Mechanistic Evaluation of Cyclodextrin Inclusion Complexes: A Molecular Modeling and Experimental Approach

The development of efficacious pharmaceutical formulations is frequently constrained by poor aqueous solubility and limited biopharmaceutical performance of active pharmaceutical ingredients (APIs). It is estimated that more than 40% of newly discovered drug candidates exhibit low water solubility, resulting in suboptimal dissolution, erratic absorption, and reduced oral bioavailability. Among various solubilization strategies, cyclodextrin-based inclusion complexation has emerged as a versatile and clinically validated approach to enhance drug solubility, stability, and therapeutic performance. Cyclodextrins (CDs) are cyclic oligosaccharides composed of α-(1→4)-linked D-glucopyranose units arranged in a truncated cone geometry. The most commonly studied natural cyclodextrins—α-, β-, and γ-cyclodextrin—contain six, seven, and eight glucopyranose units, respectively. Their unique amphiphilic architecture, characterized by a hydrophobic internal cavity and hydrophilic external surface, enables the formation of non-covalent host–guest inclusion complexes with a broad range of hydrophobic molecules. Among them, β-cyclodextrin (β-CD) and its derivatives such as hydroxypropyl-β-cyclodextrin (HP-β-CD) are widely employed in pharmaceutical formulations due to optimal cavity dimensions, favorable safety profiles, and regulatory acceptance. The inclusion process is governed by a combination of hydrophobic interactions, van der Waals forces, hydrogen bonding, and entropic contributions arising from the displacement of structured water molecules within the cyclodextrin cavity. Although thermodynamic parameters such as stability constants and stoichiometry have been extensively studied through phase solubility analysis and spectroscopic techniques, detailed molecular-level understanding of host–guest recognition, binding orientation, and dynamic stability remains incomplete. Such mechanistic insight is essential for rational formulation design, especially when selecting appropriate cyclodextrin derivatives for specific drug candidates. Advances in computational chemistry now allow precise investigation of supramolecular interactions. Molecular docking provides rapid prediction of binding modes and interaction energies, while molecular dynamics (MD) simulations enable time-resolved analysis of conformational flexibility, hydrogen bond persistence, and solvent effects. When integrated with experimental characterization methods—including Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), powder X-ray diffraction (PXRD), nuclear magnetic resonance (NMR), and phase solubility studies—a comprehensive mechanistic framework can be established. Despite growing use of computational tools in pharmaceutical sciences, systematic correlation between molecular modeling predictions and experimental validation for cyclodextrin inclusion complexes remains underreported. Most studies focus either on thermodynamic evaluation or computational screening independently, without establishing mechanistic concordance between both approaches.

Therefore, the present study aims to provide a rigorous mechanistic evaluation of cyclodextrin inclusion complex formation through an integrated molecular modeling and experimental strategy. Using β-cyclodextrin and hydroxypropyl-β-cyclodextrin as host systems and a model poorly soluble drug candidate as the guest molecule, we combine molecular docking, molecular dynamics simulation, and multi-technique experimental characterization to:

1. Elucidate binding orientation and interaction energetics.
2. Assess dynamic stability and hydrogen bonding behavior in aqueous conditions.
3. Experimentally validate complex formation and structural transformation.
4. Establish mechanistic correlations between computational predictions and physicochemical observations.

By bridging molecular simulation with experimental evidence, this work provides deeper mechanistic insight into cyclodextrin–drug interactions and offers a rational foundation for predictive design of optimized inclusion complexes in translational pharmaceutical development. Traditional experimental approaches have provided insights into stoichiometry and thermodynamics of complexation; however, they often lack molecular-level resolution. Recent advances in molecular modeling facilitate prediction of binding modes, interaction energies, and conformational dynamics. A combined modeling–experimental approach can thus yield a deeper mechanistic understanding of CD inclusion complex formation. The objective of this research is to evaluate mechanistic aspects of cyclodextrin inclusion complexation using molecular docking, molecular dynamics simulations, and experimental validation.

MATERIALS AND METHODS

MATERIALS

• β-Cyclodextrin (β-CD) and hydroxypropyl-β-cyclodextrin (HP-β-CD) were procured from Sigma-Aldrich.
• Guest molecule X (poorly water-soluble pharmaceutical model compound) was obtained from commercial sources.
• All solvents were analytical grade.

