Conformational Properties of 2,4-Methanoproline (2-Carboxy-2,4-methanopyrrolidine) in Peptides: Evidence for 2,4-Methanopyrrolidine Asymmetry Based on Solid-State X-ray Crystallography, 1H NMR in Aqueous Solution, and CNDO/2 Conformational Energy Calculations

The crystal structure of the terminally blocked amino acid A^r-acetyl-2,4-methanoproline-A^r-methylamide (Ac-2,4-MePro-NHMe) has been determined by X-ray crystallography (R = 0.05). In the solid state, the Ac-2,4-MePro peptide group is trans, and slightly nonplanar (o>₀= -178.4°). The unit cell was found to contain two conformations of Ac-2,4-MePro-NHMe which are characterized by the values of the backbone dihedral angles ϕ (±29°) and ^ (=f 114.6°), and by distortions of the side-chain 2,4-methanopyrrolidine ring conformation. These two conformations are related to each other as mirror images. 1H NMR spectroscopy was used to identify the 2,4-methanopyrrolidine asymmetry in Ac-L-Tyr-2,4-Me-Pro-NHMe in water. CNDO/2 conformational energy calculations on Ac-2,4-MePro-NHMe and related analogues indicate that the preference for asymmetric side-chain conformations and nonzero values of ϕ arises primarily from inter-residue interactions, and particularly from unfavorable interactions between the carbonyl group of the preceding peptide with the carbonyl group of the symmetric 2,4-MePro residue (e.g., unfavorable nonbonded 1–4 C'*»C' interactions). These results indicate that proper modeling of the conformational properties of 2,4-MePro in peptides requires that its backbone (ϕ) and side-chain conformational chirality be taken into account.