Polarity estimate of the hydrophobic binding sites in erythroid spectrin: A study by pyrene fluorescence

The apparent dielectric constant, epsilon, of the hydrophobic pyrene binding sites in erythroid spectrin and human serum albumin (HSA) were estimated using the linear relationship [Turro, N.J., Kuo, P.L., Somasundaran, P. and Wong, K. (1986). J. Phys. Chem. 90, 288-291] between the ratio of the first (373 nm) and the third (384 nm) vibronic peak intensities (1(1)/1(3)) and the dielectric constant of the bulk medium. Binding of the hydrophobic fluorescent probe, pyrene, to erythroid spectrin and HSA was determined from concentration dependent change in the ratio 1(1)/1(3) from the emission spectra. Pyrene binds to spectrin (K-app = 6.2 x 10(6) M-1) with a higher affinity than that of HSA (K-app = 3.7 x 10(4) M-1) and the binding in both cases are saturable. The epsilon for spectrin and HSA was estimated to be 7 +/- 2.1 and 5.4 +/- 1.6 respectively. A case study with spectrin, covalently labeled with pyrene maleimide, have been presented for aging of pyrene-labelled spectrin showing the potential of the use of vibrational peak ratios (1(1)/1(3)) in the study of polarity of microenvironments in the neighborhood of cysteine residues of a protein. Large changes in the pyrene spectral components indicated conformational changes in the cysteine microenvironment of the protein upon storage at 4 degrees C.