Prediction of binding constants of alpha-cyclodextrin complexes

Formation of β-cyclodextrin complexes in an anhydrous environment

The formation of inclusion complexes of β-cyclodextrin was studied at the melting temperature of guest compounds by differential scanning calorimetry. The complexes of long-chain n-alkanes, polyaromatics, and organic acids were investigated by calorimetry and IR spectroscopy. The complexation ratio of β-cyclodextrin was compared with results obtained in an aqueous environment. The stability and structure of inclusion complexes with various stoichiometries were estimated by quantum chemistry and molecular dynamics calculations. Comparison of experimental and theoretical results confirmed the possible formation of multiple inclusion complexes with guest molecules capable of forming hydrogen bonds. This finding gives new insight into the mechanism of formation of host-guest complexes and shows that hydrophobic interactions play a secondary role in this case. Graphical abstract The formation of complexes of β-cyclodextrin with selected n-alkanes, polyaromatics, and organic acids in an anhydrous environment is studied by differential scanning calorimetry, IR spectroscopy, and molecular modeling. The results obtained confirm the possible formation of multiple inclusion complexes with guest molecules capable of forming hydrogen bonds and give a new perspective on the mechanism of formation of host-guest complexes.

Limitations and extensions of the lock-and-key principle: differences between gas state, solution and solid state structures

The lock-and-key concept is discussed with respect to necessary extensions. Formation of supramolecular complexes depends not only, and often not even primarily on an optimal geometric fit between host and guest. Induced fit and allosteric interactions have long been known as important modifications. Different binding mechanisms, the medium used and pH effects can exert a major influence on the affinity. Stereoelectronic effects due to lone pair orientation can lead to variation of binding constants by orders of magnitude. Hydrophobic interactions due to high-energy water inside cavities modify the mechanical lock-and-key picture. That optimal affinities are observed if the cavity is only partially filled by the ligand can be in conflict with the lock-and-key principle. In crystals other forces than those between host and guest often dominate, leading to differences between solid state and solution structures. This is exemplified in particular with calixarene complexes, which by X-ray analysis more often than other hosts show guest molecules outside their cavity. In view of this the particular problems with the identification of weak interactions in crystals is discussed.

On the ease of predicting the thermodynamic properties of beta-cyclodextrin inclusion complexes

Background

In this study we investigated the predictability of three thermodynamic quantities related to complex formation. As a model system we chose the host-guest complexes of β-cyclodextrin (β-CD) with different guest molecules. A training dataset comprised of 176 β-CD guest molecules with experimentally determined thermodynamic quantities was taken from the literature. We compared the performance of three different statistical regression methods – principal component regression (PCR), partial least squares regression (PLSR), and support vector machine regression combined with forward feature selection (SVMR/FSS) – with respect to their ability to generate predictive quantitative structure property relationship (QSPR) models for ΔG°, ΔH° and ΔS° on the basis of computed molecular descriptors.

Results

We found that SVMR/FFS marginally outperforms PLSR and PCR in the prediction of ΔG°, with PLSR performing slightly better than PCR. PLSR and PCR proved to be more stable in a nested cross-validation protocol. Whereas ΔG° can be predicted in good agreement with experimental values, none of the methods led to comparably good predictive models for ΔH°. In using the methods outlined in this study, we found that ΔS° appears almost unpredictable. In order to understand the differences in the ease of predicting the quantities, we performed a detailed analysis. As a result we can show that free energies are less sensitive (than enthalpy or entropy) to the small structural variations of guest molecules. This property, as well as the lower sensitivity of ΔG° to experimental conditions, are possible explanations for its greater predictability.

Conclusion

This study shows that the ease of predicting ΔG° cannot be explained by the predictability of either ΔH° or ΔS°. Our analysis suggests that the poor predictability of TΔS° and, to a lesser extent, ΔH° has to do with a stronger dependence of these quantities on the structural details of the complex and only to a lesser extent on experimental error.

On the Possibility That Cyclodextrins' Chiral Cavities Can Be Available on AOT n-Heptane Reverse Micelles. A UV−Visible and Induced Circular Dichroism Study

The formation of reverse micelles (RMs) of sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) in n-heptane including two different beta-cyclodextrin (beta-CD) derivatives (hydroxypropyl-beta-CD, hp-beta-CD, and decenyl succinyl-beta-CD, Mod-beta-CD) is reported. Both cyclodextrins can be incorporated into AOT RMs in different zones within the aggregate, while beta-CD cannot. Using UV-vis and induced circular dichroism (ICD) spectroscopy and different achiral molecular probes (some azo dyes, p-nitroaniline and ferrocene), it was possible to determine that Mod-beta-CD is located with its cavity at the oil side of the AOT RM interface, while for hp-beta-CD the cavity is inside the RM water pool. Among the molecular probes used, methyl orange (MO) was the only one which gave the ICD signal when dissolved in the AOT RMs with hp-beta-CD, so a detailed study of MO behavior in homogeneous media was also performed to compare with the microheterogeneous media. The solvatochromic behavior of the dye depends not only on the polarity of the media but also on other specific solvent properties. A Kamlet-Taft analysis shows that the MO absorption spectrum shifts to longer wavelength with an increase in the solvent polarity-polarizability (pi*) and the hydrogen donor ability (alpha) of the medium. MO appears to be almost 3 times more sensitive to the pi* parameter than to the alpha parameter. In addition, from the MO absorption spectral changes with the hp-beta-CD concentration, the association equilibrium constants in pure water (K11W) and inside the RMs (K11RM) were computed. The results show that K11W is almost 10 times larger than the value inside the RMs. The latter can be explained considering that MO resides anchored to the RM interface through hydrogen bond interaction with the hydration bound water. This study shows for the first time that the cyclodextrin chiral cavity is available for a guest in an organic medium such as the RMs; therefore, we have created a potentially powerful nanoreactor with two different confined regions in the same aggregate: the polar core of the RMs and the chiral hydrophobic cavity of cyclodextrin.

