Pyrene excitation via resonance energy transfer from protein tryptophan reveals a fluidity gradient in liver microsomes

Membrane fluidity measurements based on excimer formation of pyrene and pyrene derivatives as a measure of lateral diffusion yield a decreased fluidity in the presence of proteins [1,2]. It was the aim of our study to investigate whether the reduced excimer formation is due to a rigidifying effect of proteins on the whole membrane or if the fluorophore mobility is hindered mainly in the immediate protein environment. Resonance energy transfer in microsomal membranes between intrinsic tryptophan residues and pyrene was used to study the excimer formation rate in the vicinity of proteins. The excimer-to-monomer fluorescence ratio after excitation via resonance energy transfer decreased compared to that observed for the direct excitation. The results suggest that, because of the reduced fluidity in the neighborhood of proteins, pyrene and pyrenedodecanoic acid do not diffuse homogeneously in the membrane plane.