Figure 1.
(A) Classical representation of the major resonance forms of the peptide bond studied; (B) N-acetyl N-methylalaninamide (Ala1); (C) Simplified model (Pep). Definitions of backbone conformation angles are shown.
Figure 2.
Pep model in vacuo: Δω′ variation as a function of ψ′.
Results from calculations at the PBE0/6-31G(d) (•) level and MP2/6-31G(d) (▪) level for the conformer φ′i+1 = 60° (see Fig. S1). On the right side, schematic drawings of conformers of a peptide model characterized by different ψ values are shown. The projections are drawn by looking along the Cα-C bond (see also Fig. S2). The S substituent stands for the CH3-CO-NH- and the CH3- group in Ala1 and Pep model, respectively.
Figure 3.
Pep model in vacuo: Minimized energy for the ψ′ = 150° conformer at different Δω′ values. Comparison of results from different QM methods.
Figure 4.
Pep model in vacuo: Δω′ (▪) and θC (•) as a function of ψ′.
Figure 5.
Dependence of Δω and θC on peptide conformation.
Δω vs. peptide conformation (A) Ala1 model in solvent (B) high resolution protein structures (resolution better than 1.6 Å; Gly and Pro residues excluded from the database); θC vs. peptide conformation (C) Ala1 model in solvent (D) high resolution protein structures (resolution better than 1.6 Å; Gly and Pro residues excluded from the database). Each experimental point is the average of at least 100 independent values.
Figure 6.
Schematic representation of Pep model molecular orbitals.
The three highest energy occupied π orbitals (A, B mainly corresponding to CO π bonding orbitals, C mainly to the Nitrogen Lone Pair) and the lowest energy unoccupied one (D, mainly corresponding to CO π* antibonding orbital) are schematically depicted for the ψ′ = 150° conformer with fixed Δω′ = 0°.
Figure 7.
NBO analysis for different Δω′ values.
Pep model: Orbital interaction energy for the ψ′ = 120° conformer (squared symbols) and the ψ′ = 150° conformer (diamond symbols) vs. Δω′. (▪ ψ′ = 120°, Cα-Cβ σ→CO π*; ♦ ψ′ = 150°, Cα-Cβ σ→CO π*; □ ψ′ = 120°, N n→CO π*; ◊ ψ′ = 150°, N n→CO π*); Ala1-Solv model: Orbital interaction energy for the (φ = −135°, ψ = 150°) conformer vs. Δω′. (• Cα-Cβ σ→CO π*; ○ N n→CO π*).