| Aggregation/dissolution of biomolecules is of fundamental importance in a large variety of biological, physical and chemical processes. Salting-in/out of biomolecules from aqueous electrolyte solution is a fundamental question in biotechnological developments and aqueous chemistry, and relates to how biomolecules cause to biological functions. Generally, at low salt concentrations, biomolecules occur a slightly salting-in behavior. However at higher salt concentrations, they occur a sharply salting-out behavior. As the simplest biomolecules and essential building blocks of more complex biomolecules, the solubility of aromatic amino acids is low.Here, it is found that an anomalously significant salting-in behavior of L-Tryptophan (Trp) at higher CuCl2 concentrations induced by Cu2+ via cation-Ï€ interactions. Meanwhile, the solubility of Trp in CuCl2 solition is related to the concentration of Cu2+ and the addition sequence of samples. It is contrary to the previous studies of observing a large amount of Trp-Cu2+-Trp precipitates when adding CuCl2 solid/solution into Trp solution. Theoretical studies show that the unexpected experimental phenomena are attributed to the cation-Ï€ interactions of Cu2+ and indole rings structure in Trp, which may significantly enhance adsorption energy between water and indole rings structure. Further experimental observations (ultraviolet differential spectrum, Raman spectrum, fluorescence emission spectrum, etc) demonstrate the existence of Cu2+-Trp cation-Ï€ interactions, and their structural mechanism. The structures and properties of Trp-Cu2+-Trp precipitates and their different producing reactions are also studied in this work.Moreover, the enhancement of Trp solubility for water induced by divalent transition-metal cations-Ï€ is universal since others (for examples, Zn2+ and Ni2+) have similar behaviors. And other Aromatic compounds (for examples, L-Phenylalanine and Theophylline) have shown similar enhancement of solubility in Cu2+ solution. The findings further illustrate the wide existence of cation-Ï€. This work is hoped to increase scientific understanding of some pathogenesis, function and mechanism of many macromolecular, and aggregation/diffusivity mechanisms in metal ions aqueous solution. |
