Determination of second-order association constants by global analysis of 1H and 13C NMR chemical shifts. Application to the complexation of sodium fusidate and potassium helvolate by beta- and gamma-cyclodextrin

Study of the Structural Chemistry of the Inclusion Complexation of 4-Phenylbutyrate and Related Compounds with Cyclodextrins in Solution: Differences in Inclusion Mode with Cavity Size Dependency

4-phenylbutyrate (PB) and structurally related compounds hold promise for treating many diseases, including cancers. However, pharmaceutical limitations, such as an unpleasant taste or poor aqueous solubility, impede their evaluation and clinical use. This study explores cyclodextrin (CD) complexation as a strategy to address these limitations. The structural chemistry of the CD complexes of these compounds was analyzed using phase solubility, nuclear magnetic resonance (NMR) spectroscopic techniques, and molecular modeling to inform the choice of CD for such application. The study revealed that PB and its shorter-chain derivative form 1:1 αCD complexes, while the longer-chain derivatives form 1:2 (guest:host) complexes. αCD includes the alkyl chain of the shorter-chain compounds, depositing the phenyl ring around its secondary rim, whereas two αCD molecules sandwich the phenyl ring in a secondary-to-secondary rim orientation for the longer-chain derivatives. βCD includes each compound to form 1:1 complexes, with their alkyl chains bent to varying degrees within the CD cavity. γCD includes two molecules of each compound to form 2:1 complexes, with both parallel and antiparallel orientations plausible. The study found that αCD is more suitable for overcoming the pharmaceutical drawbacks of PB and its shorter-chain derivative, while βCD is better for the longer-chain derivatives.

Fully Symmetric Cyclodextrin Polycarboxylates: How to Determine Reliable Protonation Constants from NMR Titration Data

Acid-base properties of cyclodextrins (CDs), persubstituted at C-6 by 3-mercaptopropionic acid, sualphadex (Suα-CD), subetadex (Suβ-CD) and sugammadex (Suγ-CD, the antidote of neuromuscular blocking steroids) were studied by ¹H NMR-pH titrations. For each CD, the severe overlap in protonation steps prevented the calculation of macroscopic pK_a values using the standard data fitting model. Considering the full symmetry of polycarboxylate structures, we reduced the number of unknown NMR parameters in the "Q-fitting" or the novel "equidistant macroscopic" evaluation approaches. These models already provided pK_a values, but some of them proved to be physically unrealistic, deceptively suggesting cooperativity in carboxylate protonations. The latter problem could be circumvented by adapting the microscopic site-binding (cluster expansion) model by Borkovec, which applies pairwise interactivity parameters to quantify the mutual basicity-decreasing effect of carboxylate protonations. Surprisingly, only a single averaged interactivity parameter could be calculated reliably besides the carboxylate 'core' microconstant for each CD derivative. The speciation of protonation isomers hence could not be resolved, but the optimized microscopic basicity parameters could be converted to the following sets of macroscopic pK_a values: 3.84, 4.35, 4.81, 5.31, 5.78, 6.28 for Suα-CD; 3.82, 4.31, 4.73, 5.18, 5.64, 6.06, 6.54 for Suβ-CD and 3.83, 4.28, 4.65, 5.03, 5.43, 5.81, 6.18, 6.64 for Suγ-CD. The pH-dependent charge of these compounds can now be accurately calculated, in support of designing new analytical methods to exploit their charge-dependent molecular recognition such as in cyclodextrin-aided chiral capillary electrophoresis.

NMR studies of inclusion complexes: naphthalene and natural cyclodextrins

Inclusion complexes of naphthalene (NP) with cyclodextrins (CD) have been investigated so far using non-NMR techniques resulting in inconsistent data. Here, the first application of high-field NMR spectroscopy in combination with a precise analysis of the results has allowed us to determine accurately the stoichiometry of complexes and their association constants. Titration measurements have been performed by ¹H NMR spectroscopy in D₂O at a magnetic field B₀ of 18.8 T. NP and αCD form a 1 : 2 complex in which a single NP molecule is closed in a capsule made up of two αCD macrocycles. NP and βCD build coexisting 2 : 1 and 2 : 2 complexes with large binding constants. Larger γCD host molecules form essentially similar complexes with NP as the βCD but corresponding binding constants are smaller.

