Fluorescence quenching studies were performed on a human serum albumin (HSA)-quercetin complex by adding six different divalent metal ions (Cu(II), Ni(II), Mn(II), Cd(II), Zn(II), and Co(II)) to form a tertiary complex. Upon binding to human serum albumin, quercetin fluoresces after excitation at 295 or 450 nm. Two different quercetin moieties in the HSA-quercetin complex were observed to fluoresce, namely QC1 and QC2. The band shape of the QC1 emission peak was relatively sensitive to the nature of the quencher and the temperature. In contrast, the emission band of QC2 was not shifted upon changing the temperature, but was shifted if tryptophan or tyrosine emission were quenched. The divalent metal ions acted as quenchers for both QC1 and QC2 emissions. Results were analyzed using the Stern-Volmer relationship by plotting the relative intensity vs. the quencher concentration. From the Stern-Volmer plots, QC1 and QC2 emission peaks were seen to be quenched by collisional quenching, with a minor contribution due to static quenching in the presence of Cu(II). When Mn(II) was added to the complex, QC1 was quenched by collision only while QC2 was quenched by the combination of both mechanisms, although collisional quenching was the dominant mechanism. Cd(II) and Co(II) acted as quenchers for both QC1 and QC2. Zn(II) enhanced QC1 and QC2. In the presence of Cd(II), a combination of static and collisional quenching was responsible for the quenching of QC1 and QC2. Collisional quenching was the dominant mechanism for QC1. For QC2, static quenching was the dominant mechanism for concentrations equal to and below 30 µM and collisional quenching was the dominant mechanism for concentrations above 30 µM. In the presence of Co(II), QC1 was quenched statically while QC2 was quenched both statically and collisionally, with static being the dominant quenching mechanism. In the case of QC2 quenching by Ni(II), the interpretation of the Stern-Volmer relationship was difficult since changing the temperature did not alter the ratio of the fluorescence intensity in the absence and presence of the quencher (F0/F) at the original temperature. Even though the change was small in the Stern-Volmer plots, it can be stated that the QC2 emission band is possibly due to complex formation with Ni(II).<br/>

Fluorescence quenching studies of the human serum albumin (HSA) - quercetin complex by addition of divalent cations

256th National Meeting (2018)

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