Several interaction modes of glycine with one Zn ϩ or Zn 2ϩ and further with one and even two H 2 O molecules in the gas phase are studied at the hybrid three-parameter B3LYP and Hartree-Fock level, respectively. On the basis of these optimized geometries, single point calculations are performed using different theoretical methods and larger basis sets. The calculated results imply that the most stable glycine-Zn ϩ isomer is a five-membered ring with Zn ϩ bound to both amino nitrogen and carbonyl oxygen ͑NO͒ of glycine, and the next most stable glycine-Zn ϩ species is a four-membered ring with Zn ϩ coordinated at both oxygen ends ͑OO͒ of the zwitterionic glycine. The binding energy of the most stable glycine-Zn ϩ is 68.5 kcal/mol calibrated at the BHLYP/6-311ϩG*//6-311ϩG* level. On the contrary with glycine-Zn ϩ isomers, the most stable glycine-Zn 2ϩ species holds the similar coordination mode to that of next most stable glycine-Zn 2ϩ complex, while the next most stable glycine-Zn 2ϩ exhibits the similar coordination mode to that of the most stable glycine-Zn ϩ . The binding strength of these glycine-Zn 2ϩ isomers are all far more than those of their corresponding counterparts of glycine-Zn ϩ isomers, such as the binding energy of the most stable glycine-Zn 2ϩ being 234.4 kcal/mol, showing stronger electrostatic interaction. The reoptimization for the two most stable modes with the different valent states ͑ϩ1,ϩ2͒ to combine a H 2 O molecule at their each end of Zn ion show that the relative energy ordering does not change, and also resembles their no-H 2 O-combined counterparts. However, an interesting and important observation has been first obtained that single hydration effect can strikingly strengthen the stability of the monovalent OO form though it is still higher by 0.1 kcal/mol in energy than the NO counterpart. Hydration effect of double waters can reverse their relative stability due to the strong hydrogen bond effect in the OO form. Different from the case of the two monovalent hydrated complexes, calculated results for the divalent zinc ion chelated complexes show that with or without single hydration hardly change the value of their relative energy, and hydration strength and glycine deformation difference induced with or without hydration in the two different modes display surprising similarity. So we predict that the further hydration basically do not yield any effect on the relative stability. The prediction for the hydration effect on the glycine-Zn ϩ /Zn 2ϩ system would be also suitable for its analogs, such as glycine-Cu ϩ /Cu 2ϩ and glycine-Ni ϩ /Ni 2ϩ systems, and even suitable for other similar transition metal ion-chelated glycine systems.

Seven stable stationary points, corresponding to three pairs of mirror-image conformers and one C s symmetry conformer, have been located on the potential energy surface (PES) of neutral glycine amide at the B3LYP/ 6-311++G** level of theory. Accurate geometric structures, relative stabilities, and harmonic vibrational frequencies have been investigated. More importantly, the intramolecular H-bond formed from the amide to the amine plays a key role in stabilizing the global minimum, as observed in alanine amide and has been discussed qualitatively from the viewpoint of the structures, charge distributions, and vibrational analyses. As an important supplement in property for glycine amide, other property parameters, such as gas-phase basicity (GB), proton affinity (PA), and ionization potential (IP), have been predicted. The Boltzmann equilibrium distributions for the seven conformers have also been discussed qualitatively through the calculations of Gibbs free energy at various temperatures. At room temperature, the equilibrium compositions are mainly composed of conformers I and II exclusively, i.e., about 75.02% and 23.28%, respectively. As a tentative study, the conformational behaviors in aqueous solution have been explored using the Onsager model within the self-consistent reaction field (SCRF) method at the same level employed in the gas phase. Computational results indicate that the global minimum should be conformer I regardless of whether in the gas phase or in aqueous solution, which is different from the previous theoretical reports. Moreover, the consistent results in relative energy using higher-level computations, including the MP2, MP3, MP4SDQ, and CCSD(T) methods employing the Dunning's correlation consistent basis set aug-cc-PVDZ, indicate that the B3LYP/6-311++G** level of theory may be applied to the analogous systems.

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 C1dO5 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 I, primarily involving the C1dO5 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.