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 bulky 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 D 2 O were ascertained using NMR with coalescence and magnetization transfer experiments. The relative populations of free C -terminal amide and hydrogen-bonded amide in the γ-turn conformation were also estimated by integrating the N−H stretch absorbances in the FT-IR spectra of 1 − 3 in CHCl 3 and CCl 4 . 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 γ-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 CHCl 3 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 γ-turn conformation, which is a favored conformer for 1 and 2 in CHCl 3 . Maps, in which the ψ- and ω-dihedral angles are plotted at 30° 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.