  The understanding of prototropic tautomerism in water and the
characterization of solvent effects on protomeric equilibrium pose significant
challenges. Using molecular dynamics simulations based on state-of-the-art deep
learning potential and enhanced sampling methods, we provide a comprehensive
description of all configurational transformations in glycine solvated in water
and determine accurate free energy profiles of these processes. We observe that
the tautomerism between the neutral and zwitterionic forms of solvated glycine
can occur by both intramolecular proton transfer in glycine and intermolecular
proton transfer in the contact ion pair (anionic glycine and hydronium ion) or
the separated ion pair (cationic glycine and hydroxide ion).
