Self-assembly is a fundamental process in the field of nanotechnology, where molecules organize into complex structures spontaneously or induced by environmental factors. Peptides, short chains of amino acids, can self-assemble into many types of nanostructures. The self-assembly of peptides is governed by noncovalent interactions, including electrostatic interactions, hydrogen bonding, hydrophobic interactions, aromatic-aromatic interactions, and van der Waals forces. By varying the amino acid sequences and manipulating environmental parameters, these interactions can be modulated to obtain diverse supramolecular nanostructures, exhibiting a wide range of physical, chemical, and biological properties. Furthermore, the ability to control these properties opens up a world of possibilities in biomedical and biocatalytic applications. From drug delivery systems to enzyme mimics, as well as cancer treatments, the potential of these self-assembling peptides is vast and continues to be a vibrant area of research. Exploiting this potential, this dissertation delves into the design, synthesis, and investigation of self-assembling peptides for a range of applications. The introductory chapters of this document lay the groundwork, providing a comprehensive overview of self-assembly and its potential in biocatalytic and biomedical domains. The focus shifts in the later chapters to drug delivery applications, particularly in the delivery of hydrogen sulfide (H2S), and its implications in cardioprotection and cancer treatment. Finally, this document details an evaluation of self-assembled peptides in the context of biocatalysis using a combined experimental and computational approach. Chapter 3 discusses the design and synthesis of peptide-H2S donor conjugates (PHDCs) with an unusual adamantyl group. Several of PHDCs studied in this chapter self-assembled into novel nanocrescent structures observed under both conventional transmission microscopy (TEM) and cryogenic TEM (cryo-TEM). By varying the C-terminal amino acid with cationic, nonionic, or anionic amino acids, the PHDC morphologies remained unaffected, offering a robust peptide design for crescent-shaped supramolecular nanostructures. Chapter 4 discusses an extension of this project, introducing a cyclohexane in PHDCs instead of an adamantyl group. In this work, we designed and fabricated four constitutional isomeric PHDCs, which self-assembled into nanoribbons with different dimensions and large nanobelts. These morphologies exhibited varying cellular uptake and in vitro H2S release amounts, influencing their protective effects against oxidative stress induced by H2O2. With the knowledge of the impact of subtle changes in PHDC structures, Chapter 5 discusses our further design of three more PHDCs with the variation of side chain capping group, from an aromatic phenyl ring to a cyclohexane unit, to an aliphatic n-hexyl chain. In this chapter, we studied how changes in the hydrocarbon tail can influence the supramolecular nanostructures and their potential ability for colon cancer treatment. A final aspect of H2S delivery in Chapter 6 involves the creation of a stable PHDC with an extended H2S release profile. By integrating the H2S donor into a β-sheet forming peptide sequence with a Newkome-like poly(ethylene glycol) dendron, this PHDC self-assembles into spherical or fibril nanostructures with or without stirring. The H2S release was further studied by triggering release with various charged thiol molecules. Finally, another facet of this document focuses on three constitutional isomeric tetrapeptides containing a catalytic functional amino acid, His. Chapter 7 discusses these tetrapeptides, which self-assembled into nanocoils, nanotoroids, and nanoribbons based on the position of the His residue in the peptide sequence. Computational studies simulating the self-assembling process revealed the distribution of His residues and hydrophobic pockets, reminiscent of natural enzyme binding sites. A tight spatial distribution of His residues and hydrophobic pocket in nanocoils provided a picture for why this morphology exhibited the highest rate enhancement in catalyzing a model ester hydrolysis reaction. This study demonstrated how subtle molecular-level changes impact supramolecular nanostructures and catalytic efficiency. The final chapter details conclusions on all the research in this dissertation and discusses further directions of self-assembling peptides in the application of drug delivery and design of catalyst mimics. / Doctor of Philosophy / Self-assembly is a fascinating process in nanotechnology, where molecular building blocks come together to form complex structures. Peptides, which are short chains made up of amino acids, can play a crucial role in this process. They can organize themselves into various shapes due to different forces acting between their amino acid building blocks. By changing the arrangement of amino acids and adjusting the environment, scientists can create a wide range of nanoscale structures with unique properties from peptides. These self-assembling peptides have enormous potential in fields like medicine and catalysis. This dissertation describes how to design and make self-assembling peptides for various uses. Chapter 1 describes the general structure of the document, and Chapter 2 discusses the basics of self-assembly and how it can be applied in medicine and other areas. Chapters 3-6 focus on using self-assembling peptides to deliver hydrogen sulfide (H2S), a noxious gaseous molecule that is now recognized as a vital signaling molecule involved in various physiological processes. Several classes of peptide-H2S donor conjugates (PHDCs) are discussed in these chapters, including PHDCs that form nanoscale crescents, twisted ribbons, fibers, and other structures. These nanostructures show promise in protecting cells from harmful substances or can act as drugs in cancer treatment. We also investigate how different modifications affect their performance in biomedical applications. The final research chapter, Chapter 7, involves using self-assembling peptides as catalysts, molecules that speed up chemical reactions. By arranging the amino acids in different ways, peptides that form nanoscale coils, toroids, or ribbons-like structures were created. These different shapes influenced how well they catalyzed reactions. Computational modeling studies helped explain how small differences in molecular design led to big impacts on their catalytic abilities. The final chapter discloses conclusions on all the research in this dissertation and discusses the further directions of self-assembling peptides as medicines and catalysts.