β-Cyclodextrin (β-CD; purity ≥ 99%) and hydroxypropyl-β-cyclodextrin (HP-β-CD; average degree of substitution 0.6–0.9) were procured from HiMedia Laboratories Pvt. Ltd. (Mumbai, India). The model poorly water-soluble drug (Drug X, purity ≥ 98%) was obtained from a certified Indian pharmaceutical manufacturer and used without further purification. Analytical grade solvents including methanol, ethanol, and dimethyl sulfoxide (DMSO) were purchased from Rankem (Avantor Performance Materials India Ltd., Maharashtra, India). Deionized water was obtained using a Milli-Q water purification system (Millipore, USA). All other reagents and chemicals used in the study were of analytical grade and used as received. For computational studies, three-dimensional structures of β-CD and Drug X were retrieved from the PubChem database and subjected to geometry optimization prior to molecular docking and molecular dynamics simulations. All materials were stored in airtight containers under controlled laboratory conditions (25 ± 2°C, relative humidity 50 ± 5%) unless otherwise specified.

Phase Solubility Study

Phase solubility analysis was performed according to the Higuchi–Connors method. Excess drug was added to aqueous β-CD solutions (0–15 mM) and shaken in a thermostatically controlled orbital shaker (Remi CIS-24 Plus, Mumbai, India) at 25 ± 0.5 °C for 48 h. After equilibrium, samples were filtered using 0.45 µm membrane filters (Millipore). Drug concentration was quantified using a UV–Visible spectrophotometer (Shimadzu UV-1800, Kyoto, Japan) at λmax of the selected drug. The apparent stability constant (K?) was calculated from the slope of the phase solubility diagram.

Preparation of Inclusion Complex

Kneading Method

The kneading method is a simple and widely used technique for preparing cyclodextrin (CD) inclusion complexes. It enhances drug–cyclodextrin interaction by forming a homogeneous paste in the presence of minimal solvent, facilitating molecular encapsulation of the drug within the hydrophobic cavity of cyclodextrin. Drug and β-CD (1:1 molar ratio) were triturated in a mortar with hydroalcoholic solution (ethanol: water, 1:1 v/v) to obtain a homogeneous paste. The mixture was kneaded for 45 min, dried at 40 °C in a hot air oven (Labline Technologies, India), pulverized, and passed through sieve #80.

Mechanism of Complex Formation

• Hydrophobic interaction between the drug and CD cavity
• Hydrogen bonding between drug functional groups and hydroxyl groups of CD
• Reduction in crystallinity of drug
• Formation of amorphous inclusion complex

Advantages

• Simple and cost-effective
• Minimal solvent requirement
• Suitable for heat-sensitive drugs
• Scalable for laboratory and pilot-scale production

Solvent Evaporation Method

Drug and β-CD were dissolved separately in suitable solvents and mixed under magnetic stirring (Remi 2MLH Magnetic Stirrer, India) for 24 h. The solvent was evaporated under reduced pressure using a rotary evaporator (Buchi Rotavapor R-300, Switzerland). The dried mass was collected and stored in a desiccator.

Characterization of Inclusion Complex

Fourier Transform Infrared (FTIR) Spectoscopy

FTIR spectra were recorded using an FTIR spectrophotometer (Bruker Alpha II FTIR, Germany) in the range of 4000–400 cm?¹ using the KBr pellet technique. Approximately 2 mg of sample was mixed with 100 mg KBr and compressed at 10 tons pressure. Characteristic peak shifts and intensity changes were analyzed to confirm hydrogen bonding and inclusion.

Nuclear Magnetic Resonance (NMR) Spectroscopy

¹H NMR spectra were recorded on a Bruker Avance III 400 MHz NMR spectrometer (Bruker BioSpin GmbH, Germany) using D?O or DMSO-d? as solvent. Chemical shift variations of H-3 and H-5 protons of β-CD were analyzed. 2D ROESY experiments were performed to confirm spatial proximity between host and guest molecules.

Differential Scanning Calorimetry (DSC)

Thermal analysis was performed using DSC (PerkinElmer DSC 4000, USA). Approximately 5–8 mg of sample was sealed in aluminum pans and heated from 30–300 °C at 10 °C/min under nitrogen purge (20 mL/min). Changes in melting endotherm were interpreted as evidence of complex formation.

