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

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