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 oftheory were carried out. geometry optimization of the dipeptide within the leucine (leu) side chainangles (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, h, 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 indicatethat the bb has the highest stability bearing 'p i (do = 180°, (1:11 (d11 ) = 180°, t2 (d6) = 150°, and 02(d4) = 210°.