Glycine-Zn+/Zn2+ and their hydrates: On the number of water molecules necessary to stabilize the switterionic glycine-Zn+/Zn2+ over the nonzwitterionic ones

Several interaction modes of glycine with one Zn+ or Zn2+ and further with one and even two H2O 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+ isomer is a five-membered ring with Zn+ bound to both amino nitrogen and carbonyl oxygen (NO) of glycine, and the next most stable glycine-Zn+ species is a four-membered ring with Zn+ coordinated at both oxygen ends (OO) of the zwitterionic glycine. The binding energy of the most stable glycine-Zn+ is 68.5 kcal/mol calibrated at the BHLYP/6-311+G*//6-311+G* level. On the contrary with glycine-Zn+ isomers, the most stable glycine-Zn2+ species holds the similar coordination mode to that of next most stable glycine-Zn2+ complex, while the next most stable glycine-Zn2+ exhibits the similar coordination mode to that of the most stable glycine-Zn+. The binding strength of these glycine-Zn2+ isomers are all far more than those of their corresponding counterparts of glycine-Zn+ isomers, such as the binding energy of the most stable glycine-Zn2+ being 234.4 kcal/mol, showing stronger electrostatic interaction. The reoptimization for the two most stable modes with the different valent states (+1,+2) to combine a H2O molecule at their each end of Zn ion show that the relative energy ordering does not change, and also resembles their no-H2O-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+/Zn2+ system would be also suitable for its analogs, such as glycine-Cu+/Cu2+ and glycine-Ni+/Ni2+ systems, and even suitable for other similar transition metal ion-chelated glycine systems.