Conformational Analysis of N- and C-Terminally Protected Tripeptide Model Glycyl-Isoleucine-Glycyl: An ab initio and DFT Study

An ab initio and density functional theory (DFT) study on conformational analysis of tripeptide model HCO-GLY-L-ILE-GLY-NH2 is presented. The tripeptide was scanned around initial, central, and final residues, separately while for every scanning procedure the two other residues had been kept in the β conformation and side chain (SC) dihedral angles were maintained on the gauche− (g‾) state (χ1, χ2 =-60). Conformers (βL, γL, γD, αD, εD), (βL, γL, γD), and (βL, γL, γD, εL) were found through scanning of the tripeptide around initial, central, and the last amino acids, respectively. The geometry optimization and frequency calculation were performed at the HF/6-31G(d) and B3LYP/6-31G(d) levels of theory. In followings, comparison of the calculated thermodynamic data presents βLβLβL as the most stable conformer among the tripeptide minima on Ramachandran map.