Macromolecular hydration compared with preferential hydration and their role on macromolecule-osmolyte coupled diffusion

Abstract

Hydration and preferential hydration of macromolecules are two distinct properties of their multicomponent aqueous solutions. We have experimentally investigated ternary diffusion in a macromolecule–osmolyte–water system in order to characterize and compare these two independent quantities and to experimentally establish their role on the phenomenon of coupled diffusion. Specifically, we report the four diffusion coefficients for the poly(ethylene glycol)–di(ethylene glycol)–water system at 25 °C using Rayleigh interferometry. In this work, the molecular weight of poly(ethylene glycol) (PEG) is 200-fold higher than that of di(ethylene glycol) (DEG). This ratio is comparable to that between proteins and low molecular-weight osmolytes. This system has been selected because both solutes are neutral hydrated species with similar chemical properties and very different size. Hence, the observed behavior of coupled diffusion can be directly related to solute hydration and size ratio and is not complicated by other factors such as ionic interactions usually encountered in protein systems. Using our multicomponent diffusion coefficients, we have found that PEG hydration is slightly smaller than its preferential hydration. The observed difference can be attributed to PEG–DEG excluded-volume interactions. Our experimental results also enable us to reveal that Onsager cross-transport coefficients are large and negative. This implies that this transport coefficient should not be neglected in multicomponent-diffusion theoretical models even when ionic interactions or chemical association between the solute species are absent. This work provides the basis for understanding coupled diffusion in more complex aqueous systems such as those containing charged proteins or nucleic acids in the presence of salts or osmolytes.