PEG hydration and conformation in aqueous solution: Hints to macromolecular crowding

The molecular structural dynamics of PEG in aqueous solution has been addressed with a series of experimental data correlating macroscopic solution properties of PEG with structural and hydration properties at molecular level, by using thermodynamic, UV Brillouin and Raman spectroscopy data. Water activity measured in PEG 600 solutions by a novel dynamical calorimetry approach shows that data reveal some non-equilibrium process for dilute solutions. Brillouin scattering data on aqueous solutions in the UV range made possible the measurement of the viscoelastic relaxation of the system with a characteristic temperature, TM, as precursor of the glass transition process at lower T, while at constant temperature, the addition of water to liquid PEG 600 first lead to a slight decrease and then to an increase in the solution viscoelastic relaxation consistent with the slowest polymer dynamics observed in the concentrated solution. Specific Raman bands corresponding to trans and gauche conformations of the C–C and C–O bonds and their sequences in the PEG chain have been identified and their relative intensities as a function of concentration evidence non-monotonous variations with a rather unusual concentration dependence for the frequency of C–O bonds in the range of physiological temperatures. The heuristic result is that the extended time-space domains approach suggests an overall quasi-regular solution behavior of semidilute PEG, with opposite concentration dependence at low and at high water content, similar to discontinuities observed in excess thermodynamic properties and eutectic phase diagram. The cross-over of these distinct behaviors occurs at the concentration close to that usually employed to mimic the cellular crowding in biomolecular systems, highlighting the interplay of water molecules in solute-solute interactions.