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The conformations of proline esters are investigated by infrared spectroscopy in supersonic slit jet expansions. Two easilyconvertible puckering variants of the pyrrolidine ring with intramolecular N-H···O contacts are shown to be particularly stable.The aggregation tendency of proline esters via intermolecular N-H···O hydrogen bonds is remarkably weak. IR differences betweenenantiopure and racemic dimers are difficult to quantify. Dehydrogenation of the pyrrolidine ring to pyrrole leads to a stableplanar carboxylic ester conformation. Its aggregation tendency is pronounced due to the planar hybridization of the nitrogenatom and leads to a symmetric,  β  sheet-like dimer with strongly red-shifting hydrogen bonds. The spectroscopic observations underscore the differences betweenintermolecular interactions of N-terminal and peptide-bound amino acids in peptide chains.

Conformation and Aggregation of Proline Esters and Their Aromatic Homologs: Pyramidal vs. Planar RR´N-H in Hydrogen Bonds