Two-Step Recrystallization of Water in Concentrated Aqueous Solution of Poly(ethylene glycol)

Abstract

Crystallization behavior of water in a concentrated aqueous solution of poly(ethylene glycol) (PEG) with a water content of 37.5 wt % was investigated by temperature variable mid-infrared (mid-IR) spectroscopy in a temperature range of 298-170 K. The mid-IR spectrum of water at 298 K showed that a large water cluster was not formed and that most of the water molecules were associated with the PEG chain. Ice formation, however, occurred as found in previous studies by differential scanning calorimetory. Ice formations were grouped into three types: crystallization at 231 K during cooling, that at 198 K during heating, and that at 210 K during heating. The latter two were just recrystallization. These ice formations were the direct transition from hydration species to ice without condensation regardless of crystallization or recrystallization. This means that the recrystallized water in the present system was not generated from low-density amorphous solid water. At a low cooling rate, nearly complete crystallization at 231 K during cooling and no recrystallization were observed. At a high cooling rate, no crystallization and two-step recrystallization at 198 and 210 K were observed. The former and latter recrystallizations were found to be generated from water associated with the PEG chains with ttg (the sequence -O-CH(2)-CH(2)-O- having a trans (t) conformation about the -C-O- bond and a gauche (g) conformation about the -C-C- bond) and random conformations, respectively. These results indicate that recrystallizable water does not have a single specific water structure.

Water Dynamics in Cytoplasm-Like Crowded Environment Correlates with the Conformational Transition of the Macromolecular Crowder

“…They suggest that PEG chain can adopt various conformations around C–C and C–O bonds of O–C–C–O unit segment. The trans–gauche–trans ( tgt ) conformation in the O–C–C–O segment adopted by solid PEG has been reported to be present even in PEG–water mixtures. ,,− The tgt conformation is a basic unit of 7 / 2 (seven chemical units and two turns) helical segment formed in PEG chains as confirmed by X-ray diffraction study . For the helical conformation of PEG, the closest oxygen–oxygen distance is similar to that between neighboring water molecules in water .…”

“…10 In water-poor solutions (70−80 wt %) however, PEG favors a compact helical structure, which is similar to that of crystalline PEG, with some random-coil segments. 21,22 Conformational Transition of PEG. To support these findings on the interplay of PEGDME conformational transition with water structure and dynamics upon increasing crowder concentration, we further compared the C−H stretching vibrations of PEGDME at ∼2919 cm −1 (associated with random-coil configuration of PEGDME) and ∼2883 cm −1 (both random-coil and tgt conformations of PEGDME) (Figure 4).…”

“…25 The change of conformation, such as C−C bonds with a trans conformation and C−O bonds with a gauche confirmation, causes the helical structure to become randomcoil so that both helical and random-coil-like conformers could coexist in solution. 18,19,21,22 PEG's ability to alter its local dipole moment by dihedral rotations along the C−C and C−O bonds modifies not only its conformations but also its interaction with surrounding molecules. This could be the reason why the number of water molecules hydrating a unit segment of PEG varies from 1 to 4.…”

Mid-Infrared Spectroscopic Investigation of the Perfect Vitrification of Poly(ethylene glycol) Aqueous Solutions

“…Our recent study using a PEG aqueous solution with a W C of 37.5 wt% revealed that the recrystallizable water in this solution had two different types of hydration structures and that their recrystallization temperatures were different. 54 The perfect vitrification of the PEGwater system was probably related to the existence of these two different types of hydration structure. In the present study, to reveal the cause of the perfect vitrification, the thermal properties of PEG aqueous solutions with various W C values in the range 3651 wt% were investigated by temperature-variable Fourier-transform mid-infrared spectroscopy (TVFT-IR).…”

How Molecular Crowding Differs from Macromolecular Crowding: A Femtosecond Mid-Infrared Pump–Probe Study

“…In contrast, macromolecular crowding does not affect interstitial water dynamics but moderately slows the dynamics of tightly associated ether water. PEGDME responds to this high crowding by adopting a crystalline structure similar to solid PEG, in which its contact with water is minimized but optimized. − This enhanced intermolecular interaction favors the helical conformation of PEGDME and water pooling. Hence, the water dynamics remain unperturbed and the water associated with ether oxygen shows a marginal dependence on crowding due to conformational flexibility.…”