1H NMR spectrum and conformation of the proline ring in propionylalanylprolineethylamide

The 400 MHz 1H NMR spectrum of propionylalanylprolineethylamide (PAPE) has been obtained in CDCI3 solution. Analysis of the spectrum for the protons of the pyrrolidine ring shows a 4:1 preference for the S, or C-γ-endo, form of the ring. This preference is more pronounced than is commonly observed for trans-proline derivatives.