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 (ΔG) from water to amino acid solvents. The ΔG of aromatic amino acids showed a different value depending on the type and the pH of amino acid solvent. Interestingly, the propensity of ΔG was completely different from the hydrophobicity of amino acids. This indicate that the ΔG describes the affinity between amino acid side chains involving the existence of water. The ΔG is a significant parameter for understanding whether amino acid side chains prefer bulk or protein condensate.