Molecular mechanics analysis of Tet repressor TRP-43 fluorescence

A 35% decrease in the fluorescence intensity of F75 TetR Trp-43 was observed upon binding of the tetracycline derivative 5a,6-anhydrotetracycline (AnTc) to the repressor. The fluorescence decay of Trp-43 in F75 TetR and in its complex with AnTc could be described by the sum of three exponential components, with lifetimes of about 6, 3, and 0.3 ns. The amplitudes, however, were markedly altered upon binding. The minimized energy mapping of Trp-43 chi 1 x chi 2 isomerization clearly indicated the existence of three main potential wells at positions (-160 degrees, -90 degrees) (rotamer I), (-170 degrees, 90 degrees) (rotamer II), and (-70, 150 degrees) (rotamer III). Our study of Trp-43 environment for each of the three rotamers suggests that the longest decay component may be assigned to rotamer II, the middle-lived component to rotamer I, and the subnanosecond component to rotamer III. The origin of the changes in the rotamer distribution upon AnTc binding is discussed. Anisotropy decays are also discussed within the framework of the rotamer model.