Investigations on the interaction of the phototoxic alkaloid coralyne with serum albumins

The interaction of the phototoxic alkaloid coralyne with bovine and human serum albumins (BSA, HSA) was investigated. Absorbance and fluorescence quenching experiments revealed the formation of strong complexes. Based on the binding parameters calculated from Stern-Volmer quenching method, coralyne has higher affinity to BSA (∼10⁵ M⁻¹) compared to HSA (∼10⁴ M⁻¹). Forster resonance energy transfer studies showed that the specific binding distances between Trp (donor) of the proteins and coralyne (acceptor) were 2.95 and 3.10 nm, respectively. The bindings were favored by negative enthalpy and a stronger favorable entropy contribution. The heat capacity values for binding to BSA and HSA were similar, indicating the involvement of similar molecular forces in the complexation. Competitive binding experiments using site markers demonstrated that coralyne binds to site I (subdomain IIA) of both proteins. The secondary structure of the proteins was altered, suggesting a small but definitive partial unfolding on complexation.