Spectroscopic and molecular docking studies of coumarin derivatives with HSA: bilirubin displacement and theranostic potential

Human Serum Albumin (HSA) plays a pivotal role in the transport, distribution, and bioavailability of pharmaceutical agents. This study investigates the interaction between HSA and a coumarin derivative, 4-((4-((benzo[d]oxazol-2-ylthio)methyl)-1H-1,2,3-triazol-1-yl)methyl)-7-methoxy-2H-chromen-2-one (C1), under physiological conditions. Binding characteristics were explored using fluorescence and absorption spectroscopy, along with molecular docking studies. Steady-state results indicate that quenching occurs via a static mechanism due to the formation of a non-fluorescent complex between HSA and C1. However, at elevated temperatures, the complex becomes destabilized, leading to a transition toward dynamic quenching as C1 interacts with the excited state of HSA. Thermodynamic parameters (ΔH, ΔG, and ΔS) confirm spontaneous binding. Circular dichroism (CD) spectroscopy demonstrated only minor changes (2-3%) in the α-helical content of HSA upon ligand binding, confirming preservation of the protein's secondary structure. Molecular docking confirms preferential binding, stabilized through hydrogen bonding and hydrophobic interactions. Importantly, a bilirubin-triggered displacement mechanism is proposed, wherein elevated bilirubin levels under pathological conditions may competitively release the bound coumarin derivative, thereby enhancing targeted therapeutic action. These findings underscore the potential of the coumarin derivative as a theranostic agent, offering biological stability through HSA binding for therapeutic applications while retaining fluorescence for bioimaging. This work advances the understanding of serum protein interactions in theranostic development.