Complexation forces in aqueous solution. Calorimetric studies of the association of 2-hydroxypropyl-b-cyclodextrin with monocarboxylic acids or cycloalkanols.

The formation of complexes between 2-hydroxypropyl-b-cyclodextrin and monocarboxylic acids or cycloalkanols has been studied calorimetrically at 298 K in phosphate buffer, pH 11.3. The forces involved in the assocn. process are discussed in the light of the signs and values of the thermodn. parameters obtained: assocn. enthalpy, binding const., Gibbs free energy, and entropy. For monocarboxylic acids, hydrophobic interactions are the primary force detg. complexation, as indicated by the small enthalpies and by the high and pos. entropies. For the cycloalkanols, instead, enthalpies are neg. and entropies pos. or neg., depending on the solvent medium employed, namely water or phosphate buffer. The most important requirement for the formation of the complex is a good spatial fit between the two interacting mols. A cavity elongation effect occurs because of the presence of the hydroxypropyl groups on the rim of the macrocycle. The relative contribution of hydrophobic and van der Waals interactions varies with the dimensions of the guest mols. A linear correlation exists between enthalpy and entropy of complexation, underlying that inclusion is a process dominated by hydration phenomena and ascribed to the modifications experienced by the solvent in the hydration shells of the interacting substances

Water-mediated interactions between benzene rings. Calorimetric studies of aromatic model compounds in aqueous solutions at 298 K

Pairwise enthalpic interaction coeffs. of the virial expansion of the excess enthalpies were detd. at 298 K by measuring the enthalpies of diln. in aq. soln. of binary aq. solns. contg. 4-hydroxyphenylacetic acid, 2-phenylethanol, 3-phenylpropanol, 3-phenylpropionic acid, L-tyrosine, and L-phenylalanine. Coeffs. obtained are compared with those already reported in the literature for other arom. substances in aq. solns. Not withstanding the similarity of the substances employed, the values of the enthalpic coeffs. range from highly neg. to highly pos., an indication that the interactions between the benzene rings are largely dependent on the nature of the functional groups. For hydroxylated substances, enhanced hydrophobic interactions are operating, probably for the simultaneous interaction between the benzene rings and the alkyl chains, forced by the hydroxyl group. On the contrary, the strength of hydrophobic interactions in the solns. of the amino acids depend on the pH of the medium and on the presence of hydroxyl group on the arom. ring. The data are discussed according to an interaction model which assumes the presence of a preferential configuration between two hydrated mols

Solvent effects on the complexation of 1-alkanols by parent and modiefied cyclodextrins. Caloimetric studies at 298K

The formation of complexes of parent and alkylated cyclodextrins (CDs) with 1-heptanol and 1-octanol has been studied calorimetrically at 298 K in water and in concentrated aqueous solutions of urea. The forces involved in the association process are discussed in the light of the signs and values of the thermodynamic parameters obtained: association enthalpy, binding constant, Gibbs free energy, and entropy. It was inferred that: (i) in water, the formation of complexes for parent and substituted a-cyclodextrins (aCDs) is determined by enthalpy. For parent and substituted b-cyclodextrins (bCDs), instead, hydrophobic interactions are the prevailing forces determining complexation, as indicated by the small and negative or positive enthalpies and by the high and positive entropies. (ii) In urea, hydrophilic interactions are attenuated. The formation of complexes with alkylated CDs does not occur. (iii) The analysis of the thermodynamic properties confirms that inclusion is a process dominated by hydration phenomena. Modifications experienced by the solvent water in the hydration shells of the interacting substances upon association determine the formation of the complexes

Thermodynamics of inclusion complexes of natural and modified cyclodextrins with propranolol in aqueous solution at 298 K

The assocn. const., std. Gibbs energy, enthalpy and entropy for formation of inclusion complexes of propranolol, a b-blocker, with various natural and modified cyclodextrins have been detd. by calorimetry at 298 K. Both natural and methyl-modified a-cyclodextrins do not form complexes, while b- and g-cyclodextrins do. Complexing ability of 2-hydroxypropyl-b-cyclodextrin depends on the av. substitution degree. For g-cyclodextrin, hydrophobic interactions play the major role in binding the guest. The assocn. of natural and modified b-cyclodextrins is ruled by van der Waals interactions and hydrogen bonding because of the tighter fit of the guest into the cavity. Decreasing pH dets. increasingly neg. values of the assocn. enthalpies