Correlation between dynamics, structure and spectral properties of human α1-acid glycoprotein (orosomucoid): a fluorescence approach

Abstract

Dynamics of proteins and membranes are usually investigated by red-edge excitation spectra and fluorescence anisotropy. In a viscous or rigid medium, the fluorescence maximum position changes with the excitation wavelength upon red-edge excitation. In addition to the shift in the emission maximum on red edge excitation, fluorescence anisotropy is also known to be dependent on the excitation and emission wavelengths in viscous media. However, this dependence has always been explained by the fact that the fluorophore is rigid, i.e. it does not diplay any residual motions. The aim of the present work was to check the validity of this latest assumption and to explain the possible origin of the dependence of the anisotropy on both the excitation and emission wavelengths. Therefore, we compared the results obtained from the fluorescence of the Trp residues of two α1-acid glycoproteins (orosomucoid). One protein was purified by chromatographic methods (orosomucoid c) and the other was obtained with ammonium sulfate precipitation (orosomucoid s). Trp residues of orosomucoid c display free motions while those of orosomucoid s are rigid. The general qualitative feature of the excitation anisotropy spectra recorded on both types of preparation is identical and resembles that obtained for other proteins containing tryptophan residue in protein. The fluorescence anisotropy measured across the emission spectra decreases for both preparations, indicating that this phenomenon is characteristic for fluorophores surrounded by a rigid microenvironment or by a microenvironment that displays motions. The fluorescence anisotropy variation across the emission and the excitation spectra is more important when the fluorophore possesses constrained motions than when it displays a high degree of freedom. Our results clearly demonstrate that the tertiary structure of the protein and the structure and dynamics of the microenvironments of the Trp residues are the origin of the dependence of anisotropy on the excitation and emission wavelengths.