Effect of cyclodextrins on the solubility and stability of candesartan cilexetil in solution and solid state

Guest-host interactions of candesartan cilexetil (CAND) with cyclodextrins (CyDs) have been investigated using phase solubility diagrams (PSD), X-ray powder diffractometry (XRPD), differential scanning calorimetry (DSC) and molecular mechanical modelling (MM). Estimates of the complex formation constant (K(11)) show that the tendency of CAND (pK(a)=6.0) to complex with CyDs follows the order: β-CyD>HP-β-CyD>γ-CyD>α-CyD. Complex formation of CAND with β-CyD (ΔG°=-31.5 kJ/mol) is largely driven by enthalpy change (ΔH°=-32.8 kJ/mol) and slightly retarded by entropy change (ΔS°=-4.6J/mol K). The HPLC results indicate that complex prepared by freeze drying method is chemically not stable due to the formation of amorphous CAND. Also it may suggest formulating CAND with β-CyD by kneading (dispersion) or co-evaporation (real inclusion complex) methods into capsule rather than compressed in tablets, where the compression enhances the instability of CAND. DSC thermograms for CAND/β-CyD complexes proved the formation of inclusion complexes with new solid phase. MM studies indicate the partial penetration of CAND into the β-CyD cavity.     

Novel inclusion complex of ibuprofen tromethamine with cyclodextrins: Physico-chemical characterization

Guest–host interactions of ibuprofen tromethamine salt (Ibu.T) with native and modified cyclodextrins (CyDs) have been investigated using several techniques, namely phase solubility diagrams (PSDs), proton nuclear magnetic resonance (¹H NMR), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR), X-ray powder diffractometry (XRPD), scanning-electron microscopy (SEM) and molecular mechanics (MM). From the analysis of PSD data (A_L-type) it is concluded that the anionic tromethamine salt of ibuprofen (pK_a = 4.55) forms 1:1 soluble complexes with all CyDs investigated in buffered water at pH 7.0, while the neutral form of Ibu forms an insoluble complex with β-CyD (B_S-type) in buffered water at pH 2.0. Ibu.T has a lower tendency to complex with β-CyD (K₁₁ = 58 M⁻¹ at pH 7.0) compared with the neutral Ibu (K₁₁ = 4200 M⁻¹) in water. Complex formation of Ibu.T with β-CyD (ΔG° = −20.4 kJ/mol) is enthalpy driven (ΔH° = −22.9 kJ/mol) and is accompanied by a small unfavorable entropy (ΔS° = −8.4 J/mol K) change. ¹H NMR studies and MM computations revealed that, on complexation, the hydrophobic central benzene ring of Ibu.T and part of the isobutyl group reside within the β-CyD cavity leaving the peripheral groups (carboxylate, tromethamine and methyl groups) located near the hydroxyl group networks at either rim of β-CyD. PSD, ¹H NMR, DSC, FT-IR, XRPD, SEM and MM studies confirmed the formation of Ibu.T/β-CyD inclusion complex in solution and the solid state.     

Study of the physicochemical properties in aqueous medium and molecular modeling of tagitinin C/cyclodextrin complexes

The inclusion complexes of tagitinin C with beta-, 2,6-di-O-methyl-beta- and gamma-cyclodextrin (CyD) was investigated in aqueous medium. The stoichiometric ratios and stability constants (K(f)) which describe the extent of formation of the complexes have been determined by UV spectroscopy and direct current tast polarography (DC(tast)), respectively. For each complex, a 1:1 molar ratio was formed in solution and the trend of stability constants was K(f) (2,6-di-O-methyl-beta-CyD)>K(f) (gamma-CyD)>K(f) (beta-CyD). The effect of molecular encapsulation on the photochemical conversion of tagitinin C was evaluated. No significant protection efficacy was noticed with beta- and gamma-CyD for the complexed drug with the respect to the free one. On the other hand, the photochemical conversion rate was slowed in presence of 2,6-di-O-methyl-beta-CyD. Data from (1)H NMR and ROESY experiments provided a clear evidence of formation of inclusion complexes. The lactone, the ester and the unsaturated ketone parts of tagitinin C inserted into the wide rim of the CyDs torus. These experimental results were confirmed by the molecular modeling using semiempirical Austin Model 1 (AM1) method.     

