Peptide Conformation. 12.¹ Conformation of Cyclo-(L-Pro₃) in Solution

The 270- and 500-MHz ¹H NMR spectra of cyclo-(L-Pro₃) have been completely analyzed. The molecule shows effective C₃ symmetry. A number of previous spectral assignments had to be revised on the basis of 4H chemical shifts and changes in these shifts caused by the addition of benzene, Me₂SO, or the shift reagent Eu(fod)₃. The ¹H coupling constants are best interpreted in terms of an α⁺ (envelope) conformation. The high mobility of the γ-CH₂ group easily transforms this structure to twist conformations.