Understanding the role of water in the aggregation of proteins and polymers in aqueous solution using near-infrared spectroscopy

Abstract

Water plays an important role in chemical and biological processes. For understanding the role of water in the aggregation of proteins and polymers, the variation of water structures in the process of aggregation was studied by near-infrared spectroscopy. The near-infrared spectra of the aqueous R2/wt and poly(N,N-dimethylaminoethyl methacrylate) solutions of different concentrations were measured at different temperatures. The spectral changes of the solutes and water with temperature were analyzed with the help of chemometric methods. In the aggregation of R2/wt, the water species with one hydrogen bond around the NH groups dissociate to initiate the change of the hydrogen bonding network of the hydration water, and then, the water molecules with two hydrogen bonds (S₂) near the hydrophobic side chains release from the R2/wt, resulting in the formation of the ordered amyloid fibers. In the aggregation process of low concentration poly(N,N-dimethylaminoethyl methacrylate) solutions, the chains of the polymer tend to form a loose hydrophobic structure below 36°C and then aggregate into a micelle at a lower critical solution temperature of around 39°C. S₂ acts as a bridge to connect the polymer chains in the loose hydrophobic structure, and the dissociation of the S₂ bridge at high temperature is the reason for the formation of the micelle. For high concentration solution, however, the spectral information of S₂ was not found in the aggregation, suggesting a direct formation of the micelle from the dehydrated chains.