Effects of cyclodextrins on chlorpromazine-induced haemolysis and central nervous system responses

Cyclodextrins (CyDs) protected the human erythrocytes from haemolysis induced with chlorpromazine (CPZ) in isotonic solution, depending upon the magnitude of the stability constant of CPZ-CyD complexes (beta-greater than-gamma-greater than alpha-CyD). From the observations of CPZ uptake into erythrocytes and changes in surface activity of CPZ, the protective effects of CyDs in vitro appeared to be due to the decrease in effective haemolytic concentration of CPZ through inclusion complex formation rather than the direct interaction of CyDs with the erythrocyte membrane. The effect of beta-CyD on some central nervous system (c.n.s.) actions of CPZ in rats was also investigated to see if there were any advantages in the use of beta-CyD complexes given by injection. The results suggest that beta-CyD does not after the time-course or magnitude of the effects of CPZ on the c.n.s.     

Interaction of L-tryptophan with alpha-cyclodextrin: studies with calorimetry and proton nuclear magnetic resonance spectroscopy

The interaction of L-tryptophan with alpha-cyclodextrin was investigated in a 0.1 M phosphate buffer at pH 7.4 with a LKB 2277 microcalorimeter, using flow mixed mode at 25 degrees C. The thermodynamic parameters for inclusion complex formation obtained are as follows; DeltaG(0) = - 7.03 kJ/mol (K = 17.0), DeltaH(0) = - 9.50 kJ/mol, DeltaS(0) = - 8.3 J/mol K. The driving force for inclusion complex formation was considered to be mainly van der Waals-London dispersion force, and the contribution of hydrogen bonding was secondary in importance. Also, from the measurements of the proton nuclear magnetic resonance spectra and the model building with Corey-Pauling-Koltum atomic models, the probable structures of the complex, together with conformational change of L-tryptophan by complexation, were determined.     

Influence of cyclodextrin complexation on the in vitro permeation and skin metabolism of dexamethasone

The influence of complexation of a model drug, dexamethasone acetate (DMA), with beta-cyclodextrin (beta-CyD) and hydroxypropyl-beta-cyclodextrin (HP-beta-CyD) on the in vitro permeation through hairless mouse skin and on skin metabolism have been investigated. Complexation with CyDs increased the amount of DMA permeated in the order of 2.0 and 3.0 times for beta-CyD and HP-beta-CyD, respectively. The partition coefficient, between stratum corneum and buffer (K(SC/buffer)), for DMA decreased when the drug was an inclusion complex, being greatest for DMA/HP-beta-CyD complex. Complexation protected the drug against skin metabolism. The increase of skin permeation and stability of the model drug in the skin suggest that the complexation with beta-CyD and HP-beta-CyD is a rational way to improve the physical-chemical properties of drugs for use in transdermal delivery systems.     

Inclusion complex of 8-anilinonaphthalene-1-sulfonate with beta-cyclodextrin.

The interaction of 8-anilinonaphthalene-1-sulfonate with beta-cyclodextrin was investigated in 0.1 M phosphate buffer at pH 7.4 by fluorescence spectrophotometry. Utilizing the fact that the fluorescence intensity of 8-anilinonaphthalene-1-sulfonate increases in the presence of beta-cyclodextrin, the thermodynamic parameters for the inclusion complex formation were determined as follows: delta G degrees = -2.52 kcal/mol at 25 degrees C, delta H degree = -1.92 kcal/mol, and delta S degree = 2.1 eu. The driving forces for the inclusion complex formation were considered to be van der Waals-London dispersion force and hydrophobic interaction. Also, from the measurements of 1H NMR spectra and from studying the Corey-Pauling-Koltun (CPK) model, the structure of the inclusion complex was discussed.     