In Search of Fully Uncomplexed Cyclodextrin in the Presence of Micellar Aggregates

The chemical behavior of beta-cyclodextrin/nonionic surfactant mixed systems has been investigated using the basic hydrolysis of N-methyl-N-nitroso-p-toluenesulfonamide as a chemical probe. The experimental results prove that at the cmc, there are significant quantities of uncomplexed beta-CD in equilibrium with the micellar aggregates. In contrast to the expected situation, the percentage of uncomplexed beta-CD in equilibrium with the micellar system increases on increasing the hydrophobicity of the surfactant molecule. This behavior is due to the existence of two simultaneous processes: complexation of surfactant monomers by cyclodextrin and the process of self-assembly to form micellar aggregates. The autoaggregation of surfactant monomers is expected to be more important than the complexation process in this mixed system. Varying the hydrophobicity of the surfactant monomer enabled us to determine that the percentages of uncomplexed cyclodextrin in equilibrium with the micellar system were in the range of 5-95%.

Design and monitoring of photostability systems for amlodipine dosage forms

Photostability of amlodipine (AML) has been monitored in several pharmaceutical inclusion systems characterized by plurimolecular aggregation of the drug and excipients with high molecular weight. Several formulations including cyclodextrins, liposomes and microspheres have been prepared and characterized. The photodegradation process has been monitored according to the conditions suggested by the ICH Guideline for photostability testing, by using a light cabinet equipped with a Xenon lamp and monitored by spectrophotometry. The formulations herein tested have been found to be able to considerably increase drug stability, when compared with usual pharmaceutical forms. The residual concentration detected in the inclusion complexes with cyclodextrins and liposomes was 90 and 77%, respectively, while a very good value of 97% was found for microspheres, after a radiant exposure of 11,340 kJm(-2).

When can cyclodextrins be considered for solubilization purposes?

This communication is intended to address the question: How can one determine, with minimum experimentation, if cyclodextrins (CDs) might be the right choice as solubilization enhancers for a given poorly water-soluble drug? The cyclodextrin utility number, U(CD), a dimensionless number, is introduced to assess the feasibility of the use of CDs in dosage forms. U(CD) is a lumped-parameter consisting of the dose of the drug, the workable amount of CD, the binding constant, and the drug solubility in the absence of CDs. U(CD) was been extended to ionizable drugs that show synergistic increase in solubility due to ionization and complexation. U(CD) is a guiding and not a predictive tool that the formulator can use.

Multimodal inclusion complexes between barbiturates and 2-hydroxypropyl-beta-cyclodextrin in aqueous solution: isothermal titration microcalorimetry, (13)C NMR spectrometry, and molecular dynamics simulation

Multiple types (structures) of inclusion complexes between barbiturates and 2-hydroxypropyl-beta-cyclodextrin (HPCD) were evaluated by isothermal titration microcalorimetry and (13)C NMR spectroscopy. The geometries of the inclusion complexes were suggested by molecular dynamics simulation. Barbituric acid (BA), barbital (B), amobarbital (AB), pentobarbital (PB), secobarbital (SB), cyclobarbital (CB), and phenobarbital (PHB) were used as barbiturates with different substituents on the barbituric acid ring and compared for inclusion types in aqueous solution. The association constants (K), stoichiometries, and thermodynamic parameters change in free energy (DeltaG) change in enthalpy (DeltaH), and change in entropy [DeltaS] for each type of complex were determined from the calorimetric data. The inclusion complexation was largely entropy driven because of hydrophobic interactions. The values of K increased in the order BA<B<AB<PB<SB<CB<PHB. Barbiturates, except B and BA, form two types of inclusion complex with a 1:1 stoichiometry in the un-ionized forms. The first type of inclusion complex with high affinity (K(1)) was characterized by small negative values of DeltaH(1) and large positive DeltaS(1), where the substituent R2 of the barbiturate was initially inserted into the cavity of HPCD through hydrophobic interactions. There was a good relationship between DeltaG(1) obtained from the calorimetric data for the first type of inclusion complex and DeltaG(R2) calculated from the changes in (13)C Nuclear Magnetic Resonance (NMR) chemical shifts for the substituent R2 of barbiturates. These types were very stable in aqueous solution at various pHs. The second type of complex, with low affinity (K(2)), was characterized by large negative values of DeltaH(2) and small positive DeltaS(2), reflecting van der Waals' interactions in the un-ionized forms of barbiturates at pH values less than pK(a). The values of K(2) were markedly decreased to <10(3) M(-1) as the barbiturates were ionized over pH 8. Thus, in the second type, the barbituric acid ring contributed to forming the complexes. The geometries were stabilized by hydrogen bond formation between the hetero atoms in the barbituric acid ring and the secondary hydroxyl groups on the rim of the cyclodextrin. The (13)C NMR chemical shifts of C4 and C6 carbons in the barbituric acid ring were moved upfield significantly by the inclusion complexation. On the other hand, B and BA could form only one type of complex, the lid-type supramolecular complex with small association constants.

