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

Abstract

Crystallization/recrystallization behaviors of poly(ethylene glycol) (PEG) aqueous solutions with water contents (WC's) of ∼36-51 wt % were investigated by temperature-variable mid-infrared spectroscopy. At a WC of 43.2 wt %, crystallization and recrystallization of water and PEG were not observed. At this specific WC value (WCPV), perfect vitrification occurred. Below and above the WCPV value, crystallization/recrystallization behaviors changed drastically. The crystallization temperature below WCPV (237 K) was ∼10 K greater than that above WCPV (226 K). Recrystallization above and below WCPV occurred in one (213 K) and two (198 and 210 K) steps, respectively. These findings resulted from the difference in the (re)crystallization behaviors of water molecules associated with PEG chains with helical and random-coil conformations. These two types of water molecules might have limiting concentrations for their (re)crystallization, indicating that perfect vitrification might have occurred when the concentrations of the two types of water molecules were less than the limiting concentrations of their (re)crystallization.

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

“…However, as the PEGDME concentration further increases beyond 60 wt %, the solute HN 3 experiences relatively homogeneous environment, suggesting a conformational change of PEGDME at around 60 wt % PEGDME (Figure 1F). 10,15,21 This is further confirmed by the observation that the width of the azido stretch band of PEGME-N 3 at higher concentration (>60 wt %) reaches an asymptotic value that is close to the bandwidth of PEGME-N 3 in dichloromethane (water less environment) (Figure 1I). The decrease in bandwidth of the azido stretch band of PEGME-N 3 with increased crowding indicates that the conformationally accessible states of the macromolecular crowder are being restricted, which is similar to what they do to biomolecules in crowded environments.…”

“…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.…”

Effect of Osmolytes on the Conformational Behavior of a Macromolecule in a Cytoplasm-like Crowded Environment: A Femtosecond Mid-IR Pump–Probe Spectroscopy Study

“…For PEGDME, the structural change could be related to an increasing heterogeneity in the conformations of O–CH 2 –CH 2 –O segments or a change in equilibrium between the populations of helical and random-coil segments. , Such conformational shift toward helical structures upon adding protecting osmolytes to the PEGDME solution is similar to the case in water-deficient solution, where the PEG macromolecules tend to adopt crystalline structures similar to solid PEG . As shown below, direct evidence of considerable change in the conformation of PEGDME as protecting osmolyte concentration increases can be inferred from the mid-IR PP measurements of IR-probe-labeled macromolecule itself (PEGME–N 3 ).…”

“…The conformational transition of PEGDME segments can be experimentally estimated by comparing 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 helical conformations of PEGDME). , Since urea does not have any vibrational modes that absorb IR fields in the frequency range from 2800 to 2960 cm –1 , the ratio of the IR absorbance at 2883 to that at 2919 cm –1 provides direct information on the PEGDME conformational propensity. Interestingly, this absorbance ratio decreases with the addition of urea (Figure c,d), which indicates a slight increase in the population of random-coil segments.…”

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.…”