Kinetics of talampicillin decomposition in solutions

Talampicillin stability in aqueous solutions was studied in a broad range of pH values using as medium solutions of hydrochloric acid (pH 0.4-1.8), phosphate buffers (pH 2.05-3.13 and 6.03-8.04), acetate buffer (pH 3.87-5.28) and borate buffer (pH 8.90-9.10) as well as sodium hydroxide solution (pH 11.48). For the determination of talampicillin concentration changes in kinetic studies, two methods were used: iodometric and spectrophotometric in UV (lambda(max) = 254.5 nm). The catalytic velocity constants (k(H+), k(x), k(o)) were established, the log k-pH profile (35 degrees C) was found, thermodynamic parameters were calculated of the hydrolysis reaction of the beta-lactam bond (k(H+): E(A)= 67.9 kJ mol(-1), delta S = -92.4 J K(-1) mol(-1), delta G = 92.6 kJ mol(-1); k(x): E(A) = 31.8 kJ mol(-1), delta S = -347.1 J K(-1) mol(-1), delta G = 131.1 kJ mol(-1); k(o), pH = 5.28: E(A) = 98.0 kJ mol(-1), delta S = -50.3 J K(-1) mol(-1), delta G = 110.3 kJ mol(-1) at 20 degrees C), and the stability of the lactone bond was studied in the medium with the highest stability of beta-lactam bond of talampicillin (pH 5.28: k(o): E(A)= 32.5 kJ mol(-1), delta S = -220.5 J K(-1) mol(-1), delta G = 94.7 kJ mol(-1) in 20 degrees C), at controlled ionic strength (mu = 0.5 mol l(-1)).     

Effect of cyclodextrins on the acid hydrolysis of digoxin

The effects of three cyclodextrins (alpha-, beta-, gamma-CyD) on the acid hydrolysis of digoxin were examined. From the high performance liquid chromatographic tracing of each of the four components (digoxin, bisdigitoxoside, monodigitoxoside, digoxigenin) in reaction mixtures, the individual rate constants (K1-K6) were determined by analogue computer simulation. The hydrolysis was suppressed by CyDs in the order of beta-great than gamma-greater than alpha-greater than-CyD, where beta-CyD inhibited the appearance rates of digoxigenin (k3, K5, and K6) significantly. In the dissolution study of digoxin tablets, the increase in dissolution rate and decrease in acid hydrolysis were attained by inclusion complexation. The data are presented suggesting that CyDs are useful for improving the oral bioavailability of digoxin.     

Spectrophotometric and electrochemical study of the inclusion complex between beta-cyclodextrin and furnidipine

Inclusion complexation between furnidipine (2,6-dimethyl-4-(2-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylic acid 3-tetrahydrofurfuryl 5-methyl diester), a calcium-channel antagonist, and beta-cyclodextrin (beta-CyD) was studied in aqueous solution by using both spectrophotometric and electrochemical measurements. The phase solubility profile was classified as A(L)-type, indicating the formation of 1:1 stoichiometric inclusion complex of furnidipine with beta-CyD. Based on the spectrophotometric absorbance's variations, a formation constant value, K(f), of 156 M(-1) was determined. Electrochemical measurements using chronocoulometric experiments were used for the determination of the diffusion coefficients. In absence of beta-CyD, a diffusion coefficient value of 4.32 x 10(-6) cm2 s(-1) was obtained for furnidipine. The addition of beta-CyD produced a decrease of 30% for the diffusion coefficient. Formation of inclusion complexes of furnidipine with beta-CyD was proved to increase more than three times the solubility of furnidipine.     

Inclusion complex formation of captopril with alpha- and beta-cyclodextrins in aqueous solution: NMR spectroscopic and molecular dynamic studies

The inclusion complex formation of alpha-cyclodextrin (alpha-CyD), beta-cyclodextrin (beta-CyD), and 2-hydroxylpropyl-beta-cyclodextrin (HP-beta-CyD) with an angiotensin converting enzyme inhibitor, captopril, in aqueous solution was studied by (1)H- and (13)C-nuclear magnetic resonance spectroscopies, including ROESY and GROESY techniques, by kinetic methods and by molecular dynamic calculations. The oxidative degradation of captopril was markedly suppressed in alpha-CyD solutions, whereas beta-CyD and HP-beta-CyD had negligible stabilizing effects. These NMR and kinetic results suggested that alpha-CyD includes preferably the propyl thioalcohol moiety of captopril, depositing the proline moiety outside the cavity. On the other hand, beta-CyD includes a whole molecule of captopril in the cavity, locating the carboxylic acid within the cavity and the terminal thiol moiety outside the cavity. These inclusion structures were supported by molecular dynamic studies.     

