A theoretical study on quadrupole coupling parameters of HRPII Protein modeled as 310-helix & α-helix structures

A fragment of Histidine rich protein II (HRP II 215-236) was investigated by 14N and 17O electric field gradient, EFG, tensor calculations using DFT. This study is intended to explore the differences between 310-helix and α-helix of HRPII both in the gas phase and in solution. To achieve the aims, the 17O and 14N NQR parameters of a fragment of HRPII (215-236) for both structures are calculated. Due to the side chain arrangements of the 310-helix, this conformation contains several hydrogen bonding contacts in comparison to the α-helix form. The resultant 14N and 17O s of peptide bonds of HRPII are affected by these contacts. Both in the gas phase and in solution, the differences in 14N s of backbone are within the uncertainties identical between two conformers but not for NH groups of backbone that the related amino acids participate in intramolecular hydrogen bond formation with side chain. In this case, the differences in 14N of backbone are avg.= 0.36 in gas phase and avg.= 0.43MHz in solution. However, differences in 17O parameters of the backbone C=O are distinguishable between two conformers, regardless of in gas phase and in solution, with and without influencing of the intramolecular hydrogen bond. These differences reveal how hydrogen bond interactions affect EFG tensors at the sites of oxygen and nitrogen nuclei.