A Fluorescence Quenching Study of the Human Serum Albumin-Quercetin Complex by Addition of Cd(II), Zn(II), and Co(II)

Fluorescence quenching studies were done on a human serum albumin-quercetin complex by adding three different divalent metal ions (Cd(II), Zn(II), and Co(II)) to form a tertiary complex. When quercetin binds to human serum albumin, two moieties of quercetin, QC1 andQC2, fluoresce after excitation at 295 and 450nm. The band shape of the QC1 emission peak was sensitive to temperature and the nature of the quencher. The band shape of the QC2 emission peak was sensitive to temperature only if tryptophan and tyrosine emissions were quenched. Cd(II) and Co(II) acted as quenchers for both QC1 and QC2. Zn(II) enhanced stern-Volmer plots were used to analyze the mechanisms for quenching 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 mu M and collisional quenching was the dominant mechanism for concentrations above 30 mu M. 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.