Typical at glance but interesting when analyzed in detail: A story of Tris hydration.

Abstract

This paper provides results of dielectric relaxation (DR) spectroscopy of aqueous solutions of tris(hydroxymethyl)aminomethane (Tris) covering frequencies of 0.05 ≤ ν/GHz ≤89. The DR spectra can be well fit by a sum of Cole-Cole relaxation, assigned to the solute, and 2Debye modes already observed for neat water. Analysis of the amplitudes reveals that Tris is hydrated by 7 H2Os up to its solubility limit. However, the rather high effective solute dipole moment of ≈12D suggests that H2O dipoles in contact with Tris should reorient independently from it. Accordingly, an alternative description of the DR spectra with a superposition of 4Debyerelaxations was attempted. In this model, the slowest mode at ∼4GHz arises from solute reorientation and that at ∼8GHz was assigned to dynamically retarded hydration water, whereas relaxations at ∼18 and ∼500GHz are again those of (rather unperturbed) bulk water. Analysis of the solvent-related modes shows that Tris indeed slows down 7-8 H2O molecules. However, the solute-solvent interaction strength is rather weak, excluding the rotation of an alleged Tris-(7-8) H2O cluster as an entity. The now derived effective dipole moment of (6.3 ± 0.5) D for the bare Tris molecule allows speculations on its conformation. With the help of computational methods, we suggest that Tris dissolved in water most likely possesses an intramolecular H-bond between the nitrogen and hydrogen atoms of amino and hydroxyl groups, respectively. In addition, computational results indicate that the seven hydration H2Os found by DR bind directly to the Tris OH groups.