Conformational preferences of aza-proline residues and their impact on the relative stability of polyproline structures

Abstract

The effects of replacement with aza groups in the proline ring on the conformational preference and prolyl cis–trans isomerization were investigated for Ac-X-OMe (X = α-azPro, β-azPro, γ-azPro, and δ-azPro) using DFT methods with an implicit solvation model in water. In addition, we studied the propensity to form the PPII structure for Ac-(Pro)₅-NMe₂ and Ac-(Hyp-Gly-Pro)₂-NMe₂ as the polyproline model peptide (PMP) and the single-strand collagen model peptide (CMP), respectively, with the incorporation of azPro residues in the middle of each model peptide in water. The calculated results indicated that the incorporation of the aza group in the proline ring induced different conformational preferences of the Pro residue depending on the substitution position. In particular, the α-azPro residue was expected to provide conformational adaptability of the α-azPro residue in the various Pro-containing peptides. Compared to the cis population of Ac-Pro-OMe, there was an increase in cis population for the α-azPro, β-azPro, and γ-azPro peptides, whereas a greater decrease was found for Ac-δ-azPro-OMe. The ΔG^‡_tc barrier to trans-to-cis isomerization decreased by 2.5–7.2 kcal mol⁻¹ due to the incorporation of azPro and the greatest decrease was found for Ac-α-azPro-OMe, which was ascribed to the weakened C–N bond of the trans and ts conformers. The substitution with all azPro residues enhanced the propensity to form the PPII structure of PMP. In the case of CMP1, the replacement of Pro with γ-azPro(d) and δ-azPro(d) enhanced the stability of the PPII structure more by 2.47 and 2.24 kcal mol⁻¹ than the canonical CMP. However, the relative propensity to form the PPII structure was diminished by 1–4 kcal mol⁻¹ for all CMP2 with the replacement of Hyp compared to the canonical CMP. Hence, we deduced that the replacement of Pro with γ-azPro(d) or δ-azPro(d) in the single strand of (Hyp-Gly-Pro)_n can enhance the preorganization of the PPII structure and may lead to an increase in triple helix stability.