Molecular interactions in remdesivir-cyclodextrin systems

Remdesivir (REM) is the first antiviral drug (Veklury™) approved by the Food and Drug Administration for the therapy of COVID-19. Due to its poor water solubility, the preparation of Veklury™ requires a suitable solubilizing excipient at pH 2 conditions. For this purpose, the final formulation contains the randomly substituted sulfobutylether-β-cyclodextrin (SBEβCD) as a complexing agent. Herein, extensive NMR spectroscopic study with various cyclodextrin (CD) derivatives were conducted to understand the interactions in SBEβCD - REM systems at the molecular level. The pK_a value of REM has been determined experimentally for the first time, as the protonation state of the aminopyrrolo-triazine moiety can play a key role in CD-REM inclusion complex formation as SBEβCD has permanent negative charges. The UV-pH titration experiments yielded a pK_a of 3.56, thus the majority of REM bears a positive charge at pH 2.0. NMR experiments were performed on β- and γCD derivatives to determine complex stabilities, stoichiometries and structures. The stability constants were determined by nonlinear curve fitting based on ¹H NMR titrations at pH 2.0, while Job's method was used to determine the stoichiometries. βCD complexes were one order of magnitude more stable than their γCD counterparts. Sulfobutylation resulted in a significant increase in stability and the single isomer derivatives showed unexpectedly high stability values (logK = 4.35 for REM - per-6-SBEβCD). In the case of βCDs, the ethylbutyl-moiety plays a key role in complexation immersing into the βCD cavity, while the phenoxy-moiety overtakes and drives the inclusion of REM in the case of γCDs. This is the first comprehensive study of REM-CD complexation, allowing the design of new CD derivatives with tailored stabilities, thereby aiding the formulation or production and even the analytical characterization of REM.

Sulfobutylether-beta-cyclodextrin-enabled antiviral remdesivir: Characterization of electrospun- and lyophilized formulations

Veklury™ by Gilead Sciences, Inc., containing antiviral drug, remdesivir (REM) has received emergency authorization in the USA and in Europe for COVID-19 therapy. Here, for the first time, we describe details of the non-covalent, host-guest type interaction between REM and the solubilizing excipient, sulfobutylether-beta-cyclodextrin (SBECD) that results in significant solubility enhancement. Complete amorphousness of the cyclodextrin-enabled REM formulation was demonstrated by X-ray diffraction, thermal analysis, Raman chemical mapping and electron microscopy/energy dispersive spectroscopy. The use of solubilizing carbohydrate resulted in a 300-fold improvement of the aqueous solubility of REM, and enhanced dissolution rate of the drug enabling the preparation of stable infusion solutions for therapy. 2D ROESY NMR spectroscopy provided information on the nature of REM-excipient interaction and indicated the presence of inclusion phenomenon and the electrostatic attraction between anionic SBECD and nitrogen-containing REM in aqueous solution.

Structural characterization of fondaparinux interaction with per-6-amino-beta-cyclodextrin: An NMR and MS study

The highly anionic synthetic pentasaccharide fondaparinux (FDPX) - representing the antithrombin binding sequence of heparin - is in clinical use as a potent anticoagulant. Contrary to the unfractionated heparin, FDPX lacks potent antidote completely reversing its anticoagulant activity, therefore it is of great importance to identify new structures exhibiting strong intermolecular interactions towards FDPX. The polycationic heptakis(6-amino-6-deoxy)-beta-cyclodextrin (NH₂-β-CD) can serve as an excellent model compound to mimic these interactions between the oppositely charged oligosaccharides. Herein, extensive NMR spectroscopic and nano-electrospray ionization mass spectrometric (nESI-MS) studies were conducted to understand the molecular-level interactions in the FDPX - NH₂-β-CD systems. NMR experiments were performed at pD 7.4 and 2.0. Job's method of continuous variation and ¹H NMR titration experiments suggested the formation of FDPX∙NH₂-β-CD complex at pD 7.4, while the presence of multiple complexes was assumed at pD 2.0. Stability constants were determined by separate ¹H NMR titrations, yielding log β₁₁=3.65 ± 0.02 at pD 7.4, while log β₁₁ ≥ 4.9 value suggested a high-affinity system at pD 2.0. 2D NOESY NMR studies indicated spatial proximities between the anomeric resonance α-l-iduronic acid residue and the cyclodextrin's methylene unit in the proximity of the cationic amino function. Acidic degradation of FDPX was investigated by NMR and MS for the first time in detail confirming that desulfation occurs involving one to two sulfate moieties. The desulfation of FDPX was inhibited by the cationic cyclodextrin in the case of equimolar ratio at pD 2.0. This is the first report on the stabilizing effect of cyclodextrin complexation on heparin degradation.

