Intramolecular Hydrogen Bond-Controlled Prolyl Amide Isomerization in Glucosyl 3′(S)-Hydroxy-5′-hydroxymethylproline Hybrids: Influence of a C-5′-Hydroxymethyl Substituent on the Thermodynamics and Kinetics of Prolyl Amide Cis/Trans Isomerization

Peptide mimics containing spirocyclic glucosyl-(3′-hydroxy-5′-hydroxymethyl)proline hybrids (Glc3′(S)-5′(CH2OH)HypHs) with a polar hydroxymethyl substituent at the C-5′ position, such as C-terminal ester Ac-Glc3′(S)-5′(CH2OH)Hyp-OMe and C-terminal amide Ac-Glc3′(S)-5′(CH2OH)Hyp-N′-CH3, were synthesized. C-Terminal esters exhibit increased cis population (23−53%) relative to Ac-3(S)HyPro-OMe (17%) or Ac-Pro-OMe (14%) in D2O. The prolyl amide cis population is further increased to 38−74% in the C-terminal amide form in D2O. Our study shows that the stereochemistry of the hydroxymethyl substituent at the C-5′ position of proline permits tuning of the prolyl amide cis/trans isomer ratio. Inversion−magnetization transfer NMR experiments indicate that the stereochemistry of the hydroxymethyl substituent has a dramatic effect on the kinetics of prolyl amide cis/trans isomerization. A 200-fold difference in the trans-to-cis (ktc) isomerization and a 90-fold rate difference in the cis-to-trans (kct) isomerization is observed between epimeric C-5′ 3 and 4. When compared to reference peptide mimics Ac-Pro-OMe and Ac-3(S)Hyp-OMe, our study demonstrates that a (13−16)-fold decrease in ktc and kct is observed for the C-5′(S), while a (5−24)-fold acceleration is observed for the C-5′(R) epimer. DFT calculations indicate that the pyrrolidine ring prefers a Cβ exo pucker in both Ac-Glc3′(S)-5′(CH2OH)Hyp-OMe diastereoisomers. Computational calculations and chemical shift temperature coefficient (Δδ/ΔT) experiments indicate that the hydroxymethyl group at C-5′ in Ac-Glc3′(S)-5′(CH2OH)Hyp-OMe forms a stabilizing intramolecular hydrogen bond to the carbonyl of the N-acetyl group in both epimeric cis isomers. However, a competing intramolecular hydrogen bond between the hydroxymethyl groups in the pyrrolidine ring and pyran ring stabilizes the trans isomer in the C-5′(S) diastereoisomer. The dramatic differences in the kinetic properties of the diastereoisomeric peptide mimics are rationalized by the presence or absence of an intramolecular hydrogen bond between the hydroxymethyl substituent located at C-5′ and the developing lone pair on the nitrogen atom of the N-acetyl group in the transition state.