Ab Initio Quantum Chemical Studies of 15N and 13C NMR Shielding Tensors in Serine and Complexes of Serine- nH2O: Investigation on Strength of the CαH…O Hydrogen bonding in the Amino Acid Residue.

In this paper, the hydrogen bonding (HB) effects on the NMR chemical shifts of selected atoms in serineand serine-nH2O complexes (from one to ten water molecules) have been investigated with quantummechanical calculations of the 15N and 13C tensors. Interaction with water molecules causes importantchanges in geometry and electronic structure of serine.For the compound studied, the most important intermolecular interaction between serine and watermolecules employ different geometrical models of numerous N…H and C-H…O bonds in the crystallinestructures. These interactions have been approximated by explicitly adding the nearest neighbors into thecalculations.At present, quantum chemistry is almost universally applicable to the interpretation of physical andchemical properties of various compoundsChemical shift calculations, geometry optimization and energies have been performed with ab initiomethod at HF/6-31G* and HF/6-31G** levels with GIAO methods.There is strong evidence that intermolecular effects are important in determining the 15N chemical shiftsof free amino acid residue to assign principal axes of the tensors and some systematic trends appear fromthe analysis of the calculated values.Formation of each interaction (in ten orientations) results in a shift of the bridging hydrogen's chemicalshifts of N…H bond that indicate the most stabilized compound.The CαH…O bond plays an important role in the interactions of amino acids residue upon the structureand function of a protein.Despite the finding of numerous CαH…O contacts in proteins, major questions remain about theirimportance. Thus it is the strength of this binding that is of most importance in understanding the possiblefactor that the CαH…O H-bond may play in the folding of proteins.This paper represents comparison between theoretical and experimental values of NMR resonances, andcalculations of HF/6-31G** level produce results in better agreement with the experimental data.