Fluorescent pseudorotaxanes of a quinodicarbocyanine dye with gamma cyclodextrin

Spectrophotometric titration of buffered solutions of gamma cyclodextrin (γCD) and 1,1'-diethyl,2,2'-dicarbocyanine (DDI) demonstrates extension of the known 1:2 host:guest complex to form a previously unreported 2:2 complex near the γCD solubility limit. Though DDI is predominantly hosted as a non-fluorescent H-aggregate, both complexes exist in respective equilibria with two secondary complexes hosting unaggregated DDI as 1:1 and 2:1 complexes. The 2:1 complex exhibits significant fluorescence emission, with a quantum yield six times that of DDI in organic solvents, but ten times lower than that of an analogous indodicarbocyanine. Fragment Molecular Orbital calculations suggest that the 2:1 complex has the tail-to-tail conformation, and that solvent access to the dye strongly favors photoisomerization. In the host-guest complex, γCD limits solvent access to the dye and hinders rotation of the quinolyl terminal groups, but nevertheless pairwise rotation of methine carbons within the γCD cavity likely remains as a significant nonradiative relaxation pathway for the excited state.

NMR, CD and UV spectroscopic studies reveal uncommon binding modes of dapoxetine to native cyclodextrins

Complex formation between the selective serotonin reuptake inhibitor drug (S)-dapoxetine (Dpx) and β-, γ-, and methylated γ-cyclodextrins (CyDs) was studied by complementary experimental techniques.

A carbohydrate-anion recognition system in aprotic solvents

A carbohydrate–anion recognition system in nonpolar solvents is reported, in which complexes form at the B-faces of β-d-pyranosides with H1-, H3-, and H5-cis patterns similar to carbohydrate–π interactions. The complexation effect was evaluated for a range of carbohydrate structures; it resulted in either 1:1 carbohydrate–anion complexes, or 1:2 complex formation depending on the protection pattern of the carbohydrate. The interaction was also evaluated with different anions and solvents. In both cases it resulted in significant binding differences. The results indicate that complexation originates from van der Waals interactions or weak CH⋅⋅⋅A⁻ hydrogen bonds between the binding partners and is related to electron-withdrawing groups of the carbohydrates as well as increased hydrogen-bond-accepting capability of the anions.

The formation of host-guest complexes between surfactants and cyclodextrins

Cyclodextrins are able to act as host molecules in supramolecular chemistry with applications ranging from pharmaceutics to detergency. Among guest molecules surfactants play an important role with both fundamental and practical applications. The formation of cyclodextrin/surfactant host-guest compounds leads to an increase in the critical micelle concentration and in the solubility of surfactants. The possibility of changing the balance between several intermolecular forces, and thus allowing the study of, e.g., dehydration and steric hindrance effects upon association, makes surfactants ideal guest molecules for fundamental studies. Therefore, these systems allow for obtaining a deep insight into the host-guest association mechanism. In this paper, we review the influence on the thermodynamic properties of CD-surfactant association by highlighting the effect of different surfactant architectures (single tail, double-tailed, gemini and bolaform), with special emphasis on cationic surfactants. This is complemented with an assessment of the most common analytical techniques used to follow the association process. The applied methods for computation of the association stoichiometry and stability constants are also reviewed and discussed; this is an important point since there are significant discrepancies and scattered data for similar systems in the literature. In general, the surfactant-cyclodextrin association is treated without reference to the kinetics of the process. However, there are several examples where the kinetics of the process can be investigated, in particular those where volumes of the CD cavity and surfactant (either the tail or in special cases the head group) are similar in magnitude. This will also be critically reviewed.

