Contributions of weak interactions to the inclusion complexation of 3-hydroxynaphthalene-2-carboxylic acid and its analogues with cyclodextrins

Apparent formation constants and thermodynamic parameters of inclusion comp lexation of 3-hydroxynaphthalene-2-carboxylic acid (3H2NA) and its analogue s with cyclodextrins in aqueous solutions were determined by the steady-sta te fluorescence Benesi-Hildbrand method at various pH values. The pH depend ence of formation constants and thermodynamic parameters for 3H2NA complexi ng with beta-cyclodextrin (beta-CD) is different from that with heptakis(2, 3,6-tri-O-methyl)-beta-cyclodextrin (TM beta-CD). Hydrogen bonding between 3H2NA (neutral or ionic form) and beta-CD or proton-acceptors such as hydro gen ion and alkoxide ion (formed by dissociation of the secondary hydroxy g roup of beta-CD) and its effect on the stability of the cyclodextrin comple x are studied systematically and discussed in detail. It can be concluded t hat hydrogen bonding plays an important role in the formation of the inclus ion complexes. In addition, enthalpy and entropy changes both contribute to inclusion complexation when the guest exists mainly as its molecular form. However, when the guests fully ionize, inclusion complexation is solely dr iven by enthalpy. The effect of host cavity size, guest diameter and positi on of substituents on the stability of the complexes is also observed. Line ar compensatory Delta H degrees vs. T Delta S degrees plots give slopes (a = 1.20 +/- 0.10 and 1.21 +/- 0.01) and intercepts (T Delta S(0)degrees = 19 .1 +/- 2.5 and 20.7 +/- 0.3 kJ mol(-1)) for native CDs and modified CDs, re spectively. On the basis of the enthalpy-entropy compensation effect, the b ig slopes of the Delta H degrees vs.T Delta S degrees plots indicate that t he structure of native and modified CDs in aqueous solution is flexible, de spite the rigid skeleton of native CDs in the solid state, while the large intercepts T Delta S(0)degrees for native and modified CDs indicate extensi ve desolvation of both the host and guest on host-guest complexation. These two different CDs may be considered as one kind on the basis of enthalpy-e ntropy compensation effect. The general validity of the enthalpy-entropy ef fect is supported by the present results.