Motions of tryptophan residues in asialylated human alpha(1)-acid glycoprotein

The fluorescence of the tryptophan residues of asialylated human alpha(1)-acid glycoprotein (orosomucoid) was investigated. Red excitation spectra did not lead to a shift of the fluorescence emission maximum of the fluorophore, i.e., motions of the Trp residues depend on their microenvironment. This result was confirmed by anisotropy studies as a function of temperature in the range of 7 to 35 degrees C (Perrin plot). In order to determine the frictional resistance to the local rotations of the tryptophan residues, the protein was dissolved in a mixture of 80% glycerol buffer, and the fluorescence anisotropy was measured in the temperature range of -45 to +20 degrees C. The Y-plot analysis of the anisotropy indicated that the mean motion of the two Trp residues buried in the protein core was different from that of the Trp residue of the surface. The average angles of rotations for buried and surface residues were 10 and 14 degrees C of are, respectively.