A multidimensional approach to the analysis of chemical shift titration experiments in the frame of a multiple reaction scheme

We present a method for fitting curves acquired by chemical shift titration experiments, in the frame of a three-step complexation mechanism. To that end, we have implemented a fitting procedure, based on a nonlinear least squares fitting method, that determines the best fitting curve using a "coarse grid search" approach and provides distributions for the different parameters of the complexation model that are compatible with the experimental precision. The resulting analysis protocol is first described and validated on a theoretical data set. We show its ability to converge to the true parameter values of the simulated reaction scheme and to evaluate complexation constants together with multidimensional uncertainties. Then, we apply this protocol to the study of the supramolecular interactions, in aqueous solution, between a lanthanide complex and three different model molecules, using NMR titration experiments. We show that within the uncertainty that can be evaluated from the parameter distributions generated during our analysis, the affinities between the lanthanide derivative and each model molecule can be discriminated, and we propose values for the corresponding thermodynamic constants.

NMR spectra and structures of oridonin derivatives complexes with β-cyclodextrin

Complexations between three oridonin derivatives and β-cyclodextrin (βCD) were studied by nuclear magnetic resonance (NMR) method. Job's plots for complexes were depicted by (1)H NMR spectra chemical shifts, which proved the 1:1 stoichiometry inclusion complex formation between each derivative and βCD. Two-dimensional rotating frame overhauser effect spectroscopy (2D ROESY) support the above conclusion and also proved that ring A of each oridonin derivative deeply enters into hydrophobic cavity from the wider rim and the other parts are outside the cavity. Apparent formation constants (Ka) of complexes between three oridonin derivatives and two CDs are calculated according to Scott's equation.

The effect of the head-group spacer length of 12-s-12 gemini surfactants in the host-guest association with β-cyclodextrin

NMR spectroscopy has been used to study and characterize the interactions in solution between β-CD and alkyl-α,ω-bis(dodecyldimethyl ammonium bromide) gemini surfactants with the following head-group spacer lengths: 2, 4, 6, 8, and 10. The application of the method of continuous variation gives as a result that 1:1 and 2:1 (β-cyclodextrin-gemini) complexes are formed; the association stoichiometry is dependent on the spacer chain length, varying from 1.5 (for s=2) to 1.8 (for s=10). Assuming a two-step mechanism, the binding constants have been computed. In general, the overall binding constant slightly increases with an increase of the number of methylene groups in the spacer. The (1)H NMR spectra of the N-(CH(3))(2) groups in β-cyclodextrin/gemini mixed solutions are split into two peaks for 12-10-12, suggesting that the gemini spacer can thread the β-cyclodextrin so that the latter is positioned between the gemini head-groups. Inspection of the ROESY spectra allowed the establishment of several spatial proximities between the protons from the β-CD and the gemini and for a spacer length of 10, the data indeed indicate that complexes are formed with the CD molecule positioned between the two charged head groups with the spacer passing through the CD molecule.

pH Dependent In−Out Isomerism of an Amino-β-cyclodextrin Derivative

An amino derivative of beta-cyclodextrin [6-(6-aminehexanamide)-6-deoxy)-beta-cyclodextrin (6-betaCD)] was synthesized, and the formation of an intramolecular inclusion complex was studied by NMR techniques. The deprotonation/protonation of the amino group stimulates an in/out movement of the pendant group toward/from the cyclodextrin cavity, the protonated species lying outside the hydrophobic cyclodextrin cavity but the unprotonated one residing inside and outside the cavity. The protonation of the amino group is a fast exchange rate NMR time-scale process, but the chain movement is a slow one. The equilibrium constants of both processes were determined from 1H NMR experiments and the kinetic constants for the slow process were determined from exchange spectroscopy (EXSY) experiments.

Stability constants: comparative study of fitting methods. Determination of second-order complexation constants by (23)Na and (7)Li NMR chemical shift titration

NMR chemical shift titration has been widely used as a method for the determination of stability constants. Systems involving metal-ligand complexation have been investigated using a number of methodologies. There are significant differences in the values reported for stability constants obtained by different experimental methods, such as calorimetry and ion selective electrode (ISE) titrations; nor has NMR chemical shift titration always yielded consistent results. Different researchers have obtained different results for the same system with results differing by as much as an order of magnitude. The chemical shift data are generally plotted against the concentration ratio of the metal and ligand for a set of solutions. A nonlinear least squares fitting method using an analytical solution of the cubic equation for the equilibrium concentration of the free ligand is used in this study and compared with methods used in the literature. Second-order association constants for the LiClO(4):12-crown-4 system in acetonitrile and the NaClO(4):12-crown-4 system in methanol are reported. Formation of both 1:1 and 1:2 metal-ligand complexes are considered. The LiClO(4):12-crown-4 acetonitrile system had been investigated previously by NMR titration but only 1:1 complexation was considered in that study. This study provides convincing evidence that both 1:1 and 1:2 complexes are important, at least, in the lithium system. A Monte Carlo investigation of the propagation of errors from the chemical shifts to the stability constants shows that the choice of data analysis methods may, in part, contribute to discrepancies and that the nonlinear nature of the model can dramatically affect the error limits on the stability constants.

