Unveiling Zwitterionization of Glycine in the Microhydration Limit

Abstract

Charge separation under solvation stress conditions is a fundamental process that comes in many forms in doped water clusters. Yet, the mechanism of intramolecular charge separation, where constraints due to the molecular structure might be intricately tied to restricted solvation structures, remains largely unexplored. Microhydrated amino acids are such paradigmatic molecules. Ab initio simulations are carried out at 300 K in the frameworks of metadynamics sampling and thermodynamic integration to map the thermal mechanisms of zwitterionization using Gly(H 2 O) n with n = 4 and 10. In both cases, a similar water-mediated proton transfer chain mechanism is observed; yet, detailed analyses of thermodynamics and kinetics demonstrate that the charge-separated zwitterion is the preferred species only for n = 10 mainly due to kinetic stabilization. Structural analyses disclose that bifurcated H-bonded water bridges, connecting the cationic and anionic sites in the fluctuating microhydration network at room temperature, are enhanced in the transition-state ensemble exclusively for n = 10 and become overwhelmingly abundant in the stable zwitterion. The findings offer potential insights into charge separation under solvation stress conditions beyond the present example.

Temperature driven transformations of glycine molecules embedded in interstellar ice

“…This is in line with recent studies on the free-energy surfaces at 300 K with biased meta-dynamics simulations, demonstrating that the zwitterionic form in the Gly(H 2 O) 4 cluster is unstable. 73 Furthermore, in the intermediate cluster sizes all the transformations reported in Section 3.1 can be observed, namely anion (Fig. 9 orange columns), proton delocalisation (red) and zwitterion glycine.…”

“…This is in line with recent studies on the free-energy surfaces at 300 K with biased meta-dynamics simulations, demonstrating that the zwitterionic form in the Gly(H 2 O) 4 cluster is unstable. 73…”

“…This is consistent with previous free energy studies that evidenced the stabilization of neutral glycine in the gas phase or surrounded by few water molecules. 25,34,73 (iii) With sufficient numbers of molecules, the efficiency of the tautomerisation process (N-Gly -Z-Gly) depends on the nature of the glycine isomers. This is also the case in the associated mechanism.…”

Prebiotic dimer and trimer peptide formation in gas-phase atmospheric nanoclusters of water

“…16,83 DLPNO-CCSD(T)/CBS calculations on MP2 geometry optimized structures suggest that 10 waters are required before Gly is unambiguously in its zwitterionic form. 84 We have optimized every structure reported in this paper in both the zwitterionic and canonical forms, and find that in every case the DLPNO/ CCSD(T)/CBS//ob97X-D/6-31++G** calculations predict that the neutral forms are the dominant structures. An illustrative example is provided in Fig.…”

Monomers of Glycine and Serine Have a Limited Ability to Hydrate in the Atmosphere

“…In primary aerosols, amino acids are introduced into the atmosphere already embedded in an aerosol, while for secondary aerosols, amino acids begin in the gas phase and become aerosols through a gas-to-particle conversion . With many sources, amino acids now make up a significant portion of the total organic material in the atmosphere, and their interactions with water have been studied, including glycine − and serine. − However, these studies of the gas-phase conformations and microsolvation of amino acids have focused on spectroscopic properties and the electronic potential energy surface (PES) under vacuum conditions. Given that most atmospheric amino acids exist in solution in the aerosol phase, their potential role in the formation of CCN starting from gas-phase free monomers is incompletely understood and presents a gap in the CCN formation model.…”