[Software development for calculation of molecular surface area and its application to hydrophobic interaction]

A novel method of calculating the water-accessible molecular surface area from the number of points generated on the molecular surface was developed. This method yielded a molecular surface area with high accuracy and speed. The molecular surface area of lecithin shows an excellent linear correlation with the logarithm of the critical micelle concentration for many lecithins having different acyl chains. The solution structure of oxyphenonium bromide estimated from the molecular surface area approach was close to that obtained from NMR. Furthermore, the change of molecular surface area, delta S(HG), with docking of host and guest was defined and its calculation method was developed. Because both the host and the guest generally consist of hydrophilic and hydrophobic atomic groups, delta S(HG) was divided into such four terms as delta Soo(HG), delta Sow(HG), delta Swo(HG), and delta Sww(HG). For instance, delta Soo(HG) is the decrease in surface area with contact between the hydrophobic surfaces of the host and the guest. When the guest molecule was moved along the symmetry axis of cyclodextrin (CyD), the structure of a complex having the maximum value of delta Soo(HG) corresponds with the crystal structure. The solution structures of several inclusion systems were predicted by this method. For various systems including alpha-CyD, beta-CyD, gamma-CyD, and aromatic and aliphatic guests, the maximum values of delta Soo(HG) showed a good correlation with the logarithms of the binding constants. This relationship will be used for the prediction of the binding constants for CyD and other host-guest systems.

Photodegradation of inclusion complexes of isradipine with methyl-beta-cyclodextrin

The paper presents results of the studies on photochemical decomposition of isradipine (IS) and its liquid inclusion complexes with methyl-beta-cyclodextrin (M-betaCD). The process of photodegradation was assessed by the methods of UV spectrophotometry, HPLC (reverse-phase) and HPTLC (normal phase) chromatographic methods. The process of photodegradation of IS was analysed in the conditions of version I of the document of International Chemical Harmonization (ICH)-HBO-200 lamp. Quantitative evaluation of the photochemical decomposition was performed on the basis of the calculated photodegradation rate constant (k), half-life period (t0.5) and time of degradation of 10% of the compound (t0.1). Formation of inclusion complexes of IS with M-betaCD was proved to increase twice the photostability of the drug. The analytical methods used were subjected to a validation procedure in which the limits of detectability and determinability as well as specificity, precision and sensitivity of the method were determined.

A 1H NMR study of cyclodextrin - hydrocarbon surfactant inclusion complexes in aqueous solutions

A 1H NMR chemical shift ( delta ) study of a homologous series of hydrocarbon (hc) (CxH2x + 1CO2Na, x = 5, 7, 9, 11, 13) surfactants (S) has been carried out in water and in binary solvent (D2O + cyclodextrin (CD)) systems at 22°C. Complementary 1H NMR chemical shift ( delta ) data of the cyclodextrins in binary (D2O + S) systems containing hc surfactants have also been obtained. Complex induced shift (CIS) values for selected host or guest protons were found to increase as the alkyl chain (Cx) length of the surfactant increased. The CIS values are found to depend on the following factors: (i) the magnitude of the binding constant (Ki, i = 1:1, 2:1), (ii) the chain length of the surfactant, (iii) the mole ratio of the host to guest species, (iv) the host-guest stoichiometry, and (v) the host-guest inclusion geometry. The CIS values of the CD-S systems have been analyzed using equilibrium models in which 1:1 complexes, 1:1 plus 2:1 complexes, and uncomplexed species are present. Differences in the binding affinity, stoichiometry, and inclusion geometry of the complexes formed between a given hc surfactant and the various cyclodextrins were observed.Key words: cyclodextrin, surfactant, NMR, chemical shift, complex, binding constant.

Binding of substituted acetic acids to alpha-cyclodextrin in aqueous solution

Complex binding constants of 23 aliphatic acids with alpha-cyclodextrin in aqueous solution were measured by potentiometry, solubility, or competitive spectrophotometry at 25 degrees C. All systems formed 1:1 acid:cyclodextrin complexes, and some of them also formed 1:2 complexes. The conjugate acids formed stronger complexes than did the conjugate bases (except for glycine). Empirical correlations of complex stabilities are shown with partition coefficients, surface areas, molar refraction, and other descriptors. Complex stability appears to result from the hydrophobic effect, the dispersion interaction, and interaction of the carboxylic acid group with the cyclodextrin.