The solubility of N-acetyl amino acid amides in organic acid and alcohol solutions: Mechanistic insight into structural protein solubilization

Structural proteins such as spider silk and silkworm silk are generally poorly soluble in aqueous and organic so-lutions, making them difficult to manipulate in manufacturing processes. Although some organic acids and alco-hols, such as formic acid and hexafluoroisopropanol (HFIP), effectively solubilize poorly soluble proteins, little is known about their protein solubilization mechanism. In this study, the solubility of N-acetyl amino acid amide compounds in organic solvents & mdash;formic acid, acetic acid, HFIP and isopropanol & mdash;was measured to clarify the pro-tein solubilization mechanism at the amino acid residue level. On the basis of thermodynamic analyses of the sol-ubility in terms of the transfer free energy (from water to organic solvents), every organic solvent was found to be effective in thermodynamically stabilizing hydrophobic amino acid side chains in the liquid phase. Formic acid and HFIP were comparably effective in the stabilization of the polypeptide backbone, whereas acetic acid and isopropanol were ineffective. Therefore, the significant solubilizing effect of formic acid and HFIP on the structural proteins was attributed to their favorable interactions with hydrophobic amino acid side chains and with the polypeptide backbone of the proteins. The present findings are useful for the optimization of protein manipula-tion and amino acid sequence design. (c) 2021 Elsevier B.V. All rights reserved.

Automated summary

This study found that organic solvents can effectively stabilize hydrophobic amino acid side chains in the liquid phase, with formic acid and HFIP being comparably effective in stabilizing the polypeptide backbone, while acetic acid and isopropanol were ineffective.