Improvement of solubility and oral bioavailability of rutin by complexation with 2-hydroxypropyl-beta-cyclodextrin

The object of this study was to enhance the solubility, dissolution rate, and oral bioavailability of rutin by complexation with 2-hydroxypropyl-beta-cyclodextrin (HP-beta-CyD). The interaction of rutin with cyclodextrins (CyDs) was evaluated by the solubility, and ultraviolet (UV) and circular dichroism (CD) spectrophotometries. The chemical and enzymatic stability of rutin was examined in an alkaline buffer solution and in rat small intestinal homogenates, respectively. Dissolution rates of rutin and its CyD complexes were measured by the dispersed amount method. In vivo absorption studies of rutin after oral administration via conventional tablet containing rutin alone or its beta-CyD complexes was performed on beagle dogs. The stability constants calculated from the phase solubility method increased in the order of HP-gamma-CyD < G2-beta-CyD < beta-CyD < HP-beta-CyD. Spectroscopic studies also revealed that HP-beta-CyD and beta-CyD formed a relatively more stable inclusion complex with rutin. The dissolution rates of rutin increased by the complexation with CyDs in the order of rutin alone < HP-beta-CyD < or = beta-CyD. HP-beta-CyD inhibited the hydrolysis of rutin in the alkaline buffer solution and the small intestinal homogenates of rats, suggesting that HP-beta-CyD may stabilize rutin in a gastrointestinal tract after oral administration. When the tablet containing rutin or its beta-CyD complexes was administered to beagle dogs, the plasma levels of homovanillic acid (HVA) (a major stable metabolite of rutin) after oral administration of HP-beta-CyD complex were much higher than in either that of rutin alone or in its beta-CyD complex. The in vivo absorption study suggests that HP-beta-CyD increased the oral bioavailability of rutin from the gastrointestinal tracts of beagle dogs because of the increase in solubility, faster dissolution rate, and gastrointestinal stability. HP-beta-CyD has a significant advantage with respect to providing high aqueous solubility while maintaining a lack of toxicity in oral pharmaceutical preparations of rutin.     

In vitro and in vivo evaluation of an amylobarbitone/hydroxypropyl-beta-cyclodextrin complex prepared by a freeze-drying method

The complex formation of amylobarbitone (AMB) with 2-hydroxypropyl-beta-cyclodextrin (HP-beta-CD) was investigated in aqueous solution and in the solid state. The apparent stability constant for complex formation (Kc) calculated by phase solubility and spectral shift methods was 524 M-1 and 568 M-1, respectively. The stoichiometric molar ratio of the complex was estimated to be 1:1 and the solubility of AMB in water was increased about 3 fold. The solid dispersion system of AMB/HP-beta-CD in 1:1 molar ratio was prepared by a freeze-drying method. Differential scanning calorimetry (DSC), x-ray diffractometry, (IR) and 1H NMR spectroscopy were used to confirm that inclusion between the drug and HP-beta-CD occurred. The dissolution behavior of the drug as a physical mixture as well as the prepared complex, showed enhanced drug dissolution properties of the prepared complex compared to the physical mixture or the drug alone. The dissolution rate appeared in the first 2 min, 25 times greater for the complex than for the drug alone. Furthermore, in-vivo study revealed that the duration and hypnotic activity of AMB after its oral administration to mice were improved by inclusion.     

Thermodynamic characterization of three polymorphic forms of piracetam

Combustion calorimetry, solution calorimetry, and differential scanning calorimetry (DSC) were used to determine the standard (p° = 0.1 MPa) molar enthalpies of formation of Forms I, II, and III piracetam at 298.15 K, namely, Δ(f) H(m)° (C(6)H(10)O(2)N(2), cr I) = -520.6 ± 1.6 kJ·mol(-1), Δ(f) H(m)° (C(6)H(10)O(2)N(2), cr II) = -523.8 ± 1.6 kJ·mol(-1), and Δ(f) H(m)° (C(6)H(10)O(2)N(2), cr III) = -524.1 ± 1.6 kJ·mol(-1). The enthalpy of formation of gaseous piracetam at 298.15 K was also derived as Δ(f) H(m)° (C(6)H(10)O(2)N(2), g) = -401.3 ± 2.1 kJ·mol(-1), by combining the standard molar enthalpy of formation of Form II piracetam with the corresponding enthalpy of sublimation, Δ(sub) H(m)° (C(6) H(10) O(2) N(2), cr II) = 122.5 ± 1.4 kJ·mol(-1), obtained by drop-sublimation Calvet microcalorimetry and the Knudsen effusion method. The Δ(f) H(m)° (C(6)H(10)O(2)N(2), g) value was used to assess the corresponding predictions by the B3LYP/cc-pVTZ (-335.3 kJ·mol(-1)), G3MP2 (-388.7 kJ·mol(-1)), and CBS-QB3 (-402.8 kJ·mol(-1)) methods, based on the calculation of the atomization enthalpy of piracetam. Finally, the results of the solution and DSC experiments indicate that the stability hierarchy of Forms I, II, and III piracetam at 298.15 K, for which there was conflicting evidence in the literature, is III > II > I.     

