Complexation of polyvalent cyclodextrin ions with oppositely charged guests: Entropically favorable complexation due to dehydration

Thermodynamic parameters for complexation of polyvalent cyclodextrin (CD) c ation and anion with oppositely charged guests have been determined in D2O containing 0.02M NaCl by means of H-1-NMR spectroscopy. Protonated heptakis (6-amino-6-deoxy)-beta-CD (per-NH3+-beta-CD) forms stable inclusion complex es with monovalent guest anions. The enthalpy (Delta H) and entropy changes (Delta S) for complexation of per-NH3+-beta-CD with p-methylbenzoate anion (p-CH3-Ph-CO2-) are 3.8 +/- 0.7 kJ mol(-1) and 88.6 +/- 2.2 J mol(-1) K-1, respectively. The Delta H and Delta S values for the native beta-CD-p-CH3- Ph-CO2- system are -8.6 +/- 0.1 kJ mol(-1) and 15.3 +/- 0.7 J mol(-1) K-1, respectively. The thermodynamic parameters clearly indicate that dehydratio n from both the host and guest ions accounts for the entropic gain in inclu sion process of p-CH3-Ph-CO2- into the per-NH3+-beta-CD cavity. The fact th at the neutral guests such as 2,6-dihydroxynaphthalene and p-methylbenzyl a lcohol hardly form the complexes with per-NH3+-beta-CD exhibits that van de r Waals and/or hydrophobic interactions do not cause the complexation of th e polyvalent CD cation with the monovalent anion. The acetate anion is not included into the per-NH3+-beta-CD cavity, while the butanoate and hexanoat e anions form the inclusion complexes. The complexation of the alkanoate an ions is entropically dominated. Judging from these results, it may be concl uded that Coulomb interactions cooperated with inclusion are required for r ealizing the large entropic gain due to extended dehydration. Entropically favorable complexation was also observed for the anionic CD-cationic guest system. The present study might present a general mechanism for ion pairing in water.