Intramolecular proton transfer induced by divalent alkali earth metal cation in the gas state

Abstract

Interactions between divalent alkali earth metal (DAEM) ions M (MABe,Mg, Ca, Sr, Ba) and the second stable glycine conformer in the gas phase, which can transfer into the ground-state glycine-M 2ϩ (except the glycine-Be 2ϩ ) among each corresponding isomers when these divalent metal ions are bound, are studied at the hybrid three-parameter B3LYP level with three different basis sets. Proton transfers from the hydroxyl to the amino nitrogen of the glycine without energy barriers have been first observed in the gas phase in these glycine-M 2ϩ systems. The interaction between the glycine and these DAEM ions except beryllium and magnesium ion only create an amino hydrogen pointing to the original hydroxyl due to their weaker interaction relative to those divalent transition metal (DTM) ion-bound glycine derivatives, being obviously different from that between the glycine and DTM ions, in which two amino hydrogens point to the original hydroxyl oxygen when these metal-chelated glycine derivatives are produced. The interaction energy between the glycine and divalent magnesium would be the boundary of one or two amino hydrogens pointing to the hydrogyl oxygen, i.e., the Ϫ170.3 kcal/mol of binding energy is a critical point. Similar intramolecular proton transfer has also been predicted for those DTM ion-chelated glycine systems; however, that in the gas state has not been observed in the monovalent metal ion-coordinated glycine systems. The binding energy between some monovalent TM ion and the glycine is similar to that of the glycineBa 2ϩ , which has the lowest binding strength among these DAEM-ion chelated glycine complexes. The difference among them only lies in the larger electrostatic and polarized effects in the latter, which favor the stability of the zwitterionic glycine form in the gas phase. According to these observations, we predict that the zwitterionic glycine would exist in the field of two positive charges in the gas phase.

Gas‐phase interactions of organotin compounds with glycine

“…While it is well established that the zwitterionic form of glycine is not stable in the gas phase, preformed complexes in solution between metal ions and zwitterionic glycine may be directly transferred in the gas phase by ESI. Furthermore, many theoretical studies have shown that zwitterionic glycine can be strongly stabilized through the interaction with metal ions, so that this particular conformation, usually named salt bridge, is generally the global minimum and can play a key role in the unimolecular reactivity of M 2+ /glycine complexes …”

The regulatory roles of metal ions (M+/2+= Li+, Na+, K+, Be2+, Mg2+, and Ca2+) and water molecules in stabilizing the zwitterionic form of glycine derivatives

“…Due to the importance of information about the gas-phase intrinsic properties for the solution-phase measurements, 21 a number of attempts have been made at stabilizing and characterizing the gas-phase zwitterions. These efforts have concentrated on the hydrated, [22][23][24] or the metal-ion cationized zwitterionic systems, [25][26][27][28][29] etc. For the hydration of the zwitterionic glycine, Jensen et al 30 have determined that two water molecules can stabilize the glycine zwitterion.…”

“…For the hydration of the zwitterionic glycine, Jensen et al 30 have determined that two water molecules can stabilize the glycine zwitterion. Metal cationization can not only stabilize zwitterionic systems [25][26][27][28][29] but can also result in many biological functions. For example, the osmotic equilibrium of cells, the electrical excitability of nerves and muscles, the active transport of glucides and amino acids all involve the alkali metal cations.…”

“…A special study on the combination modes from Hoyau et al 35 confirmed this point. For the monovalent cations, the product with the OO mode is a local minimum, not the global one on its potential energy surface (PES), 25,26,28 while the same product form in some divalent metal cationized systems becomes the global minimum on the PES. In these interactions, the electrostatic effect between the glycine and the metal ion plays a significant role, especially for the divalent metal-ion coupled systems.…”

The binding site dependence of binding energy in both metalated and protonated diglycine and triglycine peptides

“…The BE of G 2 K1 + obtained by Wong et al [19] is 33.1 kcal/mol, in good agreement with our result. Similarly, the BEs of the charge-solvated GBe 2+ and GMg 2+ are about À300.3 and À170.8 kcal/mol [30], respectively, lower than their G 2 Be1 2+ (À315.4 kcal/mol) and G 2 Mg1 +/2+ (À185.8 kcal/mol) counterparts. Thus the extension of peptide chain between two cations is favorable to the binding of the metal-ion, which confirms, from another point of view, that extension of peptide chain can effectively decrease the electronic repulsive effect between two cations (H + and M +/2+ ) in the G 2 H + Mx +/2+ and weaken the positive BEs of the M-O bonds.…”