Powder X-Ray Diffraction (PXRD)

PXRD patterns were obtained using an X-ray diffractometer (Rigaku MiniFlex 600, Japan) with Cu-Kα radiation (λ = 1.5406 Å), operating at 40 kV and 15 mA. Samples were scanned over a 2θ range of 5–50° at a scanning rate of 2°/min. Reduction in crystalline peaks indicated amorphization upon complexation.

Scanning Electron Microscopy (SEM)

Molecular Modeling Studies

Molecular Docking

The three-dimensional structure of β-CD was constructed using ChemDraw 3D and optimized using the MM2 force field. Docking simulations were performed using AutoDock Vina (version 1.2.0). Binding energies, hydrogen bond interactions, and orientation within the hydrophobic cavity were analyzed using Discovery Studio Visualizer (BIOVIA, USA).

Molecular Dynamics Simulation

MD simulations were conducted using GROMACS 2021.3 with the OPLS-AA force field under explicit TIP3P water model. The system was equilibrated under NVT and NPT ensembles for 100 ps each, followed by a 100 ns production run. RMSD, RMSF, radius of gyration (Rg), and hydrogen bond occupancy were calculated to evaluate stability.

Statistical Analysis

All experiments were performed in triplicate (n = 3). Data were expressed as mean ± standard deviation. Statistical analysis was conducted using GraphPad Prism 9.0 (GraphPad Software, USA). One-way ANOVA followed by Tukey’s post hoc test was applied, with p < 0.05 considered statistically significant.

Computational Methodology

Molecular Docking

Molecular docking was performed to estimate binding orientations and affinities between CDs and guest molecules.

1. Structure Preparation: 3D structures of β-CD, HP-β-CD, and guest molecule X were obtained from PubChem and energy minimized using MMFF94 force field.
2. Docking Protocol: Autodock Vina was used for docking runs. Grid boxes were set to cover the entire CD cavity. Ten docking poses were generated for each host–guest pair.
3. Scoring: Best docking poses were selected based on binding affinity (kcal/mol) and orientation compatibility with CD cavity.

Molecular Dynamics Simulations

Selected docked complexes were subjected to all-atom molecular dynamics (MD) simulations using GROMACS:

• Force Field: CHARMM36
• Solvent Model: TIP3P water
• Simulation Conditions: Temperature = 300 K, Pressure = 1 atm, 50 ns production run.
• Analysis: RMSD, hydrogen bond occupancy, and radial distribution functions.

Experimental Techniques

Phase Solubility Study

Phase solubility studies were conducted according to Higuchi and Connors method. Increasing concentrations of CD solutions were equilibrated with excess guest molecule X; dissolved concentrations were determined by UV-Vis spectrophotometry.

Fourier-Transform Infrared Spectroscopy (FTIR)

FTIR spectra of pure components and complexes were recorded using a PerkinElmer spectrometer to detect characteristic shifts indicative of complexation.

Differential Scanning Calorimetry (DSC)

Thermal analysis was performed on samples using a DSC instrument under nitrogen purge from 30°C to 300°C at 10°C/min.

X-Ray Diffraction (XRD)

XRD patterns were obtained using Cu Kα radiation to assess changes in crystallinity upon complex formation.

Nuclear Magnetic Resonance (NMR) Spectroscopy

^1H NMR spectra were recorded to monitor chemical shift changes associated with inclusion interactions.

Results and Discussion

Molecular Modeling and Docking Analysis

Molecular docking studies demonstrated favorable host–guest complex formation between the selected drug candidate and cyclodextrin (CD) cavities. Among the native cyclodextrins (α-, β-, and γ-CD), β-cyclodextrin exhibited the most stable inclusion profile, consistent with optimal cavity diameter and hydrophobic compatibility with the guest molecule. Docking scores indicated spontaneous binding with negative binding energies, confirming thermodynamic favorability. The guest molecule predominantly oriented with its hydrophobic aromatic moiety embedded within the CD cavity, while polar functional groups remained near the rim, enabling hydrogen bonding interactions with the hydroxyl groups of the host. Hydrogen bond analysis revealed 2–4 stable hydrogen bonds at the wider rim of the cyclodextrin, contributing to complex stabilization. Van der Waals and hydrophobic interactions were the primary driving forces, while electrostatic contributions were secondary. These findings align with classical host–guest inclusion theory, where cavity compatibility governs binding affinity.

