Spectroscopic studies of the interaction between hypocrellin B and human serum albumin Original Research Article

Previous work has proved that hypocrellin B (HB) binds to human serum albumin (HSA) at a specific site instead of distributed randomly on the surface of a protein. In the current work, further investigation by using bilirubin as a site I marker indicates that HB can compete for the same site with bilirubin, suggesting that the HB binding site is located at sub-domain IIA (site I) of HSA. Moreover, bound to HSA, the HB fluorescence was found to be pH sensitive in physiological range (pH 6.0–8.0). The increasing of binding constant of HB to HSA in the pH range 6–8 also indicates that the N ↔ B transition modulates the microenvironment changes of the binding site and influences considerably the binding between HB and HSA. Furthermore, picosecond time-resolved fluorescence spectra of HB–HSA complex in PBS indicate an additional short-lived component compared to that for HB in benzene, which may be assigned to the process of electron transfer from Trp-214 to HB.