Improved competitive indicator methods for the study of alpha-cyclodextrin complexes

Characterization of Cyclodextrin/Volatile Inclusion Complexes: A Review

Cyclodextrins (CDs) are a family of cyclic oligosaccharides that constitute one of the most widely used molecular hosts in supramolecular chemistry. Encapsulation in the hydrophobic cavity of CDs positively affects the physical and chemical characteristics of the guests upon the formation of inclusion complexes. Such a property is interestingly employed to retain volatile guests and reduce their volatility. Within this scope, the starting crucial point for a suitable and careful characterization of an inclusion complex is to assess the value of the formation constant (K_f), also called stability or binding constant. This task requires the application of the appropriate analytical method and technique. Thus, the aim of the present paper is to give a general overview of the main analytical tools used for the determination of K_f values for CD/volatile inclusion complexes. This review emphasizes on the advantages, inconvenients and limits of each applied method. A special attention is also dedicated to the improvement of the current methods and to the development of new techniques. Further, the applicability of each technique is illustrated by a summary of data obtained from the literature.

Theoretical and experimental study of the inclusion complexes of the 3-carboxy-5,6-benzocoumarinic acid with cyclodextrins

The 3-carboxy-5,6-benzocoumarinic acid (BzCum) presents important emission properties that could make it suitable to be used as a fluorescence marker for proteins. Since the guest-cyclodextrin interaction can be used as a model for the ligand-protein interaction, the host-guest complexes of BzCum with alpha-, beta-, gamma-, 2-hydroxypropyl-beta-cyclodextrin (2-HP-beta-CD) and 2-hydroxypropyl-gamma-cyclodextrin (2-HP-gamma-CD) were investigated by means of steady-state fluorescence measurements. As the guest may exist either in anionic or in neutral form, the experiments were performed at two different pH values, 9.48 and 2.11. The association constants and the stoichiometry of the complexes were estimated by non-linear regression analysis. A theoretical study of the complexes was made using molecular mechanics (MM) calculation methods. The electrostatic and van der Waals contributions to the energy were estimated and a discussion was made on this basis.

Binding of cyclodextrins to alicyclic and aromatic substrates: complex formation of alpha-, beta-, and gamma-cyclodextrins with substituted cyclohexanecarboxylic acids and phenylalkanoic acids

Complex binding constants of the three native cyclodextrins with seven cyclohexane derivatives (all possessing the carboxylic acid group) and with the series C6H5(CH2)nCOOH (n = 0 to 4) were measured in aqueous solution at 25 degrees C by potentiometry and the solubility method. These results, combined with literature data, indicate that alpha- and gamma-cyclodextrins bind with comparable strength to both the cyclohexyl and phenyl moieties, with beta-cyclodextrin binding significantly more strongly. These acid series are compared with several series CH3(CH2)nX, where X is CH3, COOH, COO-, OH, SO3-, etc., and it is concluded that the X group (for X other than methyl) contributes appreciably to complex stability, perhaps by means of an extracavity interaction. The COOH group provides a further augmentation of complex stability. NMR CIS and ROESY results indicate the presence of isomeric complexes in both the cyclohexyl and phenylalkanoic series, and clearly demonstrate the existence of intracavity inclusion. An NOE study of the alpha-cyclodextrin: cyclohexanecarboxylate system provides evidence for inclusion combined with interaction outside (that is, at the rim of) the cavity.

Determination of association constants in cyclodextrin/drug complexation using the Scatchard plot: application to beta-cyclodextrin-anilinonaphthalenesulfonates

The appropriate Scatchard equation was developed for a system involving the formation of 1:1 and 1:2 substrate:cyclodextrin complexes. Simulation of this system was performed under the most common experimental conditions encountered in this type of study. The use of the equation allows for nonlinear least-squares estimation of the association constants. The interaction of the model compounds 1-anilino-8-naphthalenesulfonate (1,8-ANS) and 2(p-toluidinyl)-6-naphthalenesulfonate (2,6-TNS) with beta-cyclodextrin (beta-CD) was used to evaluate the theoretical model. Binding experiments were performed using either potentiometric titration or fluorimetric detection. The experimental data for 1,8-ANS/beta-CD fit well to the 1:1 binding model, with an association constant of 87 +/- 1 M-1. The association constants of the 1:1 and 1:2 2,6-TNS/beta-CD complexes utilizing direct potentiometry were 3737 +/- 6 and 149 +/- 2 M-1. It is shown that fluorimetry can give biased estimates for the association constants of the complexation 2,6-TNS/beta-CD, since the assumption of an equivalent quantum yield of bound species is not valid.

Binding studies of ions with cyclodextrins using ion-selective electrodes

The use of ion-selective electrodes (ISEs) to study the binding of various organic ions to alpha- and beta-cyclodextrins (CDs) is described. The ISEs respond selectively, directly, and continuously to the activity of the free ion of interest in solution. Binding parameters (intrinsic binding constants and number of binding sites) are calculated with a computer program which performs a nonlinear fit of the Scatchard model to the experimental data (electrode potential versus total ion concentration) of a titration of a CD solution with the ion of interest. Electrodes selective to chloramine-T, naproxenate, p-nitrophenolate, and chlorpromazine ions were used as models. One binding site was found for all cases, with binding constants varying from 0.67 x 10(3) M-1 (naproxenate to beta CD) to 1.27 x 10(4) M-1 (chlorpromazine to beta CD) at 25 degrees C and a CD concentration of 0.010 M. Precision for the estimations of the binding parameters was 2.0-7.5% within run and 8-10% between run (n = 3). Nonspecific binding was observed for CAT: beta CD and CPM: beta CD interactions. The effects of pH, temperature, CD concentration, and ionic strength on the binding are also presented. The overall binding phenomenon is discussed in light of the estimates of the thermodynamic parameters in relation to the size of the guest molecule, the possible mechanism(s) involved, and the forces participating in the interaction of ions with CDs.

Spectral displacement techniques for studying the binding of spectroscopically transparent ligands to cyclodextrins

A spectroscopic displacement method is used to determine association constants of beta-cyclodextrin with compounds that are spectroscopically transparent. These compounds are adamantanecarboxylate and structurally related compounds. Association constants obtained are compared to values obtained by other methods. It is shown that for all types of displacement techniques a distinction must be made between free and total concentrations of ligand in cases of strong binding.