Inclusion Complexation Thermodynamics of Acridine Red and Rhodamine B by Natural and Novel Oligo(ethylenediamine) Tethered Schiff Base β-Cyclodextrin

Abstract

A series of Schiff base β-cyclodextrin derivatives 2–5 with an oligo(ethylenediamine)tether have been newly synthesized and their inclusion complexation behavior has beenassessed and discussed thermodynamically, employing acridine red (AR) and rhodamine B (RhB) as representative guests. Fluorescence spectrophotometric titrations have been performed in methanol-water (1 : 2) phosphate buffer solution (pH = 7.20) at 25.0–45.0 °C in order to obtain the complex stability constants (KS) and the thermodynamic parameters (Δ H° and TΔ S°) for the stoichiometric 1 : 1 inclusion complexation of two guests with the native and modified β-cyclodextrins ( 1 and 2–5). As compared with theparent β-cyclodextrin 1, all of the chemical modifications to the primary side of β-cyclodextrins examined led to substantial decreases for rhodamine B and marked increases for acridine red in complex stability, which are elucidated in terms of the induced-fit interaction and the complementary geometrical relationship between the host β-cyclodextrins and guest molecules, as well as the length of the linking chain of β-CD derivatives. The induced circular dichroism spectral analyses of these β-cyclodextrin derivatives indicated that the aromatic moiety in modified β-cyclodextrins is not embeded into the hydrophobic cavity ofcyclodextrin. The inclusion complexation of 2–5 with acridine red possess higher binding constants than that with rhodamine B, which are solely attributed to the increased enthalpic gain. Thermodynamically, the inclusion complexation with the modified β-cyclodextrins 2–5 is absolutely enthalpy-driven for acridine red, while for complexation with rhodamine B is mainly entropy-driven.