Secondary structure of peptides. 17—Cis/trans isomerism of solid proline‐containing oligopeptides as revealed by 13C NMR CP/MAS spectroscopy

Abstract

AbstractThe 75.4 MHz 13C NMR CP/MAS spectra of five classes of solid proline peptides were measured: cyclodipeptides, unprotected dipeptides, N‐tert‐butoxycarbonyl‐ (Boc) or N‐benzyloxycarbonyl‐ (Z)‐proline derivatives and N‐protected peptides with an aminoacyl—proline bond. In the case of solid N‐protected proline derivatives, the shift difference (Δδ) of the C‐β and C‐γ signals mainly reflects the cis/trans isomerism of the XPro bond, with Δδ = 3‐5 ppm being characteristic of trans and 7‐11 ppm of cis isomers. However, in the case of solid cyclodipeptides and unprotected dipeptides, the individual crystal lattices also affect the conformation of the side chain, so that the C‐β and C‐γ‐signals are not reliable indicators of the cis/trans isomerism.When the N‐protected proline derivatives were crystallized slowly from various solvents, the N‐Boc‐ and N‐Z‐proline derivatives exclusively yielded the cis isomer, even when the cis/trans equilibrium in solution favoured the trans isomer. In the case of N‐aminoacylprolines, the trans isomer crystallized almost exclusively. Oligopeptides containing two proline units, such as ZProProOH or ZGlyProGly GlyProAlaOH, may possess a cis and a trans bond in the same molecule.