Malic acid-based deep eutectic solvent and its application in Insulin's structural stability

Deep eutectic solvents (DESs) and Ionic Liquids (ILs) have been recently used as a solvent for insulin delivery via subcutaneous, transdermal, oral, and nasal ways. However, due to their high viscosity, DESs are usually impractical for delivery applications in their pure solution. The addition of water decreases the viscosity of DESs, but it can also influence the structural stability of the proteins. Hence, DES-water solutions and their effect on insulin's structural stability need to be investigated. In this study, molecular dynamics simulations on insulin dimer in DES consisting of malic acid and choline chloride (MA-ChCl) with varying amounts of water were performed. Insulin is strongly stabilized in the DES and its aqueous solutions as compared to water. However, in the DES solutions with 50 % and 75 % water content, malic acid was observed to interact readily with insulin dimer and its binding site, causing the separation of the two monomers. Therefore, while the insulin dimer is stable in the DES with low water content, in the presence of higher amounts of water it is destabilized and the monomers are separated. Considering that insulin's active form is monomeric, and the decomposition of insulin into monomers before reaching the bloodstream can increase the pharmacokinetics of insulin delivery, the use of the aqueous DES for rapid insulin delivery is highly relevant.