Unraveling the binding mechanism and enhanced performance of indigo/β-cyclodextrin inclusion complex: An integrated experimental and computational study.

Indigo (IN) has significant value in dyeing and potential pharmacological applications, but its poor water solubility severely limits bioavailability and processing efficiency. This study investigated the formation, structure, and binding mechanism of inclusion complexes (ICs) between IN and β-cyclodextrin (β-CD) through integrated experimental and computational methods. Phase solubility studies confirmed stable 1:1 stoichiometric ICs, with apparent stability constants (Ks) of 586.73 M-1 and 477.52 M-1 for complexes with two indigo derivatives (IN1 and IN2), respectively, significantly enhancing aqueous solubility. Characterization by SEM, XRD, and FT-IR provided direct evidence of successful encapsulation and new solid phase formation. In dyeing applications, the ICs enabled a 25% reduction in reducing agent (sodium metabisulfite) dosage while improving color depth (K/S value) on cotton fabric. Molecular docking revealed the optimal binding conformation, with IN's hydrophobic moiety encapsulated in the β-CD cavity and a docking binding affinity of -6.1 kcal/mol. The Independent Gradient Model based on Hirshfeld partition (IGMH) analysis identified van der Waals interactions and a specific hydrogen bond as key stabilizing forces. This work elucidates the host-guest interaction mechanism and demonstrates β-CD complexation as an effective strategy for overcoming indigo's solubility limitations, promoting more efficient and sustainable applications.