Host-guest complexation of curcumin and coumarin 6 with PAMAM-OH: Insight from fluorescence spectroscopy and molecular dynamics simulations

The 1:1 host-guest association between hydroxyl-terminated poly(amido amine) dendrimers of the fourth generation (G4-PAMAM-OH) and two model fluorescent dyes, curcumin (CUR) and coumarin 6 (CU6), was studied from fluorescence intensity experiments and fully atomistic molecular dynamics (MD) simulations. Our results account for experimental binding constants of 3.45 × 10⁵ and 2.97 × 10⁵ mol⁻¹L for CUR-G4-PAMAM-OH and CU6-G4-PAMAM-OH systems, respectively. The potential of dendrimer complexation to increase dye photostability was evaluated from fluorescence intensity experiments in 5–100 μmol L⁻¹ dye solutions containing 100 μmol L⁻¹ of G4-PAMAM-OH under visible light irradiation. Our results revealed that G4-PAMAM-OH enhances the photostability of CUR at 1:1, 1:2 and 1:20 dye-dendrimer ratios, whereas CU6 is effectively protected only at 1:20 dendrimer excess conditions. MD simulations predicted a deep encapsulation of CUR within innermost dendrimer branches and an outer association of CU6 to surface dendrimer cavities, which can be related to the enhanced photoprotection of CUR at lower dye-dendrimer ratios. MM/GBSA binding free energy estimates suggest the predominance of van der Waals interactions between apolar dye segments and hydrophobic dendrimer pockets as the main binding forces driving dye-dendrimer supramolecular association in the systems under study.