Intramolecular and Water Mediated Tautomerism of Solvated Glycine

Abstract

Understanding tautomerism and characterizing solvent effects on the dynamic processes pose significant challenges. Using enhanced-sampling molecular dynamics based on state-of-the-art deep learning potentials, we investigated the tautomeric equilibria of glycine in water. We observed that the tautomerism between neutral and zwitterionic glycine can occur through both intramolecular and intermolecular proton transfers. The latter proceeds involving a contact anionic-glycine−hydronium ion pair or separate cationic-glycine− hydroxide ion pair. These pathways with comparable barriers contribute almost equally to the reaction flux.

Hydroxide and Hydronium Ions Modulate the Dynamic Evolution of Nitrogen Nanobubbles in Water

“…As revealed in Figure a, the dipole vectors of H 3 O + ions are predominantly aligned parallel to the interfacial normal vector with cos θ = ∼1.0, where θ is the angle between the normal vector of the interface and the dipole vector of the self-ions. The orientation distribution can be attributed to the preference of H 3 O + ions to lie flat on the uppermost water surface with their hydrophobic oxygen atom facing the N 2 nanobubble and their hydrophilic protons neighboring H 2 O molecules. ,, In contrast, the orientation of OH – is more random (cos θ = −1.0 to 1.0) because its oxygen atom with more delocalized lone pair electrons is a much better HB acceptor than both oxygens of H 3 O + and water molecule, and thus, OH – ions can stay in a deeper water layer of the interface to form abundant HB network with surrounding water molecules …”

“…However, there is a lack of MD simulations that explicitly consider the presence of OH – and H 3 O + ions in mimicking alkaline and acidic environments, respectively. The main challenge lies in accurately capturing the diffusion of OH – and H 3 O + in water using nonreactive force fields because proton hopping plays a crucial role in ionic diffusion. , The proton hopping in water is a reactive process via the Grotthuss mechanism, − involving the formation and breakage of oxygen–hydrogen bonds. Describing this process requires using ab initio MD methods, which are resource-intensive and constrained for nanoscale spatial and temporal resolutions.…”