The effects of isomerism and side chain mutation on binding energy and NMR/NQR tensors of L-methionylasparagine and L-asparagylmethionine

Density functional theory methods(DFT) and natural bond orbital (NBO) analysis were used to investigate the effects of isomerism and side chain mutation at a microscopic level on the stability, binding energy and NMR/NQR tensors of structural isomers, L- methionylasparagine (Met-Asn) and L- asparagylmethionine (Asn-Met) in the gas phase. The results represented that the isomerism and side chain mutation were caused to change the relative stability, binding energy and the thermodynamics parameters of peptide bond in the considered compounds. Therefore, Asn-Met had higher binding energy and relative stability than Met-Asn. On the other hand, NMR and NQR calculations at B3LYP/6-311+G (d, p) level of theory on the optimized structures of Met-Asn and Asn-Met indicated that the isotropic chemical shielding (σiso) values of oxygen and nitrogen nuclei in two structures with similar positions were different considerably and nitrogen nuclei were more shielded than oxygen nuclei in both dipeptides. In addition, amino nitrogens (N10 nuclei) had the highest values of chemical shielding (σiso) and the nuclear quadrupole coupling constant (χ) among nitrogen nuclei and the order of chemical shielding values of nitrogen nuclei in two structural isomers was amino nitrogen> amidic nitrogen> peptide nitrogen (N10>N9>N1). The mentioned order for chemical shielding values was exactly the opposite of the order of resonance energies values of nitrogen lone pair electrons in two dipeptides. In other words, by increasing contribution of nitrogen lone pair electrons in intra-molecular resonance interactions, NMR chemical shielding around nitrogen nuclei were decreased.