Steric effects on the amide isomer equilibrium of prolyl peptides. Synthesis and conformational analysis of N-acetyl-5-tert-butylproline N'-methylamides

The influence of a bully 5-position substituent on the amide isomer equilibrium N-terminal to proline has been explored via the synthesis and analysis of N-(acetyl)proline N'-methylamide (1) and its respective cis- and trans-5-tert-butylproline amide diastereomers 2 and 3. The relative populations of the amide cis- and trans-isomers as well as the energy barriers for amide isomerization of 1-3 in D2O were ascertained using NMR with coalescence and magnetization transfer experiments. The relative populations of free C-terminal amide and hydrogen-bonded amide in the gamma-turn conformation were also estimated by integrating the N-H stretch absorbances in the FT-IR spectra of 1-3 in CHCl3 and CCl4. In the prolyl peptides, the 5-tert-butyl substituent was found to exhibit profound effects on the amide isomer equilibrium, on the energy barrier for amide isomerization, and on the stability of the gamma-turn conformation. Steric interactions between the 5-position substituent and the N-acetyl group disfavor the amide trans- and augment the cis-isomer population: 25% in 1, 48% in 2, and 66% in 3. In the case of cis-5-tert-butylproline 2, the energy barrier for amide isomerization is observed to be 3.9 kcal/mol lower than that of 1, On the other hand, the amide isomerization barrier for trans-5 tert-butylproline 3 is similar to that for 1, Only a single amide N-H stretch band is observed at 3454 cm(-1) in the FT-IR spectrum of 3 in CHCl3 and indicates that the NH group is free of intramolecular hydrogen bonding. Hence, trans-5-tert-butylproline amide 3 does not adopt a seven-membered gamma-turn conformation, which is a favored conformer for 1 and 2 in CHCl3. Maps, in which the psi- and omega-dihedral angles are plotted at 30 degrees intervals against the calculated energy of the local minimum conformation, predict qualitatively and display clearly all of the observed effects of the 5-tert-butyl substituent on the amide isomer in the N-(acetyl)-proline N-methylamides. The results of this study suggest the use of 5-tert-butylprolines to prepare both X-Pro cis-amide isomers and twisted amide surrogates for examining prolyl residue conformations in bioactive peptides.