Among the proteinogenic amino acids, proline asserts a singular influence over protein structure and function due to its unique ring structure. Unlike other amino acids, proline's side chain is covalently bonded to the nitrogen, thereby forming a characteristic pyrrolidine loop. Consequently, proline can disrupt and terminate both alpha helices and beta sheets as well as form a distinct type of protein secondary structure known as a polyproline helix. Furthermore, beyond its biological functions, proline can act as a catalyst in important organic reactions such as the aldol and Mannich reactions. As with other amino acids, proline can exist in multiple conformers, which can be categorized by their intramolecular hydrogen bonding patterns as shown in Figure . Type I features an N-H•••O=C interaction, Type II features an H-N•••H-O interaction, and Type III features an N-H•••O-H interaction. Types I and III are characterized by hydrogen bonded N-H stretches and free O-H stretches, while Type II is characterized by a free N-H stretch and a hydrogen bonded O-H stretch. Typically, Type I and Type III conformers are highly similar in their vibrational spectra, and we here collectively refer to them as Type I/III. The ring structure of proline introduces an additional source of conformational isomerism. Specifically, the pyrrolidine ring can pucker either up or down, with the lower-case letters a and b used to refer to the lower and higher energy ring puckers, respectively. The energy ordering of these conformers can vary depending on the amino acid. For proline, past experimental and theoretical investigations indicate that the Type II conformer is the lowest in energy, and that the lowerenergy ring-pucker conformers dominate under the experimental matrix conditions. Amino acid conformers can be investigated with vibrational spectroscopy.