THERMODYNAMICS OF ALPHA-CYCLODEXTRIN-P-NITROPHENYL GLYCOSIDE COMPLEXES - A SIMPLE SYSTEM TO UNDERSTAND THE ENERGETICS OF CARBOHYDRATE INTERACTIONS IN WATER

Thermodynamic studies of the binding of a series of p-nitrophenyl glyc osides (PNPGly) of varying stereochemistry to alpha-cgclodextrin (alph a-CD) were performed at three different temperatures (25, 35, and 42 d egrees C) using a microcalorimetric technique, The system p-nitropheno l (PNP) at pH = 3 and alpha-CD was also studied for the sake of compar ison. All these complexes were found to be enthalpy driven with a favo rable enthalpic term clearly dominant over an unfavorable entropic ter m. A clear enthalpy-entropy compensation effect was observed at all th e temperatures, with a slope close to unity (alpha = 1.02) and an inte rcept T Delta S degrees(0) = 2.91 kcal mol(-1). This thermodynamic pat tern is in agreement with those usually found for lectin-carbohydrate associations and for the binding processes of several host-guest syste ms. This pattern is explained in terms of the contribution of primaril y two driving forces: the van der Waals interactions between the host and the guest, and the solvation/desolvation processes which accompany the association reaction, The presence of the carbohydrate molecule i n the PNP ring causes a slight destabilization of the complex at 25 de grees C with respect to the alpha-CD-PNP (pH = 3) complex, although a different behavior has been observed depending on the axial/equatorial configuration of the glycoside and the temperature. This behavior is modulated by the stereochemistry of the glycoside. Differences were ob served between the deoxy-derivatives (LAra and LFuc) and those derivat ives with a hydroxymethyl group (Glc, Gal, Man). Delta C-p(degrees) va lues were obtained from the dependency of Delta H degrees an temperatu re (=(partial derivative Delta degrees/partial derivative T)(p)). Thes e values are small and negative except for alpha Man complex. For the latter complex, discrepancy between the calorimetric and the calculate d van't Hoff enthalpies was observed. Parallels are drawn between the thermodynamics of our model and those proposed for carbohydrate-protei n associations.