The binding free energy profiles of the PA clusters containing Na + and Cl -ions were investigated. The left panel of Figure presents the gas-phase binding free energies of PA with Na + and Cl -. In contrast to the positive binding free energies of the hydrated clusters containing PA, LA, ProA, and diol as discussed above, we generally observe the hydrated PA clusters containing ions to be thermodynamically favorable. Especially the (PA) 1 (Na + ) 1 (w) 0-5 and the (PA) 1 (Na + ) 1 (Cl -) 1 (w) 0-5 clusters are seen to decrease markedly at smaller water content and then flatten out at larger water content. This indicates that the stabilizing effect is diminishing with increasing water content. On the other hand, the (PA) 1 (Cl -) 1 (w) 0-5 clusters are observed to be relatively constant, just below zero in normalized binding free energy, which indicates that Cl -is not able to thermodynamically stabilize the hydrated clusters in the gas-phase as favorably as the hydrated clusters containing Na + . Generally, the hydration of the pyruvic acid-containing clusters in the presence of ions is negative and thus the ions may compensate for the lack of binding observed in the pure PA-water clusters. In the case of (PA) 1 (Cl -) 1 (w) 0-5 , the binding free energy is nearly zero as the negative free energy of the (Cl -) 1 (w) 0-5 clusters cancel with the positive free energy of the (PA) 1 (w) 0-5 clusters. In the case of the (PA) 1 (Na + ) 1 (w) 0-5 clusters, the presence of the Na + ion still leads to a low free energy.