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

摘要

This paper provides results from dielectric relaxation (DR) spectroscopy of aqueous solutions of tris(hydroxymethyl)aminomethane (Tris) covering frequencies of 0.05{less than or equal to}nu/GHz{less than or equal to}89. The DR spectra can be well fit by a sum of a Cole-Cole relaxation, assigned to the solute, and two Debye modes already observed for neat water. Analysis of the amplitudes reveals that Tris is hydrated by 7 H 2 Os up to its solubility limit. However, the rather high effective solute dipole moment of similar to 12 D suggests that H 2 O dipoles in contact with Tris should reorient independently from it. Accordingly, an alternative description of the DR spectra with a superposition of 4 Debye relaxations was attempted. In this model, the slowest mode at similar to 4 GHz arises from solute reorientation, that at similar to 8 GHz was assigned to dynamically retarded hydration water, whereas relaxations at similar to 18 and similar to 500 GHz are again those of (rather unperturbed) bulk water. Analysis of the solvent-related modes shows that Tris indeed slows down 7-8 H 2 O molecules. However, the solute-solvent interaction strength is rather weak, excluding the rotation of an alleged Tris-(7 divided by 8) H 2 O cluster as an entity. The now derived effective dipole moment of (6.3{plus minus}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 respectively hydroxyl groups. In addition, the computational results indicate that the 7 hydration H 2 Os found by DR bind directly to the Tris OH groups. Published under an exclusive license by AIP Publishing.