Theoretical Analysis on the Conformational Features of the HCO—Gly—L—Leu—NH2 Protected Dipeptide Motif: Ab initio and DFT Exploratory

For better understanding of conformational stability of the dipeptide model HCO—Gly—L—Leu—NH2, ab initio and DFT computations at HF/6-31G(4 6-311++G(d,p) and B3LYP/6-31G(d) levels of theory were carried out. Geometry optimization of the dipeptide within the leucine (Leu) side chain angles (x2 ,x2) resulted in three stable conformations as followings: anti-anti, the most stable one, (Xi = 180°, x2 = 180°), Gauche (+)-trans (xi= 60°, x2 = 210°) and 270°-Gauche (-)(x4 = 270°, x2 = 3001. The thermodynamic properties E, G, and S by changing dihedral angles '111 (DO and (Di (D11) of glycine (Gly), ‘1J2 (D6), and (62 (D4) of Leu and keeping the SC dihedral angles of the anti-- anti conformer were obtained by frequency calculations at the same levels. The calculations indicate that the BB has the highest stability bearing 'P I (DO = 180°, (1:11 (D11 ) = 180°, T2 (D6) = 150°, and 02 (D4) = 210°.