The proton resonance spectra and conformations of the prolines

Abstract

The dihedral angles in a regular pentagon are calculated as functions of the amount of buckle for the two symmetric deformations of the ring. Applying these results, together with an equation of the Karplus type to the observed proton coupling constants in trans hydroxy-L-proline and allo hydroxy-L-proline, the conformations of these molecules in solution are found. These molecules exist in the ‘C_S’ or envelope conformation, with C₄ projecting out of the plane of the other ring atoms in the trans compound and C₅ in the allo compound. The angles of buckle of the ring are 53° and approximately 70° respectively. This determination provides a quantitative test of the Karplus equation. It is found that an equation of this form with experimentally determined coefficients exactly describes the observed coupling constants.