¹⁵N‐nmr spectroscopy. 20. Cis/trans isomerism and neighboring residue effects of proline‐containing peptides

Five N ‐protected tetrapeptide esters of the structure Gly‐Pro‐ X ‐ X *‐ O ‐methyl were synthesized in such a way that one of the two variable amino acid residues ( X ) was isotopically enriched in 15 N (denoted by*). The variable amino acids are glycine, alanine, leucine, valine, and phenylalanine. For the natural abundance 15 N‐nmr spectra of these tetrapeptide derivatives in methylene chloride only the signals of the Gly‐Pro trans isomer were found. In a 2:1 mixture of acetone and dimethylsulfoxide, signals for both the cis and trans isomers were observed. Three of the five tetrapeptide derivatives show cis / trans splitting of all four nitrogen signals. The 15 N‐nmr spectra of Z ‐Pro‐Pro‐OH and of ( D , L ‐proline) n were measured in a 2:1 mixture of acetone and dimethylsulfoxide as well as in water. The effects of solvents and neighboring residues and the influence of the cis / trans isomerism on the nmr spectra are discussed. The determination of the cis / trans equilibria and the assignment of the 15 N‐nmr signals of all oligopeptides were achieved by selective isotopic enrichment and by means of 13 C‐nmr spectra.