Density Functional Theory Study On The Structures, Stabilities And Solvent Effects Of Na～+-AD Ion-complexes

B3LYP method of Density Functional Theory (DFT) with the basis set of 6-311++G** was employed to study the interaction between Na~+ and alanine dipeptide(AD), the structures and stabilities of Na~+-AD ion-complexes and solvent effects of water and ether.Six conformers of Na~+-AD ion-complexes were obtained, including two bidentate (C7AB and C7'AB) and four monodentate ones (C7eqB, C5A, C7axB andα'B). The relative energies and chemical hardnesses are studied to indicate the stabilities of the six conformers of Na~+-AD ion-complexes as: C7AB > C7'AB > C7eqB > C5A > C7axB >α'B. The two bidentate Na~+-AD ion-complexes are much more stable than the four monodentate ones, indicating that the bidentate structures are preferably formed from the interaction between Na~+ and AD. The influences of the inclusion of Na~+ into each conformer (C7AB, C7'AB, C7eqB, C5A, C7axB orα'B) of AD molecule on the Ramachandran angles (φandψ) of molecular backbone and the hydrogen bonds in AD moiety were discussed in detail. It was found that in the forming process of bidentate structures of Na~+-AD ion-complexes, the original H-bonds in AD molecule were fully broken down, accompanying with the new formation of the two Na-O bonds. In the forming processes of four monodentate structures of Na~+-AD ion-complexes, the original H-bonds in AD either become stronger or are replaced by the newly-formed and stronger H-bonds. The interactions between Na~+ and the different conformers of AD molecule were obtained based on the BSSE correction. Also enclosed are the changes of enthalpy and Gibbs free energy.The influences of the inclusion of 1 or 2 H2O molecules on the structures and stabilities of six conformers of Na~+-AD ion-complexes were studied with supermolecular model. 24 Structures of Na~+-AD ion-complexes with 1 or 2 H2O molecules were obtained. It was found that H2O can bond with Na~+ in the structure of Na~+-AD to form Na-O van der Waals bond. H2O molecule can also bond with N or O atoms in AD moiety to form H-bonds as N-H…Ow or O…Hw-Ow. The former process takes place more easily. The most stable structures of Na~+-AD ion-complexes with 1 and 2 H2O molecules are W-C7AB and 2W-C7AB, respectively.α'B conformer of Na~+-AD can not exist in the presence of 2 H2O molecules.Also PCM, IPCM and IEFPCM solvent models were applied to study the solvent effects of water and ether on the structures and relative stabilities of six conformers of Na~+-AD ion-complexes. It was found that the IPCM model is not suitable for probing the solvent effects of water and ether on Na~+-AD ion-complexes, while PCM and IEFPCM models give the similar results, quite close to the results of supermolecular model study and what obtained from the fully optimized structures of six conformers of Na~+-AD in ether. The solvention energies of water and ether on Na~+-AD structures are all minus, indicating that the Na~+-AD ion-complexes are stabilized in these solvents.