Binding of sulforhodamine B to human serum albumin: A spectroscopic study

• Increase in fluorescence is observed for sulforhodamine B (SRB) after binding to human serum albumin (HSA).
• The binding of SRB to HSA proceeds with 1:1 complexation and binding constant of 1.6 × 105 M–1.
• The spectroscopic studies imply that the SRB binds to Sudlow site I of HSA with hydrophobic interaction.

Sulforhodamine B (SRB) is a widely used fluorescent dye in biological sciences including in vivo studies. In this study, the binding interaction between SRB and human serum albumin (HSA) was investigated by fluorescence and circular dichroism spectroscopy. Complexation with HSA leads to an increase in dye fluorescence, from which the binding constant (1.6 × 105 M−1 at 25 °C), binding stoichiometry (1:1), and thermodynamic parameters of the SRB–HSA interaction (ΔH = 5.2 kJ/mol, ΔS = 116.5 J/mol K) were obtained. The large positive ΔS and the small positive ΔH indicate an entropy-driven binding process, suggesting that SRB binds to Sudlow site I of HSA by a dominant hydrophobic interaction. Circular dichroism revealed some degree of conformational changes in HSA upon binding.

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