摘要:

IntroductionCyclodextrins (CDs) are known to complex a wide variety of molecules in their hydrophobic interior.1–6The most likely mode of binding consists of inclusion of the less polar portion of the guest within the cavity while the polar or hydrophilic part of the guest remains solvent exposed.6,7It has been recognized that the binding forces involved in inclusion complexation are hydrophobic interactions, hydrogen bonding, van der Waals forces, the relief of high energy water from the CD cavity and the relief of conformational strain upon guest inclusion.1–6It is important to study these weak interactions, to further our understanding of the nature of the formation of supramolecules and to predict new approaches in pharmacy, analysis and material science,etc.6,8–10Generally, molecular association is made possible through the simultaneous cooperation of several weak interactions. The thermodynamics of inclusion complexation by CDs are a consequence of the measured contributions of these interactions.6Furthermore, the observation of the linear compensatory enthalpy–entropy relationship has been extended to the cyclodextrin field.11–16Inoue and Liuet al. have proposed that the slope (α) and the intercept (TΔS°0) of the compensatory ΔH°vs. TΔS° plot are characteristic of the ligand–host topology and can be used as a quantitative measure of the ligand–host’s conformational change and of the extent of cation–ligand or guest–host desolvation caused by complex formation. It is noted that only a little work has been done systematically on the contributions of hydrogen bonding to the stability of inclusion complexes.7,17,18Buvahiet al.studied the complex formation of β-CD with different guests and the results showed that the formation constants would relatively increase if hydrogen bonding between the hydroxy groups of β-CD and the guest is possible.17Ross and Rekharsky determined the complexation thermodynamics of CDs with several pairs of structurally related aromatic guests, either with or without a phenolic hydroxy group and reported the increments in ΔH ° and ΔG ° that can be ascribed to hydrogen bonding interactions.18The aim of this work is to investigate the function of noncovalent weak interactions in inclusion complexation involving cyclodextrins. We choose β-CD, TMβ-CD and γ-CD as hosts for their different hydrophobicity, abilities to form hydrogen bonds and cavity size. We select 3H2NA and some analogues of it (seeFig. 1) as guests for they all have aromatic rings, a rigid rectangular shape and two adjacent groups which may form intermolecular and intramolecular hydrogen bonds. They are suitable for the study of the effects of polarity, hydrogen bonding, position of the substituent and steric hindrance on the host–guest system. First, we determined the formation constants of the complexation of 3H2NA with β-CD and TMβ-CD at many different pH values, since the hydrophobicity and the ability of 3H2NA (pKa = 2.6) 19and β-CD (pKa = 12.2) 5to form hydrogen bonds are changeable in this pH range, while those of TMβ-CD are constant because it is unable to dissociate. Then we report the first measurements of thermodynamic parameters for inclusion complexation of the above host–guest systems at several pH values. Because of the low solubility 20of the guest at certain pH values and the interference of buffers on the inclusion equilibrium which has been demonstrated in our recent work,21it is not by calorimetric titration 7,11,12,22–25but through van’t Hoff analysis 26–29of the equilibrium constants that the thermodynamic parameters reported were determined, although calorimetric titration is generally thought to be more accurate. In addition, we observe that the host cavity size, the guest diameter and the position of substituent have great effect on the stability of the complexes. Finally, we discuss the host–guest inclusion caused by the weak interactions in terms of the enthalpy–entropy compensation effect.7,11–16Guest molecules.

ISSN:1472-779X    

出版商:RSC    

年代:1999

数据来源: RSC