Mircea Bogdan

Inclusion of the Niflumic Acid Anion in β-cyclodextrin: A Solution NMR and X-ray Structural Investigation

We report parallel solution and solid state studies of the inclusion of the anionic form of the non-steroidal anti-inflammatory drug niflumic acid (2-[[3-(trifluoromethyl)phenyl]-amino]-3-pyridinecarboxylic acid) in the host g -cyclodextrin ( g -CD). 1 H NMR data for the interaction between host and guest in aqueous solution recorded at 300 MHz indicated a strong preference for insertion of the trifluoromethylphenyl residue, rather than the pyridinecarboxylate moiety, in the host cavity. A 1:1 complex stoichiometry was determined by the continuous variation method utilising chemical shifts of both host and guest protons. Analysis of the data using a new flexible program developed for this purpose yielded an overall association constant K of 336 M m 1 at 298 K. The NMR data indicate a dynamic equilibrium between complexed and uncomplexed species but do not distinguish guest entry from the primary and secondary sides of the host. Reaction between the Cs + salt of niflumic acid and g -CD yielded the crystalline complex ( g -CD) 2 ·(Cs + niflumate m ) 4 ·22H 2 O whose single crystal X-ray structure was determined. A novel inclusion mode for this host, namely entry of guest trifluoromethylphenyl residues from both the primary and secondary sides, was revealed by the X-ray analysis.

Evidence of a Bimodal Binding between Diclofenac-Na and β-Cyclodextrin in Solution

The complexation between diclofenac sodium and β-cyclodextrin was investigatedin solution by 1D and 2D proton NMR. The continuous variation method was usedto establish the stoichiometry. The obtained results indicate that simultaneous inclusionof both rings occur giving rise to two isomeric 1:1 complexes. The view of abimodal binding between diclofenac sodium and β-cyclodextrin was also supportedby ROESY experiments. The association constants for the two 1:1 complexeswere calculated by non-linear least-squares regression analysis, applying an iterationprocedure. The geometry of the two 1:1 complexes, according to the obtainedNMR data is given.

Cyclodextrin inclusion of four phenylurea herbicides: determination of complex stoichiometries and stability constants using solution 1H NMR spectroscopy

An inclusion complex formation between α- and β-cyclodextrin and four phenylurea analogues, namely metobromuron, monolinuron, monuron and fenuron, is reported. Complex formation was established using solution 1H NMR spectroscopy. Complex stoichiometries were determined by the method of continuous variation using the chemically induced shifts of both the host and guest protons. An analysis of the spectroscopic data revealed the stoichiometry as 1:1 in all cases while a further analysis of the same data yielded values for the association constant K ranging from 208 to 2749 M− 1. From the observed chemical shifts it was deduced that in all cases, only the guest aromatic ring enters the host cavities, the substituted urea moiety protruding from the secondary rim in the case of α-cyclodextrin, but from the primary rim in the case of β-cyclodextrin.

NMR spectroscopic characterization of β-cyclodextrin inclusion complex with vanillin

The inclusion of vanillin by β-cyclodextrin was investigated by 1H NMR. The continuous variation technique was used to evidence the formation of soluble 1:1 complex in aqueous solution. The association constant of vanillin with β-cyclodextrin has been obtained at 298 K by fitting the experimental chemical shifts differences, Δδobs = δfree – δobs of the observed guest and host protons, with a non-linear regression method. Besides the effective association constant, the fitting procedure allows a precise determination of all chemical shift parameters characterizing the pure complex. They can by used for an analysis of the geometry of the molecular complex in solution.

Inclusion of parabens in β-cyclodextrin: A solution NMR and X-ray structural investigation

A parallel study was conducted of the inclusion of alkyl parabens (guests) in the host β-cyclodextrin (β-CD). 1H NMR data indicated an insertion of the guest phenyl ring into the β-CD cavity. The stoichiometry of each complex was 1:1, as determined by a continuous variation method that utilises the chemical shifts of the host protons. These chemical shifts were additionally used to determine the association constant yielding K values of 1631, 938, 460 and 2022 M− 1 at 298 K for the methyl-, ethyl-, propyl- and butyl paraben solution state complexes, respectively. NOE experiments conducted on the methyl paraben solution complex indicated that the phenolic group of the guest was located at the secondary rim of the cyclodextrin cavity. Solid state structure analyses of the methyl and propyl paraben β-CD complexes were performed. Both complexes crystallised at ambient temperature in the space group C2, Z = 4 with a host to guest ratio of 1:1. Additionally, a second crystal structure between methyl paraben and β-CD is reported. This complex crystallised at 7oC in the space group P1, Z = 2 with a 1:1 host–guest stoichiometry. 1H NMR and solid state structure analyses were conducted on the inclusion of alkyl parabens in the host β-cyclodextrin. Both indicated an insertion of the guest phenyl ring into the β-CD cavity.

