From the molecular structure point of view, TMA is an intriguing cation, with very low charge density. TMA is often denoted as the simplest "hydrophobic ion", being closer to hydrophobic species in certain respects than simple ions. For instance, choline binding sites are often rich in hydrophobic and aromatic residues rather than negatively charged groups. A recurring question thus appears in the literature: whether TMA behaves more like an ion or a hydrophobic species, e.g., in terms of a hydration structure. It is still unclear whether the geometry of the ion pair between TMA and an acetate molecular anion would resemble that of a simple atomic ion pair with the carboxylate group, or whether TMA would rather interact with the acetate leaning towards the hydrophobic CH 3 moiety.