Enthalpy-related parameters of interaction of sucrose, lactose and their monosaccharide constituents with the mebicar (N-tetramethylglycoluril) drug in an aqueous medium at 298.15 K

Abstract

The molar enthalpies of dissolution of N-tetramethylglycoluril, a known psychotropically active pharmaceutical mebicar (Meb) (solute x), in aqueous solutions of d-monosaccharide glucose (Glu), fructose (Fru) and galactose (Gal) as well as disaccharide sucrose (Suc) and α-lactose (Lac) (cosolute y), were measured by the precision calorimetry method at T = 298.15 K and ambient pressure. Unlike the heat absorption in aqueous monosaccharides or Lac, the endothermicity of Meb dissolution in {water (W) + Suc} decreases as the sugar content becomes higher. The enthalpy-heterotactic coefficients for pair interactions between the solute and cosolute molecules, hxy, were determined using the McMillan-Mayer theory formalism. Similar to the homotactic parameters hyy, the hxy values (in J·kg·mol−2) were found to be positive, increasing in the series Suc (86) ≪ Lac (329) < Fru (456) < Glu (545) < Gal (663), despite the existing unusually large (in magnitude) negative hxx value, which is about −2000 J·kg·mol−2. Overall, the number of hydroxy groups and their axial(a)/equatorial(e) positions as well as the peculiarities of a glycoside bond and intramolecular H-bonding in cosolute saccharide molecules cause a stereospecificity of Meb hydration manifested in pair-interaction enthalpic coefficients hxy. So, the inequality hxy > hyy was found to be characteristic of monosaccharide containing aqueous Meb solutions (down to hyy ≈ 0.2hxy for Gal), and hxy < hyy – to be related to disaccharide containing aqueous Meb solutions. Thus, if hxy for (W + Lac + Meb) is ca. 0.65hyy, then it is only about 0.15hyy in the case of (W + Suc + Meb).