Rose Bengal (RB) is a xanthene dye used as a sensitizer to convert triplet (³O₂) to singlet oxygen (¹O₂). This photophysical property makes it one of the most used dyes in photodynamic therapy. Thus, understanding its interaction with biomacromolecules can provide helpful information about its mode of action and application. The protein chosen for this study was human serum albumin (HSA), which has nine binding sites for fatty acids (FA), and at least three sites for interactions of drugs (DS). The complexation of HSA with RB caused a maximum bathochromic shift in its absorption. From this displacement and the application of the Benesi–Hildebrand model, the ligand–protein association constant (3.90 ± 0.08 × 10⁵ M⁻¹) was obtained. Applying the Job’s Plot method resulted in a 6:1 (ligand-protein) stoichiometry. The determination of preferred binding sites was performed by measuring the association constant in the presence of drugs for which their binding sites in HSA are already well established, such as warfarin (DS1), ibuprofen (DS2 and FA6), digitoxin (DS3), diazepam (DS2), and diflunisal (DS2 and FA6). From these studies, it was found that RB is able to bind at DS1, DS3, and FA6 sites but not at DS2. Subsequently, molecular docking studies using the 2BX8 and 2BXE crystallographic structures were performed and corroborated the experimental results. The lowest energy poses were −52.13, −58.79, and −67.55 kcal mol⁻¹ at DS1, DS3, and FA6, respectively. Conversely, DS2 was the lower affinity binding site. In conclusion, HSA has a high affinity for RB, being able to bind up to six dye molecules.

Coumarins are known for interacting with proteins and exhibiting diverse biological activities. This study investigates the interaction between coumarin-343 (C343) and human (HSA) and bovine (BSA) serum albumins. Fluorescence spectroscopy and theoretical simulations, including density functional theory (DFT) and molecular docking, were used to analyze the ligand–protein complex formation. The fluorescence quenching data revealed that C343 binds to both proteins, with binding constants of 2.1 × 10⁵ mol·L⁻¹ (HSA) and 6.5 × 10⁵ mol·L⁻¹ (BSA), following a 1:1 stoichiometry. Binding site markers identified drug site I (DS1), located in subdomain IIA, as the preferential binding region for both proteins. Computational results supported these findings, showing high affinity for DS1, with binding energies of −69.02 kcal·mol⁻¹ (HSA) and −67.22 kcal·mol⁻¹ (BSA). While complex formation was confirmed for both proteins, differences emerged in the induced circular dichroism (ICD) signals. HSA displayed a distinct ICD profile compared to BSA in both intensity and absorption maximum. Molecular Docking revealed that the C343 conformation differed between HSA and BSA, explaining the variation in ICD signals. These results highlight the importance of protein structure in modulating ligand interactions and spectral responses.