The thermodynamic behavior of hydrogen-bond (H-bond) in bio-systems has been subject of intense study because of several mechanisms, peculiar to biomolecules, strongly related to hydration water behavior .
In this framework, cyclodextrins (CDs) are used as a model system to study the interactions of biomolecules with water. They are natural cyclic oligosaccharides composed by a variable number of glucose units. Since the glucopyranose units are in the chair conformation, CDs assume the characteristic truncated cone shape, with an internal hydrophobic cavity, able to form host−guest supramolecular structures, and a hydrophilic outer surface, which makes them soluble in water . The regular H-bond network in pure water gets modified when CD molecules come in contact with it. An H-bonds rearrangement occurs around these molecules allowing them to be dissolved in water.
Water plays also a crucial role in governing the properties of the CDs and their derivatives . To better understand the origin of such role, we have investigated the structural rearrangement of CD–water H-bonds in aqueous solutions by a joint combination of UV Raman and Brillouin scattering. All the measurements have been performed at IUVS Beamline@Elettra-Sincrotrone Trieste as function of temperature and solute concentration.
In the Raman data analysis, the attention was focused on the changes occurring in the O-H stretching band of water, which represents a marker of the structural rearrangement occurring in the H-bonding network of water molecules . The ability of CD to form hydrogen bonds with water leads to modify the regular network of water–water H-bonds in the solvent molecules placed around the solute.
In addition, UV Brillouin scattering measurements have been performed to carried out information on the collective dynamics of the cyclodextrin-water solution. Inelastic UltraViolet Scattering (IUVS) gives the possibility to study disordered systems in the mesoscopic transfer region not accessible by other spectroscopic techniques. Its frequency window matches the water relaxation process time, so it is satisfied the condition ωτ ≈ 1. This peculiarity allows us to probe the characteristic timescale 0.1–10 ps, which is associated to the average timescale of the intermolecular bonds lifetime, typically observed in liquid H-bond systems .
The overall results presented here confirm the potentiality of the joint use of inelastic scattering techniques to provide a comprehensive molecular view on the solvation dynamics in water-cyclodextrin solutions.
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