Hydration and Dynamic Behavior of Cyclodextrins in Aqueous Solution

Abstract

The hydration state and dynamics of plain and chemically modified cyclodextrins (CDs) in aqueous solution were investigated by using dielectric relaxation measurements at 25 degrees C over a wide frequency range up to 20 GHz, which is the relaxation frequency of pure liquid water molecules. The obtained dielectric relaxation spectra were decomposed into two major and one minor relaxation modes with relaxation times of approximately 8.3, 20-25, and 1000-2500 ps, respectively, depending on the CD species. The two major modes, fast and medium, were attributed to a rotational relaxation process of water molecules belonging to the bulk (free) state and an exchange of water molecules hydrated to CDs owing to hydrogen bond formation. The hydration numbers of the CDs strongly depend on the number of hydroxy (OH) groups controlled by chemical modification such as methylation. Increasing the number of methoxy or 2-hydroxypropoxy groups increases the hydration number of CD molecules, and results in higher solubilities of the chemically modified CDs than those of the plain CDs. The minor, slow mode was assigned to overall rotational relaxation for CDs with finite permanent dipole moments, which also depends on the number of OH groups.

Effect of α-Cyclodextrin on Drug Distribution Studied by Electrochemistry at Interfaces between Immiscible Electrolyte Solutions

“…The large concentration of R-CD has an impact on water structure and thereby also on the solvation of the analyte ions. [34][35][36] It has, for example, been shown that approximately 35 water molecules are closely associated to R-CDs in solution 35 which inevitably will affect the solvation of ions. In fact, the presence of structure-creating solutes (also called kosmotropes) based on carbohydrates has previously been found to affect the permeation of analytes through lipid membranes.…”

Microscopic Investigation of the Hydration Properties of Cyclodextrin and Its Substituted Forms

“…The dynamics of pure and chemically modified forms of CDs in aqueous solution have also been studied recently using dielectric relaxation experiments. 24 The spectra obtained from these measurements indicate the presence of three relaxation modes with time scales of 8-10 ps, 20-25 ps, and 1-2.5 ns. In an important work, Li and McGown 25 detected the formation of nanotube aggregates of β-and γ-CDs in solution and on solid surfaces.…”

Vibrational Properties of Cyclodextrin–Water Solutions Investigated by Low-Frequency Raman Scattering: Temperature and Concentration Effects

“…[20][21][22][23][24] On the contrary, less literature has regarded till now the properties of water molecules surrounding CD in aqueous solution [25][26][27] due to the difficulties to reveal separate solute and solvent dynamics, especially in case of diluted solutions. In this context, recently, depolarized light scattering has been used by some of us as a powerful tool to distinguish spectral features coming from solute rotational dynamics, hydrogen bonding solvent dynamics, and vibrational intermolecular modes of saccharides and proteins aqueous solutions (Rossi et al, submitted for publication).…”