In the past few decades, there has been an increasing number of therapeutic peptides entering the market. Unfortunately, peptide presents several critical challenges in stability where peptides are short amino acids that can easily degrade and aggregate. Aggregation is a significant challenge in the formulation and stability of parenteral peptide formulations. Peptide aggregation is a common instability issue where individual peptide molecules join together to form larger structures such as dimers, oligomers, or insoluble particles. As such, peptides are often unstable in aqueous formulation and require lyophilization to make a stable solid formulation, such as GlucaGen R, 279 TERLIVAZ, 280 Anidulafungin, 281 Caspofungin acetate, 282 Human secretin, 283 Dactinomycin, 284 Oritavancin, Rezafungin, In addition to developing stable peptide drug products via the drying technique, it is important to understand the mechanism of peptide aggregation. This investigation could offer the opportunity to redesign the molecules and optimize the formulation with re-tuned propensity of aggregation. For example, glucagon and insulin regulate the blood glucose levels. While insulin remains stable for several days in its solution form, glucagon quickly forms fibrils when exposed to acidic pH necessary for solubility. Glucagon with an isoelectric point of 7.1, exhibits insolubility in water under physiological pH conditions (pH 4-8). When the pH is 3 or below, glucagon is initially able to dissolve, but after a few hours, it will aggregate together to create a gel. The gelled glucagon primarily consists of beta-sheets or fibril structures and has the potential to obstruct the thin tubing of an infusion set. Early-stage aggregation of glucagon has been investigated by HDX MS and MD simulation (Figure ), suggesting the mechanistic role of a few critical intermolecular contacts. 289 Solid-state NMR was used to determine the atomic-resolution structure of fibrils of synthetic human glucagon grown at pharmaceutically relevant low pH, as shown in Figure ). Unexpectedly, two sets of chemical shifts are observed, indicating the coexistence of two β-strand conformations. Those two conformations have distinct water accessibilities and intermolecular contacts, indicating that they alternate and hydrogen bond in an antiparallel fashion along the fibril axis. Two antiparallel β-sheets assemble with symmetric homodimer cross sections. This amyloid structure is stabilized by numerous aromatics, cation-π, polar and hydrophobic interactions, suggesting mutagenesis approaches to inhibit fibrillization to improve this important drug. Therefore, glucagon is so far produced as lyophilized powder and must be reconstituted in an acidic solution just before use.