Solid-state characterization and dissolution profiles of the inclusion complexes of omeprazole with native and chemically modified beta-cyclodextrin.

The aim of this work was to investigate the formation of the inclusion complex between omeprazole (OME), a benzimidazolic derivative and a methylated cyclodextrin, methyl-beta-cyclodextrin (MbetaCD), with an average degree of substitution of 0.5. Inclusion complex between OME and beta-cyclodextrin (betaCD), a natural cyclodextrin, was used as reference. In aqueous media, apparent stability constants (K(s)), which describe the extent of formation of the complexes, have been determined by UV spectroscopy and 1H NMR experiments. The stoichiometry of the complexes was found to be 1:1 mol:mol OME:cyclodextrin (CD) and the value of K(s) was higher for OME:MbetaCD than for OME:betaCD inclusion complexes. Solid binary systems of OME and CDs were prepared by different techniques, namely kneading, spray-drying and freeze-drying. The formation and physicochemical characterization of solid inclusion complexes were investigated by differential scanning calorimetry (DSC), Fourier transform-infrared (FTIR), X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The results show that freeze-drying method produces true inclusion complexes between OME and both CDs. In contrast, crystalline drug was detectable in kneaded and spray-drying products. The dissolution of OME from the binary systems was studied to select the most appropriate system for the development of a buccal drug delivery formulation. It was concluded that the preparation technique played an important role in the dissolution behaviour of the drug and the inclusion complex between OME and MbetaCD obtained by spray-drying and freeze-drying allowed better performances.     

Development of parenteral formulation for a novel angiogenesis inhibitor, CKD-732 through complexation with hydroxypropyl-beta-cyclodextrin

The effect of hydroxypropyl-beta-cyclodextrin (HP-beta-CyD) on the aqueous solubility and chemical stability of O-(4-Dimethylaminoethoxycinnamoyl)fumagillol (CKD-732), a new angiogenesis inhibitor, was investigated with an aim of preparing a stable and effective parenteral formulation. The CKD-732/HP-beta-CyD inclusion complex was obtained in solid state by freeze-drying and characterized in solution by proton nuclear magnetic resonance (1H NMR). Then, the pharmacokinetic profile in rats and the in vivo tumor growth inhibitory activity in mice following the parenteral administration of aqueous CKD-732/HP-beta-CyD complex were compared to those of CKD-732.hemioxalate solution having an equivalent concentration. The aqueous solubility of CKD-732 was markedly increased by the combination of pH adjustment and HP-beta-CyD complexation through a soluble 1:1 inclusion complex formation, which was supported by NMR spectroscopy. The hydrolysis of CKD-732 following pseudo first-order kinetics was decelerated moderately but significantly in acidic and basic solutions in the presence of HP-beta-CyD. The stability of lyophilized CKD-732/HP-beta-CyD complex was also drastically improved after storage in various conditions. The intravenous pharmacokinetic profile and the subcutaneous in vivo tumor growth inhibitory activity of aqueous CKD-732/HP-beta-CyD complex were not significantly different from those of CKD-732.hemioxalate solution with the favorable reduction of irritation. These results demonstrate that the CKD-732/HP-beta-CyD complex is an attractive formulation for use in the parenteral delivery of CKD-732.     

