Density functional studies on conformational behaviors of glycinamide in solution

Conformational behaviors and relative stabilities for four glycinamide conformers in aqueous solution have been investigated at the B3LYP/6-311++G** level of theory employing the self-consistent isodensity polarized continuum (SCIPCM) model within the framework of the self-consistent reaction field (SCRF) theory. Overall, the peptide bond (C1-N4) in all of the conformers is strengthened while the C1=O5 bond is weakened upon solvation though the structural parameters have no significant changes except for the dihedral angles in conformer IIA. The relative stabilities among glycinamide conformers still remain as that in the gas phase except that two minor conformers reverse their order. The characteristic frequencies of amide 1, primarily involving the C1=O5 stretching mode, are red-shifted in all conformers, and IR intensities of mostly absorption bands are calculated to become more intense in going from the gas phase to solution. The microscopic model, which is simulated by studying the interactions of four glycinamide conformers with one water molecule in the gas phase, has also been performed to explore the microhydrated effects. All of the results, which are based on the SCIPCM model, microscopic model, and combination of them as well, are in agreement with one another, especially for the prediction of the relative stabilities among all of the glycinamide conformers. Comparison of the SCIPCM model employed in the present study and Onsager model used previously indicates that the latter may be used to study larger analogous systems that are too expensive to treat with the SCIPCM model computationally. Additionally, the tautomeric equilibria between glycinamide and glycinamidic acid have been assessed with and without the participation of one water molecule. Calculated results show that the presence of solvent disfavors the tautomeric process, whereas the water-assisted proton transfer plays a positive role in the tautomeric process.