Complexation of natural and methylated β-cyclodextrins with long-chain Carboxylic acids in aqueous solutions. Calorimetric studies at 298 K

Extracavity Effect in Cyclodextrin/Surfactant Complexation

Cyclodextrin (CD) complexation is a convenient method to sequester surfactants in a controllable way, for example, during membrane-protein reconstitution. Interestingly, the equilibrium stability of CD/surfactant inclusion complexes increases with the length of the nonpolar surfactant chain even beyond the point where all hydrophobic contacts within the canonical CD cavity are saturated. To rationalize this observation, we have dissected the inclusion complexation equilibria of a structurally well-defined CD, that is, heptakis(2,6-di- O-methyl)-β-CD (DIMEB), and a homologous series of surfactants, namely, n-alkyl- N, N-dimethyl-3-ammonio-1-propanesulfonates (SB3- x) with chain lengths ranging from x = 8 to 14. Thermodynamic parameters obtained by isothermal titration calorimetry and structural insights derived from nuclear magnetic resonance spectroscopy and molecular dynamics simulations revealed that, upon inclusion, long-chain surfactants with x = ≥10 extend beyond the canonical CD cavity. This enables the formation of hydrophobic contacts between long surfactant chains and the extracavity parts of DIMEB, which make additional favorable contributions to the stability of the inclusion complex. These results explain the finding that the stability of CD/surfactant inclusion complexes monotonously increases with the surfactant chain length even for long chains that completely fill the canonical CD cavity.

A spectral displacement study of cyclodextrin/naphthenic acids inclusion complexes

The spectral displacement technique has been used to obtain 1:1 β-cyclodextrin (β-CD)/carboxylate anion equilibrium binding constants (K2) for some complex mixtures of naphthenic acids (NAs) and some examples of single-component NAs in aqueous solution. Three specific examples of single-component NAs were chosen with variable Z values as follows: 2-hexyldecanoic acid (Z = 0; S1), trans-4-pentylcyclohexanecarboxylic acid (Z = –2; S2), and dicyclohexylacetic acid (Z = –4; S3). The estimated K2 values for S1, S2, and S3 are as follows: 1.42 × 103 M–1, 52.2 × 104 M–1, and 13.1 × 104 M–1, respectively. The corresponding K2 values are 2.34 × 104 M–1 and 1.27 × 104 M–1 for commercial (Fluka) and industrial (Syncrude) sourced NAs, respectively. The magnitude of K2 for 1:1 complexes formed between β-CD and S1, S2, or S3 did not correlate with the degree of hydrogen deficiency (Z-series) but there was a correlation with the size of the guest molecules (n) examined in this study. The correlation between complex stability and the relative size of the lipophilic fragments of the guest molecule are related to the importance of the hydrophobic effect for inclusion of such carboxylic acid guest molecules within β-CD.

The influence of cosolvents on hydrophilic and hydrophobic interactions. Calorimetric studies of parent and alkylated cyclomaltooligosaccharides in concentrated aqueous solutions of ethanol or urea

Heats of dilution in water and in aqueous 7 mol kg(-1) urea and 3 mol kg(-1) ethanol of binary solutions containing cyclomaltohexaose, cyclomaltoheptaose, cyclomaltooctaose, 2-hydroxypropyl-cyclomaltohexaose (HPαCD), 2-hydroxypropyl-cyclomaltoheptaose (HPβCD), methyl-cyclomaltohexaose (MeαCD), methyl-cyclomaltoheptaose (MeβCD) and 2-hydroxypropyl-cyclomaltooctaose (HPγCD) have been determined at 298.15K by flow microcalorimetry. The purpose of this study is to gain information about the influence of urea and ethanol, which have different effects on water structure, on hydrophilic and hydrophobic interactions. The pairwise interaction coefficients of the virial expansion of the excess enthalpies were evaluated and compared to those previously obtained for binary solutions of cyclomaltohexaose and cyclomaltoheptaose. The particular behaviour of cyclomaltooligosaccharides in water is put in evidence with respect to that shown by simple oligosaccharides. The values of the interaction coefficients greatly change in dependence of the solvent medium. They are negative in water for unsubstituted cyclomaltooligosaccharides, and positive for the alkyl-substituted ones, thus marking the major role of the hydrophobic interactions. In concentrated aqueous ethanol, coefficients are negative, while they are positive in concentrated aqueous urea. Urea solvates the hydroxyl group provoking the attenuation of hydrophilic and hydrophobic interactions. Instead, the presence of the cosolvent ethanol, which lowers the relative permittivity of the medium, enhances the strength of hydrophilic interactions.