Metal Ions Enhance the Affinities of Flavonoids for Human Serum Albumin in Vitro

The affinities of apigenin, chrysin, daidzein and quercetin for human serum albumin (HSA) were studied in the presence and absence of Pb2+, Ni2+, Zn2+, Mg2+ and Mn2+. The fluorescence intensities of HSA decrease remarkably with increasing concentration of these four flavonoids. Adding apigenin and chrysin resulted in blue-shifts of HSA from lambda (em)=336 to 332 nm and 330 nm, respectively. However, quercetin showed an obvious red-shift of HSA from lambda (em)=336 to 347 nm whereas daidzein hardly affected the lambda (em) of HSA. The lambda (em) shifts induced by flavonoids in the presence of mental ions were much bigger than those in the absence of these ions. Pb2+, Ni2+, Zn2+, Mg2+ and Mn2+ increased the quenching constants of these four flavonoids for HSA by 19.2 % to 43 %, 47.7 % to 117 %, 23.3 % to 64.4 %, 9.29 % to 42.2 % and 18 % to 55.6 %, respectively. The affinities of apigenin, chrysin, daidzein and quercetin for HSA increased about 9.49 %, 3.63 %, 5.73 % and 2.32 %, respectively, in the presence of Pb2+. Ni2+ improved the affinities of apigenin, chrysin, daidzein and quercetin for HSA by about 4.79 %, 0.85 %, 11.91 % and 10.55 %, respectively. Zn2+ enhanced the affinities of apigenin, chrysin, daidzein and quercetin for HSA by about 1.03 %, 1.34 %, 1.96 % and 13.14 %, respectively. Mg2+ increased the affinities of apigenin, chrysin, daidzein and quercetin for HSA by about 2.03 %, 0.7 %, 1.39 % and 2.07 %, respectively. Mn2+ increased the affinities of apigenin, chrysin, daidzein and quercetin for HSA by about 2.46 %, 6.71 %, 12.3 % and 4.10 %, respectively.