Gas Phase Intramolecular Proton Transfer in Cationized Glycine and Chlorine Substituted Derivatives (M–Gly, M=Na+, Mg2+, Cu+, Ni+, and Cu2+): Existence of Zwitterionic Structures?

Abstract

The intramolecular proton transfer in cationized glycine and chlorine substituted derivatives with M = Na+, Mg2+, Ni+, Cu+, and Cu2+ has been studied with the three parameter B3LYP density functional method. The coordination of metal cations to the oxygens of the carboxylic group of glycine stabilizes the zwitterionic structure. For all monocations the intramolecular proton transfer occurs readily with small energy barriers (1-2 kcalmol(-1)). For the dication Mg2+ and Cu2+ systems, the zwitterionic structure becomes very stable. However, whereas for Mg2+, the proton transfer process takes place spontaneously, for Cu2+ the reaction occurs with an important energy barrier. The substitution of the hydrogens of the amino group by chlorine atoms decreases the basicity of nitrogen, which destabilizes the zwitterionic structure. For monosubstituted glycine complexed with Na+, the zwitterionic structure still exists as a minimum, but for disubstituted glycine no minimum appears for this structure. In contrast, for Mg2+ complexed to mono- and disubstituted glycine, the zwitterionic structure remains the only minimum, since the enhanced electrostatic interaction with the dication overcomes the destabilizing effect of the chlorine atoms.

Complexation of glycine by manganese (II) in the gas phase: A theoretical study

“…In contrast, extensive studies on gaseous glycine have shown that zwitterions (salt bridge forms) are not even local minima on the potential energy surface of glycine [31,[45][46][47][48][49][50][51][52]. One of the ways to induce the natural to zwitterion transition in the gas phase is favoring the zwitterion by attaching a charged species such as an atomic metal cation [13,32,[34][35][36][37]43,44,53]. Such studies have established that for complexation of glycine by doubly charged cations the salt bridge structure, in which the metal ion interacts with the carboxylate end of zwitterionic glycine, is preferred in most cases.…”

“…Most of these structures have already been considered in previous studies on cationized glycine [13,[31][32][33][34][35][36][37][38][39][40][41][42][43][44]. According to previous reports, three coordination modes maximize the metal cation-glycine interaction [13,[31][32][33][34][35][36][37][38][39][40][41][42][43][44]. Note that B3LYP and MP2 optimized geometries are very similar, with Mn-O and Mn-N distances being slightly larger at the MP2 level, by 0.03-0.05 Å.…”

“…The structure of glycine cationized with singly charged metal cations has been analyzed in previous theoretical works [13,[31][32][33][34][35][36][37][38][39][40][41][42][43][44]. It has been found that the lowest isomer of cationized glycine is a charge solvated structure.…”

Effect of regular hydration on gas phase structural stability of [zwitterionic alanine+M+] (M+=Li+, Na+, K+) complexes: A quantum chemical study

“…A number of studies [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] have been done to investigate the role of monovalent and divalent metal cations and also with explicit addition of water molecules on the structural stability of Ala and ZAla in both, gas phase as well as aqueous medium. In our recent study [43], the theoretically calculated Raman spectra of hydrogen bonded complexes of ZGly with 1-5 water molecules were compared to the experimentally recorded Raman spectra of Gly in aqueous medium and we observed that the calculated Raman spectra of ZGly with five water molecules were found to be in good agreement with the experimentally recorded Raman spectra in aqueous medium.…”

Conformational analysis of alkali metal complexes of anionic species of aspartic acid, their interconversion and deprotonation: A DFT investigation

“…Complexes of glycine, alanine and praline with alkali metal ions have been studied by DFT method [10][11][12]. Gas-phase intramolecular proton transfer in various metal cation complexes with glycine and chlorine substituted derivatives have been studied [13]. Non-zwitterionic and zwitterionic forms of glycine and their complexes with alkali, alkaline earth and transition metal ions hydrated with two and five water molecules were studied by DFT methods [14].…”