Hydrogen bond dynamics and water structure in glucose-water solutions by depolarized Rayleigh scattering and low-frequency Raman spectroscopy

Abstract

The effect of glucose on the relaxation process of water at picosecond time scales has been investigated by depolarized Rayleigh scattering (DRS) experiments. The process is assigned to the fast hydrogen bonding dynamics of the water network. In DRS spectra this contribution can be safely separated from the slower relaxation process due to the sugar. The detected relaxation time is studied at different glucose concentrations and modeled considering bulk and hydrating water contributions. As a result, it is found that in diluted conditions the hydrogen bond lifetime of proximal water molecules becomes about three times slower than that of the bulk. The effect of the sugar on the hydrogen bond water structure is investigated by analyzing the low-frequency Raman (LFR) spectrum sensitive to intermolecular modes. The addition of glucose strongly reduces the intensity of the band at 170 cm(-1) assigned to a collective stretching mode of water molecules arranged in cooperative tetrahedral domains. These findings indicate that proximal water molecules partially lose the tetrahedral ordering typical of the bulk leading to the formation of high density environments around the sugar. Thus the glucose imposes a new local order among water molecules localized in its hydration shell in which the hydrogen bond breaking dynamics is sensitively retarded. This work provides new experimental evidences that support recent molecular dynamics simulation and thermodynamics results.

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“…This low frequency band is usually attributed to the damped stretching intermolecular modes of hydrogen bonded species and is sensitive to the different structures of water. 13,14 The increase in the intensity of the 170 cm −1 is associated with enhanced water structuring and increased tetrahedral ordering of water. 25,26 The intermolecular stretching band at 170 cm −1 has a collective nature similar to the O-H band at 3200 cm −1 and the intensity of that band was also enhanced in the gel state of agarose.…”

“…However, limited studies have been carried out on exploring effects on the timescale of picoseconds 9,11 which is relevant for intermolecular modes of a hydrogen bonded water network and is the time scale of hydrogen bond formation and cleavage. 13,14 To explore these processes at this timescale, analysis of the low-frequency region (50-300 cm −1 ) of the spectrum is required but has been difficult to obtain due to historic limitations in instrumentation that resulted from measuring Raman spectra close to the exciting line. However, recent advances in the development of extremely narrow-band holographic notch filters that provide very selective, high-extinction of the excitation laser now permit reliable and consistent measurements of this region.…”