Conformational effects of the valine side chain on the β_Lβ_Lβ_L extended in the HCO-Gly-L-Val-Gly-NH_2 tripeptide motif: An ab initio and DFT exploratory study

摘要

Ab initio and DFT molecular computations were carried out on the HCO-Gly-L-Val-Gly-NH_2 tripeptide at the HF/6-31G(d) and B_3LYP/6-31G(d) levels of theory. The study of conformation in HCO-Gly-L-Val-Gly-NH_2 tripeptide, which is in repeating sequences of parent elastin-mimetic polypeptide, were performed with varying of side chain torsional angle (X) and backbone dihedral angles (φ_2,ψ_2), to finding the most stable conformer. At first the side chain torsional angle (X) as a variable of the energy function was changed at 30° intervals from 0° to 360°. Three minima, gauche (-), gauche (+) and anti, were obtained that among of them, gauche (-) conformer has the lowest energy. Afterward the two terminal glycine moieties were kept in the β_L conformations while the conformation of the central valine was varied for three states, gauche (-), anti and gauche (+). In addition to electronic energy (E), the key thermodynamic functions: enthalpy (H), Gibbs free energy (G) and entropy (S) were obtained at B_3LYP/6-3 IG(d) level of theory. The most stable conformation for three states of valine, gauche (+), anti and gauche (-) is β_Lβ_Lβ_L conformation and also in gauche (+), anti and gauche (-) states 8, 7 and 6 conformers were found respectively. The obtained β_Lγ_Lβ_L, β_Lγ_Dβ_L, β_Lε_Dβ_L and β_Lα_Dβ_L Conformations in this three states, are more stable in anti state than other two states. According to obtained results dipole-dipole attraction interactions and also entropy play the main role in forming a more stable conformer.