1H NMR investigation of self-association of vanillin in aqueous solution

A self-association of vanillin have been studied by 1H NMR spectroscopy using the analysis of proton chemical shifts changes in aqueous solution as a function of concentration. The experimental results have been analysed using indefinite non-cooperative and cooperative models of molecular self-association, enabling the determination of equilibrium constants, parameters of cooperativity and the limiting values of vanillin proton chemical shifts in the complex. It was found that the dimer formation creates energetically favourable conditions for subsequent molecular association.

Calorimetric and spectroscopic studies of the interaction between zidovudine and human serum albumin

A quantitative analysis of the interaction between zidovudine (AZT) and human serum albumin (HSA) was achieved using Isothermal titration calorimetry (ITC) in combination with fluorescence and 1H NMR spectroscopy. ITC directly measure the heat during a biomolecular binding event and gave us thermodynamic parameters and the characteristic association constant. By fluorescence quenching, the binding parameters of AZT-HSA interaction was determined and location to binding site I of HSA was confirmed. Via T1 NMR selective relaxation time measurements the drug-protein binding extent was evaluated as dissociation constants Kd and the involvement of azido moiety of zidovudine in molecular complex formation was put in evidence. All three methods indicated a very weak binding interaction. The association constant determined by ITC (3.58×102M-1) is supported by fluorescence quenching data (2.74×102M-1). The thermodynamic signature indicates that at least hydrophobic and electrostatic type interactions played a main role in the binding process.

NMR study and computational assays of meclofenamic Na salt and β-cyclodextrin inclusion complex

Complexation in solution between meclofenamate sodium (MCF) and β-cyclodextrin was studied using one- and two-dimensional 1H NMR spectroscopy and molecular docking coupled with ab initio calculations. The large variation of chemical shifts from protons located inside the hydrophobic cavity of β-cyclodextrin, provided clear evidence of inclusion complexation. The stoichiometry of the inclusion complex was determined to be 1:1, using the method of continuous variation. To ascertain the solution geometry of the host–guest complex a ROESY experiment was carried out. The results suggested a preferential binding of the dichlorophenyl moiety of the guest molecule within the β-cyclodextrin cavity, conformation which also sustained by the theoretical computational investigations. The association constant of the inclusion complex was determined using 1H NMR titration method in solution followed by a non-linear least-square regression implemented in CONSTEQ, a software developed in our group. After a preliminary molecular docking investigation, the most probable conformation was subjected to ab initio calculations using SIESTA software package, to characterize more precisely the stabilization energy of the 1:1 inclusion complex.

Inclusion complex of benzocaine and β-cyclodextrin: 1H NMR and isothermal titration calorimetry studies

The supramolecular structure of the inclusion complex of β-cyclodextrin with benzocaine in aqueous solution has been investigated by 1H NMR spectroscopy and isothermal titration nanocalorimetry (ITC). Analysis of 1H NMR data by continuous variation method indicates that the benzocaine: β-cyclodextrin inclusion complex occurs and has a 1:1 stoichiometry. Rotating frame NOE spectroscopy (ROESY) was used to ascertain the solution geometry of the host-guest complex which indicates that the benzocaine molecule was included with the aromatic ring into the cyclodextrin cavity. Although the affinity of benzocaine for cyclodextrin is relatively high, the association constant cannot be measured using ITC due to the low solubility of benzocaine in water.

Investigation of the inclusion of the herbicide cycluron in native cyclodextrins by X-ray diffraction, nuclear magnetic resonance spectroscopy and isothermal titration calorimetry

The formation of inclusion complexes between the native cyclodextrins (CDs) and the urea herbicide cycluron has been investigated both in solution and in the solid state. Single-crystal X-ray structures of both the uncomplexed guest and the β-CD·cycluron complex were determined while powder X-ray diffraction was used to confirm complexation between γ-CD and cycluron in the solid state. Solution-state complexation between the herbicide and α-, β- and γ-CD was established using 1H NMR spectroscopy and isothermal titration calorimetry (ITC). From the 1H NMR spectroscopic studies 1:1 complex stoichiometry was indicated in all cases and association constant values (K) were determined as 228, 3254 and 155 for the complexes α-CD·cycluron, β-CD·cycluron and γ-CD·cycluron, respectively. Assigning a 1:1 host–guest ratio, the ITC technique produced K values of the same order as those determined using the spectroscopic method. The thermodynamic parameters ΔH, ΔS and ΔG obtained using ITC provide insights into the driving forces involved during complex formation.