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 ¹⁵N (denoted by*). The variable amino acids are glycine, alanine, leucine, valine, and phenylalanine. For the natural abundance ¹⁵N-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 ¹⁵N-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 ¹⁵N-nmr signals of all oligopeptides were achieved by selective isotopic enrichment and by means of ¹³C-nmr spectra.