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

Abstract

Recent studies of saccharides' peculiar anti-freezing and anti-dehydration properties point to a close association with their strong hydration capability and destructuring effect on the hydrogen bond (HB) network of bulk water. The underlying mechanisms are, however, not well understood. In this respect, examination of the complex dielectric constants of saccharide aqueous solutions, especially over a broadband frequency region, should provide interesting insights into these properties, since the dielectric responses reflect corresponding dynamics over the time scales measured. In order to do this, the complex dielectric constants of glucose solutions between 0.5 GHz and 12 THz (from the microwave to the far-infrared region) were measured. We then performed analysis procedures on this broadband spectrum by decomposing it into four Debye and two Lorentz functions, with particular attention being paid to the β relaxation (glucose tumbling), δ relaxation (rotational polarization of the hydrated water), slow relaxation (reorientation of the HB network water), fast relaxation (rotation of the non-HB water), and intermolecular stretching vibration (hindered translation of water). On the basis of this analysis, we revealed that the hydrated water surrounding the glucose molecules exhibits a mono-modal relaxational dispersion with 2-3 times slower relaxation times than unperturbed bulk water and with a hydration number of around 20. Furthermore, other species of water with distorted tetrahedral HB water structures, as well as increases in the relative proportion of non-HB water molecules which have a faster relaxation time and are not a part of the surrounding bulk water HB network, was found in the vicinity of the glucose molecules. These clearly point to the HB destructuring effect of saccharide solutes in aqueous solution. The results, as a whole, provide a detailed picture of glucose-water and water-water interactions in the vicinity of the glucose molecules at various time scales from sub-picosecond to hundreds of picoseconds.

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

“…Although the imaginary part of the complex dielectric constants exhibits significant concentration dependence over this frequency range, the real part of albumin aqueous solution was almost unchanged from that of pure water. This strong concentration dependence in the imaginary compared with the real part has been confirmed in other biological aqueous solutions (45)(46)(47)(48)69,70). In these previous studies, the characteristics of the water HB network around biomolecules were discussed by decomposing the measured complex dielectric constantεðuÞ into the constituent complex susceptibilities of the slow relaxationc slow ðuÞ at~0.02 THz, the fast relaxationc fast ðuÞ at~0.6 THz, the intermolecular stretching vibrationc S ðuÞ at~5 THz, and the librationc L ðuÞ at~18 THz.…”

“…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:…”

“…Subsequently, the complex dielectric constantεðuÞ was calculated from RðuÞ and 4ðuÞ based on the Fresnel's equation. All the THz TD-ATR measurements were carried out by a TAS7500 spectrometer (Advantest, Miyagi, Japan): our experimental setup andεðuÞ derivation procedure have been detailed elsewhere (45)(46)(47)(48).…”

“…Then, on the basis of Fresnel's equation, the complex dielectric constantεðuÞ was derived from RðuÞ and 4ðuÞ. Details of the K-K transform have been described elsewhere (47,48). Third, the frequency range between 18 and 120 THz was measured using a MIR FT-ATR spectrometer, FT/IR-4600 (Jasco).…”

“…Thus, the dynamics and structural changes in the water HB network can be ''selectively'' observed by THz spectroscopy, free from interference from the protein itself. Consequently, by measuring both real (polarization index) and imaginary (absorption index) parts of the complex dielectric constant, the characteristic of water around the solute can be quantitatively discussed in terms of the hydration state and the HB network stability (i.e., HB fragmentation and structural distortion) (45)(46)(47)(48). Therefore, THz spectroscopy is expected to selectively measure characteristics of the water HB network around the protein.…”