Affinity of aromatic amino acid side chains in amino acid solvents.

The affinity between amino acid and water is important for understanding how proteins behave in aqueous solutions. For example, the hydrophobicity of amino acid side chains determines a protein's solubility. However, the affinity of amino acid side chains in amino acid solvents should be determined in order to understand the propensity of protein condensates induced by multivalent amino acid interactions. Here we measured the transfer free energy of amino acid side chains (ΔGSC) from water to amino acid solvents. The ΔGSC of aromatic amino acids showed a different value depending on the type and the pH of amino acid solvent. Interestingly, the propensity of ΔGSC was completely different from the hydrophobicity of amino acids. This indicate that the ΔGSC describes the affinity between amino acid side chains involving the existence of water. The ΔGSC is a significant parameter for understanding whether amino acid side chains prefer bulk or protein condensate.

Solubility Parameters of Amino Acids on Liquid-Liquid Phase Separation and Aggregation of Proteins.

The solution properties of amino acids determine the folding, aggregation, and liquid-liquid phase separation (LLPS) behaviors of proteins. Various indices of amino acids, such as solubility, hydropathy, and conformational parameter, describe the behaviors of protein folding and solubility both in vitro and in vivo. However, understanding the propensity of LLPS and aggregation is difficult due to the multiple interactions among different amino acids. Here, the solubilities of aromatic amino acids (SAs) were investigated in solution containing 20 types of amino acids as amino acid solvents. The parameters of SAs in amino acid solvents (PSASs) were varied and dependent on the type of the solvent. Specifically, Tyr and Trp had the highest positive values while Glu and Asp had the lowest. The PSAS values represent soluble and insoluble interactions, which collectively are the driving force underlying the formation of droplets and aggregates. Interestingly, the PSAS of a soluble solvent reflected the affinity between amino acids and aromatic rings, while that of an insoluble solvent reflected the affinity between amino acids and water. These findings suggest that the PSAS can distinguish amino acids that contribute to droplet and aggregate formation, and provide a deeper understanding of LLPS and aggregation of proteins.