Fluorescence correlation spectroscopy, a tool to investigate supramolecular dynamics: inclusion complexes of pyronines with cyclodextrin

The control of supramolecular systems requires a thorough understanding of their dynamics on a molecular level. We present fluorescence correlation spectroscopy (FCS) as a powerful spectroscopic tool to study supramolecular dynamics with single molecule sensitivity. The formation of a supramolecular complex between beta-cyclodextrin (beta-CD) as host and pyronines Y (PY) and B (PB) as guests is studied by FCS. Global target analysis of full correlation curves with a newly derived theoretical model yields in a single experiment the fluorescence lifetimes and the diffusion coefficients of free and complexed guests and the rate constants describing the complexation dynamics. These data give insight into the recently published surprising fact that the association equilibrium constant of beta-CD with PY is much lower than that with the much bulkier guest PB. FCS shows that the stability of the complexes is dictated by the dissociation and not by the association process. The association rate constants are very similar for both guests and among the highest reported for this type of systems, although much lower than the diffusion-controlled collision rate constant. A two-step model including the formation of an encounter complex allows one to identify the unimolecular inclusion reaction as the rate-limiting step. Simulations indicate that this step may be controlled by geometrical and orientational requirements. These depend on critical molecular dimensions which are only weakly affected by the different alkyl substituents of PY and PB. Diffusion coefficients of PY and PB, of their complexes, and of rhodamine 110 are given and compared to those of similar molecules.

NMR spectroscopic characterization of metoprolol/cyclodextrin complexes in aqueous solution: Cavity size dependency

The inclusion complex formation of a water-soluble beta(1)-selective adrenoreceptor antagonist Metoprolol (Met) with alpha-cyclodextrin (alpha-CyD), beta-cyclodextrin (beta-CyD), gamma-cyclodextrin (gamma-CyD), and 2-hydroxypropyl-beta-cyclodextrin (HP-beta-CyD) in aqueous solution was studied by ultraviolet (UV), circular dichroism (CD), and nuclear magnetic resonance (NMR) spectroscopies and the modes of interaction were assessed. Continuous variation plots revealed that Met forms the inclusion complexes with alpha-CyD, beta-CyD, and HP-beta-CyD in a stoichiometry of 1:1, whereas gamma-CyD forms the 2:1 complex where two Met molecules are included in one gamma-CyD cavity. NMR spectroscopic studies, including ROESY and GROESY techniques, clearly indicated that alpha-CyD with the small cavity includes the methoxyethylbenzene moiety of Met molecule shallowly in the cavity, depositing the benzene and the methoxy moieties around the secondary and primary sides, respectively, of the cavity. In the case of the beta-CyD complex, the methoxyethylbenzene moiety is more deeply included in the cavity, and it is feasible that Met may be able to enter from both primary and secondary hydroxyl sides of the cavity, forming the 1:1 complex. On the other hand, two Met molecules are included probably in an antiparallel orientation in the large gamma-CyD cavity, and the benzene moieties of Met are in contact with each other.

Complexation of several fungicides with beta-cyclodextrin: determination of the association constants and isolation of the solid complexes

The formation of inclusion complexes with beta-cyclodextrin was studied for several popular fungicides of different types: prochloraz, 2-phenylphenol, thiophanate methyl, 8-hydroxyquinoline, and benalaxyl. Phase solubility diagrams showed that in all cases complexation takes place, leading to an important increase of water solubility in prochloraz and benalaxyl. Equilibrium association constants could be determined from the phase solubility data and from NMR titrations in the case of 2-phenylphenol. Because of the low solubility of the complex formed between 8-hydroxyquinoline and beta-cyclodextrin, the corresponding association constant could not be determined. The solid complexes of fungicide-cyclodextrin were prepared and isolated by different methods. The isolation of real complexes and not physical mixtures was confirmed in the cases of prochloraz, 2-phenylphenol, and benalaxyl by differential scanning calorimetry.