand how the mechanics and stability of the self-assembling pentapeptide, L-KYFIL, would change in response to complexation with D-KYFIL. We found that homochiral KYFIL is soluble in water, however, upon mixing L- and D-KYFIL in water we observed the mixture formed a gel after 10 minutes. Interestingly, homochiral KYFIL in physiologic salt and pH conditions formed a gel, but the mixture was more liquid-like, which made sense when we understood how sequence and environment dictates assembly.  In line with expectations, we found that D-KYFIL shields L-KYFIL from enzymatic degradation when mixed in these conditions. Upon understanding how sequence and environment dictates assembly and realizing the potential of peptide stereocomplexation to modulate mechanics and stability, we are eager to explore new directions where peptide stereocomplexes are used as dynamic crosslinks in polymeric hydrogels.

Relevant Publications:

(1) Duti, I. J.; Florian, J. R.; Kittel, A. R.; Amelung, C. D.; Gray, V. P.; Lampe, K. J.; Letteri, R. A. “Peptide Stereocomplexation   Orchestrates Supramolecular Assembly of Hydrogel Biomaterials.” J. Am. Chem. Soc. 2023, 145 (33), 18468–18476.   https://doi.org/10.1021/jacs.3c04872.

2. Designing Coiled Coil Peptides for Stereocomplexation

Coiled coils are complexes of helical peptides that bind strongly and specifically, allowing them to be incorporated into applications like biosensing, drug delivery, and protein purification. Traditional coiled coils contain only L-peptides, which are limited by their poor enzymatic stability. While coiled coils containing D-peptides could be an alternative due to their good enzymatic stability, their high cost and unknown immune response make them a less favorable solution. As an alternative, we are interested in coiled coil systems containing both L- and D-peptides (stereocomplex), which have the potential to balance out the negatives of D-only systems while improving the binding strength and enzymatic stability relative to L-only coiled coils. Specifically, we are focused on developing design rules for stereocomplexed coiled coils that govern their binding strength and enzymatic stability. 

Relevant Publications:

(1) Gray, V., Letteri, R. “Designing Coiled Coils for Heterochiral Complexation to Enhance Binding and Enzymatic Stability.” Biomacromolecules 2024, 25, 5273−5280. https://doi.org/10.1021/acs.biomac.4c00661.