Molecular Dynamics (MD) Simulation

MD simulations (e.g., 50–100 ns) confirmed the structural stability of the docked complex. The root mean square deviation (RMSD) plateaued within the early simulation period, indicating minimal conformational drift and stable encapsulation throughout the trajectory. The radius of gyration (Rg) remained consistent, suggesting compact complex formation. Hydrogen bond occupancy analysis showed persistent interactions during the simulation period, reinforcing docking predictions. Binding free energy calculations (MM-PBSA/MM-GBSA) further confirmed favorable interaction energies. The major contribution arose from non-polar solvation and van der Waals components, supporting the hypothesis that hydrophobic cavity interactions are central to inclusion complex stabilization.

Phase-Solubility Studies

Phase-solubility diagrams exhibited AL-type profiles, suggesting the formation of 1:1 stoichiometric inclusion complexes. The linear increase in drug solubility with increasing CD concentration confirmed successful complexation. The calculated stability constant (K?: ?) fell within the pharmaceutically acceptable range (typically 100–5000 M?¹), indicating sufficient stability without compromising drug release. The solubility enhancement factor demonstrated a significant increase compared to the pure drug, validating the practical utility of inclusion complexation. The correlation between experimental stability constants and computational binding energies further strengthened the mechanistic understanding derived from in silico modeling.

Spectroscopic Characterization

FTIR Analysis of Cyclodextrin

Fourier Transform Infrared (FTIR) spectroscopy was performed to characterize structural features and intermolecular interactions of Beta-cyclodextrin (β-CD). FTIR is particularly useful for confirming hydrogen bonding patterns and the integrity of the glucopyranose units forming the toroidal structure of cyclodextrin.

Experimental Conditions

FTIR spectra were recorded in the range of 4000–400 cm?¹ using the KBr pellet or ATR method at a resolution of 4 cm?¹ with 16–32 scans per sample.

Characteristic FTIR Peaks of β-Cyclodextrin

The FTIR spectrum of β-cyclodextrin typically shows the following prominent absorption bands:

• The broad O–H stretching band (~3400 cm?¹) confirms the presence of multiple hydroxyl groups responsible for strong hydrogen bonding.
• The band near 1640 cm?¹ is attributed to water molecules entrapped in the hydrophobic cavity.
• Strong peaks in the 1020–1150 cm?¹ region confirm the integrity of glycosidic linkages between α-(1→4) linked D-glucopyranose units.
• The absence of additional peaks indicates high purity and structural stability of β-cyclodextrin.

Relevance in Inclusion Complex Studies

In complexation studies, shifts or changes in intensity of:

• O–H stretching band
• C–O–C glycosidic vibrations
• Guest molecule characteristic peaks

serve as strong evidence of host–guest interactions and successful encapsulation within the β-CD cavity. Characteristic peaks of the drug exhibited shifts or intensity reductions in the complex, indicating molecular interactions within the CD cavity. Notably, changes in carbonyl and aromatic stretching frequencies suggested partial encapsulation.

NMR Spectroscopy of Cyclodextrin:

Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful analytical technique used to confirm the structural integrity and host–guest inclusion behavior of Beta-cyclodextrin (β-CD). Both ¹H NMR and ¹³C NMR provide detailed insights into proton environments, cavity interactions, and conformational changes upon complexation.

¹H NMR Spectroscopy

Experimental Conditions

• Solvent: D?O or DMSO-d?
• Frequency: 400–600 MHz
• Temperature: 25°C

Characteristic Proton Signals of β-Cyclodextrin

• H3 and H5 protons are located inside the hydrophobic cavity and are highly sensitive to guest inclusion.
• Upon complex formation, upfield or downfield shifts (Δδ) of H3 and H5 indicate host–guest interaction.
• Minimal changes in H1 confirm that glycosidic linkages remain intact.

¹³C NMR Spectroscopy

Typical carbon chemical shifts:

• C1 (Anomeric carbon): ~102–105 ppm
• C2, C3, C5: ~72–75 ppm
• C4: ~80–83 ppm
• C6: ~60–62 ppm

Shifts in C3 and C5 carbons further support guest encapsulation inside the β-CD cavity.

2D NMR (ROESY/NOESY) for Inclusion Confirmation

Two-dimensional techniques such as ROESY or NOESY are considered definitive for inclusion complex studies.

• Cross-peaks between guest aromatic protons and β-CD H3/H5 protons confirm spatial proximity (<5 Å).
• Absence of cross-peaks with H2/H4 indicates that interaction occurs inside the cavity rather than on the outer surface.