UC Riverside

Peptides are short chains of amino acids connected by amide bonds and play essential roles in numerous biochemical processes. With the advent of automated peptide synthesis, the production of structurally diverse peptides has become significantly more accessible. Like proteins, peptides exhibit structure-dependent functions, yet they are more cost-effective and more straightforward to produce. These properties have led to their widespread use in fields such as drug delivery, drug discovery, protein mimicry, and therapeutics.Proteins owe their functional specificity to their ability to fold into complex conformations, a feature governed by their amino acid sequences and side-chain functionalities. While linear peptides are easier to synthesize, they often lack the conformational rigidity necessary for more targeted interactions. Cyclization strategies have been shown to enhance peptide stability, improve protein binding, and increase resistance to enzymatic degradation. As such, macrocyclic peptides hold considerable promise for advancing therapeutic and diagnosticapplications. This dissertation seeks to explore how macrocyclic peptides can be leveraged and expanded for biomedical applications.Chapter 2 focuses on EPP6, a previously reported cell-penetrating peptide (CPP) known for its efficient cellular uptake and ability to deliver various cargos in specific cells. In one example, EPP6 was conjugated to Gemcitabine HCl, resulting in enhanced cytotoxicity against several cancer cell lines. In another example, EPP6 demonstrated potential for oral delivery of therapeutic peptides such as insulin.Chapter 3 first examines the structure–activity relationship of EPP6 to guide optimization strategies. In the second part, a complete 5-mer EPP library was synthesized and screened, resulting in the identification of multiple new EPP candidates with uptake efficiencies exceeding that of EPP6, thus expanding the scope of this novel class of CPPs.Chapter 4 details the design and synthesis of a new series of bicyclic peptides with a unique distribution of functional residues, cyclization strategies, and conjugation handles. A novel reaction scheme employing two consecutive copper-catalyzed azide–alkyne cycloaddition (CuAAC) reactions was developed, replacing the previously used CuAAC–RCM combination. This new approach proved to be efficient and generalizable. A series of 23 bicyclic (BX) peptides was synthesized as proof of concept. These peptides were further functionalized with fluorescent dyes and linkers, demonstrating their adaptability for future studies.