Reservation Energy Bonds and Structural Stability of Series of Multihydrated (n_H2O = 1−10) Glycine−H⁺M⁺ (M = Li, Na, or K) Complexes

A systematic study with the density functional theory (DFT/B3LYP) on the reservation energy bonds (REBs) and structural stability of series of biologically relevant multihydrated (n_H2O = 1−10) glycine−H⁺M⁺ (M = Li, Na, or K) complexes in the gas phase has been presented. The results show that the bonds (REBs) of (H₂O)_n-x-glycine−H⁺...M⁺(H₂O)_x (for x = 0−4, x < n) can release energy if they are broken. Calculations confirm that each of these REBs can be yielded without suffering any energy barrier only when the carboxyl oxygen of the glycine molecule is first bound by the metal ion M⁺ (or its hydrate) and then its amino nitrogen is protonated. Stepwise multiple hydrations can reduce charge distributions on the glycine−H⁺M⁺ gradually and, thus, increase the binding strength of REBs and favor the stability of the entire hydrated system. Consequently, the effects of reservation energy of these REBs are weakened. However, their energy decreases are not limitless. The lower limit of decrease, being not less than 20.0 kcal/mol for these complexes, is first predicted in the paper. In other words, the formation of a similar REB in a biological system can reserve energy of ≥20 kcal/mol. All the dissociation free energies are more than their corresponding electronic dissociation energies. Different from those dissociation energies, calculations show that these dissociation free energies change less with the addition of attached water molecules. All these reveal that two free reactants in a hydrate system finally become a regular complex and involve more energy in the new complex. These are very consistent with the real biological phenomenon. The small activation energy barrier (<3.0 kcal/mol) in the process of cleavage of the REB is also revealed in the present study, which also is in good agreement with behavior of a real biological system. Correspondingly, the affinity strength of the latter attached water molecules becomes weaker and weaker with further hydration, however, ultimately showing a tendency toward invariableness. The ordering of affinity strengths between these different ions (or ion hydrates) and its corresponding ligand is Li−nW > Na−nW > K−nW (where n = 1−10 and W represents a water molecule), whereas the magnitude ordering of REB energies of these different metal-ion−involved glycine−H⁺ hydrates reverses. In general, these different M⁺-involved complexes do not present very large differences in regard to the effect on reservation energy; however, they behave differently in coupling water molecules, using their various active sites. For example, only a water molecule can be attached to the Na⁺ end of the glycine−H⁺Na⁺ complex (GHNa) to form the most-stable Na−W₁ monohydrate among its isomers, whereas the most-stable K−W₁ monohydrate is generated by attaching a water molecule to the frontal amino hydrogen of the glycine−H⁺K⁺ complex (GHK). Comparisons of the relative energies of these Li⁺/Na⁺-involved glycine−H⁺ hydrates indicate that the carboxyl moiety that is coupling the M⁺ of the glycine is preferred to be hydrated, whereas the relative energies of the isomers of the K⁺-involved glycine−H⁺ hydrates imply that the protonated amino moiety of the glycine is the optimal combination site.