Hydration and Hydrogen Bond Network of Water during the Coil-to-Globule Transition in Poly(N-isopropylacrylamide) Aqueous Solution at Cloud Point Temperature

Abstract

Aqueous solutions of poly(N-isopropylacrylamide), P-NIPAAm, exhibit a noticeable temperature responsive change in molecular conformation at a cloud point temperature (Tcp). As the temperature rises above Tcp, the extended coil-like P-NIPAAm structure changes into a swollen globule-like conformation as hydration levels decrease and hydrophobic interactions increase. Though water plays an important role in this coil-to-globule transition of P-NIPAAm, the behavior of water molecules and the associated hydrogen-bond (HB) network of the surrounding bulk water are still veiled in uncertainty. In this study, we elucidate changes in the hydration state and the dynamical structure of the water HB network of P-NIPAAm aqueous solutions during the coil-to-globule transition by analyzing the complex dielectric constant in the terahertz region (0.25-12 THz), where bulk water reorientations and intermolecular vibrations of water can be selectively probed. The structural properties of the water HB network were examined in terms of the population of the non-HB water molecules (not directly engaged in the HB network or hydrated to P-NIPAAm) and the tetrahedral coordination of the water molecules engaged in the HB network. We found the hydration number below Tcp (≈10) was decreased to approximately 6.5 as temperature increased, in line with previous studies. The HB network of bulk water becomes more structured as the coil-to-globule phase transition takes place, via decreases in non-HB water and reduction in the orderliness of the tetrahedral HB architecture. Together these results indicate that the coil-to-globule transition is associated with a shift to hydrophobic-dominated interactions that drive thermoresponsive structural changes in the surrounding water molecules.

Hydrogen Bond Network of Water around Protein Investigated with Terahertz and Infrared Spectroscopy

“…6), indicating the fragmentation of the water HB network is inhibited and thus the ''less defective'' HB structure is formed around the albumin solute. Our previous THz studies of biological aqueous solutions have shown that hydrophilic groups tend to increase the population of NHB water, but N NHB decreases as the ratio of the hydrophobic moiety becomes larger (46)(47)(48)74). These results suggest hydrophobic groups inhibit the HB fragmentation of water, which is consistent with the recent Monte Carlo simulation study (86).…”

“…As a consequence, theεðuÞ of albumin aqueous solution can be also reasonably decomposed intoc slow ðuÞ,c fast ðuÞ,c S ðuÞ, andc L ðuÞ in the just the same way as with pure water. Thus the experimental resultεðuÞ of both pure water and albumin aqueous solutions, from 0.25 to 12 THz, were fitted (46)(47)(48)(72)(73)(74) as follows:…”

“…Under this condition, the hydration number N hyd (the number of hydration water per albumin molecule) can be quantitatively determined from the decrease in the bulk water relaxation strength ðDε slow þ Dε fast Þ (46)(47)(48)74). It should be noted that all hydration water molecules around the hydrophilic and/or hydrophobic surface, as well as the internally immobilized water molecules, will be counted as ''hydration water'' in our definition (76).…”

“…However, in f NHB , the NHB water induced by an albumin solute molecule cannot be distinguished from that inherent in the bulk-phase away from the albumin surface. Then, to isolate the induced NHB water species, we calculated the number of NHB water molecules induced by albumin ðN NHB Þ as follows (46)(47)(48)74):…”

Broadband dielectric spectroscopy of glucose aqueous solution: Analysis of the hydration state and the hydrogen bond network

“…To resolve this problem, we set ϕ ∞ as the value that is best fitted to the THz TD-ATR measurement result between 2.8 and 3.0 THz, in order to smoothly connect the THz TD-ATR and FIR FT-ATR results with the deviations smaller than ±0.003 for both real and imaginary parts. 41,42 Once the ϕ s (ω) was successfully determined, the experimental complex reflection coefficientr s (ω) =  R s (ω)e −iϕ s (ω) was substituted into the Fresnel equation to calculate the complex dielectric constant of the sample.…”