Effect of PEG Chain Length on the Water Solubility of Monodisperse PEG-Appended Molecules

Polyethylene glycol (PEG) is a nonionic and highly water-soluble polymer, and PEGylation is widely used as a versatile chemical strategy to enhance the water solubility of otherwise poorly soluble molecules. The rational design of PEG-appended materials, therefore, requires an estimation of the PEG chain length necessary to achieve the desired level of solubility. Here, we investigated the relationship between the PEG chain length and the water solubility of PEG-appended molecules using a combination of experimental and computational approaches. We focused on monodisperse PEG with a discrete molecular weight, which enables us to identify the relationship between chain length and physicochemical properties, as represented by water solubility. Then, we designed a series of PEG-appended molecules bearing monodisperse PEG chains of varying lengths, and measured their water solubilities experimentally. In parallel, molecular dynamics-based alchemical free energy calculations yielded thermodynamic trends that reproduced the observed dependence of solubility on PEG chain length. Comparison of experimental and computational results showed that the calculations not only distinguished between water-soluble and insoluble PEG-appended molecules but also captured the solubility trends for moderately soluble species. These findings demonstrate the potential of molecular-dynamics-based free energy calculations for a priori prediction of the water solubility of PEG-appended molecules.