Analysis of time-resolved fluorescence anisotropy in lipid-protein systems

The subnanosecond fluorescence and motional dynamics of the tryptophan residue in the bacteriophage M13 coat protein incorporated within pure dioleoylphosphatidylcholine (DOPC) as well as dioleoylphosphatidylcholine/dioleoylphosphatidylglycerol (DOPC/DOPG) and dimyristoylphosphatidylcholine/dimyristoylphosphatidylglycerol (DMPC/DMPG) bilayers (80/20 w/w) with various L/P ratio have been investigated. The fluorescence decay is decomposed into four components with lifetimes of about 0.5, 2.0, 4.5 and 10.0 ns, respectively. In pure DOPC and DOPC/DOPG lipid bilayers, above the phase transition temperature, the rotational diffusion of the protein molecules contributes to the depolarization and the anisotropy of tryptophan is fitted to a dual exponential function. The longer correlation time, describing the rotational diffusion of the whole protein, shortens with increasing temperature and decreasing protein aggregation number. In DMPC/DMPG lipid bilayers, below the phase transition, the rotational diffusion of the protein is slowed down such that the subnanosecond anisotropy decay of tryptophan in this system reflects only the segmental motion of the tryptophan residue. Because of a heterogeneous microenvironment, the anisotropy decay must be described by three exponentials with a constant term, containing a negative coefficient and a negative decay time constant. From such a decay, the tryptophan residue within the aggregate undergoes a more restricted motion than the one exposed to the lipids. At 20 degrees C, the order parameter of the transition moment of the isolated tryptophan is about 0.9 and that for the exposed one is about 0.5.