The proton resonance spectra and conformations of the prolines

Abstract

The proton resonance spectra of trans hydroxy-L-proline and allo hydroxy-L-proline in D₂O are obtained and completely analysed to give the coupling constants between, and chemical shifts of, all the non-exchangeable protons in the molecules. The values of the coupling constants between protons on adjacent carbon atoms in the five-membered ring vary from 1 to 10 c.p.s. Also a long-range coupling over four bonds is observed between one C₃ proton and one C₅ proton in both compounds. From the values of the proton-proton couplings, it is proposed that the ring in both compounds is considerably buckled in solution. The ring in the trans hydroxy proline has a conformation in which C₄ projects out of the plane of the other ring atoms in the opposite direction to the carboxyl group. In the allo compound no such simple conformation is found.