High-field nuclear magnetic resonance techniques for the investigation of a beta-cyclodextrin:indomethacin inclusion complex

The inclusion complex of indomethacin sodium salt in beta-cyclodextrin has been studied by proton NMR at high magnetic field. The continuous variation technique was used to evidence the formation of a soluble 1:1 complex in aqueous solution at physiological pH. The effective association constant was determined by the Benesi-Hildebrand procedure to be 760 M-1 at 303 K. This technique requires NMR measurements in the presence of a very large excess of one of the complex components and, since both beta-cyclodextrin and the sodium salt of indomethacin are sparingly soluble in water, NMR spectrometers operating at very high magnetic fields were used. Besides the effective association constant, the Benesi-Hildebrand approach allows a precise determination of all NMR parameters of the pure inclusion complex which may be used for a complete analysis of the geometry of this complex in solution.     

Comparative studies of the enhancing effects of cyclodextrins on the solubility and oral bioavailability of tacrolimus in rats

The enhancing effects of cyclodextrins (CyDs) on the solubility, the dissolution rate, and the bioavailability of tacrolimus after oral administration to rats were examined and compared with those after administration of a PROGRAF capsule containing the solid dispersion formulation of tacrolimus. Here we used natural CyDs and the hydrophilic beta-CyD derivatives; that is, randomly methylated-beta-cyclodextrin (RM-beta-CyD), heptakis(2,6-di-O-methyl)-beta-cyclodextrin (DM-beta-CyD), 2-hydroxypropyl-beta-cyclodextrin (HP-beta-CyD), and sulfobutyl ether beta-cyclodextrins (SBE-beta-CyDs). Of the natural CyDs, the solubility of tacrolimus increased in the addition of beta-CyD, indicating that the cavity of beta-CyD comfortably fits the drug. Of the beta-CyD derivatives, DM-beta-CyD had the greatest solubilizing activity and gave the A(p) type phase solubility curve as defined by Higuchi and Connors, suggesting the formation of higher-order complexes. The result of van't Hoff plot suggests that the enthalpy is dominant for the complexation of tacrolimus with DM-beta-CyD. The dissolution rate of tacrolimus was markedly augmented by the complexation with DM-beta-CyD, reflecting its solubilizing activity. An in vivo study revealed that DM-beta-CyD increased the bioavailability of tacrolimus with low variability in the absorption after oral administration of the tacrolimus suspension to rats. The present results suggest that DM-beta-CyD is particularly useful in designing oral preparations of tacrolimus with an enhanced bioavailability and a reduced variability in absorption.     

Study of retinoic acid polymorphism

Some authors recently hypothesized the existence of a new retinoic acid (RA) phase in addition to the two already known polymorphs. We investigated RA polymorphism and our results exclude the presence of new modifications and refine the properties of the known forms. By comparison of simulated and acquired X-Ray Powder Diffraction (XRPD) it was possible to identify only the known monoclinic (I) and the triclinic (II) modifications; the same were also characterized by DSC, IR, and Raman spectroscopy. A solubility study associated to DSC allowed establishing an enantiotropic relationship between the two forms, with form II being less stable (DeltaGII/I=0.71 kJ/mol at 37 degrees C) below the transition temperature (136.6 degrees C; DeltaH=3.2 kJ/mol). The intrinsic dissolution rate (IDR) (I=61 microg/cm2xmin-1; II=125 microg/cm2xmin-1) confirmed this energetic relationship. The kinetics of solid transition I-->II was examined and its activation energy estimated (356 kJ/mol). The attempts to produce new phases allowed the development of methods to obtain the two polymorphs with high chemical and polymorphic purity. A validated DSC method is presented that enables detection of the presence of form I at a level of 1% (w/w) when in mixture with form II.     

Preparation and study the 1:2 inclusion complex of carvedilol with beta-cyclodextrin

An inclusion complex of beta-cyclodextrin with carvedilol was prepared by using a convenient new method of microwave irradiation. Phase-solubility studies demonstrated the ability of beta-cyclodextrins to complex with carvedilol and increase drug solubility. The structure of inclusion complex was determined by fluorescence spectroscopy and 1H NMR, 13C NMR measurements in solution. The solid inclusion was characterised by infrared spectroscopy, differential scanning calorimetry (DSC) and element analysis. These experimental results confirmed the existence of 1:2 inclusion complex of carvedilol with beta-cyclodextrin, the formation constant of complex was determined by the fluorescence method. Molecular modeling predicted the energy-minimized structure of the complex.