simulations to probe at the molecular level the interactions between two model ions-TMA as a model for the tri-methylated Lys or the choline headgroup-and acetate as a proxy for carboxylate groups, as shown in Figure . As a key result, we obtain an experimental signature of the ion pair and refine the molecular interpretation of the experimental signal with the help of force field and ab initio molecular dynamics simulations, providing a comprehensive molecular picture of the ion pair structure and stability.