Coupling of intramolecular hydrogen bonding to the cis-to-trans isomerization of a proline imide bond of small model peptides

A relationship between intramolecular hydrogen bonding and the cis-trans is omerization of a proline imide bond for proline-containing short peptides w ere studied by proton NMR and infrared spectroscopy using DMSO-d(6)/CDCl3 m ixed solvents. The percentage of the trans form increases with increasing f raction of CDCl3 in the mixed solvents except for compounds without possibi lity of intramolecular hydrogen bonding. Chemical shift variations of amide protons with solvent mixing ratios were found to be useful for judging whe ther the amide protons take part in the intramolecular hydrogen bonding to a considerable degree or not. These results and infrared spectra were used to specify intramolecularly hydrogen bonded structures of the peptides. For mation of the 10-membered or 13-membered hydrogen bonded ring which include s the carbonyl group precedent to the prolyl residue facilitates the cis-to -trans isomerization and these hydrogen bonded rings are strong enough to r estrict the proline imide bond to the trans form in CDCl3 solution. On the other hand, a 7-membered hydrogen bonded ring is not so effective in restri cting the proline imide bond. (C) 2000 Elsevier Science B.V. All rights res erved.