Anisotropy decays tumbling

The results from fitting the anisotropy decay support the above conclusions. Wells and Lakowicz(200) resolved two exponential components in the anisotropy decay. They obtained ro = 0.11, t r = 0.3 ns, rj = 0.15, and t = 18.5 ns for the sample with no added Mg2+, and ro = 0.05, t R = 0.4 ns, r J = 0.17, and t"R = 17.4 ns for a sample with 10 mM Mg2+. Here r 0 and r o are the amplitudes of the fast and slow components. The longer rotational relaxation time corresponds to overall tumbling of the tRNA, although its amplitude is reduced by much more rapid local motions. The shorter relaxation time corresponds directly to a rapid local motion. Upon addition of Mg2+, the relative amplitude of die rapid local motion decreases, while that of the overall tumbling increases. This implies that the wyebutine base is held in a more rigid or constrained state, such as a 3 stack, in the presence of Mg2+. In that state, the amplitude of local angular motion is substantially diminished in comparison with that in the alternate state that prevails in the absence of Mg2+. As noted before, the exact nature of these conformation(s) is unresolved. 

The time dependence of the anisotropy r(t) depends on the underlying dynamics of reorientational motion. For rotational diffusion (tumbling) of a spherical object, the expected anisotropy decay is exponential with a rotational diffusion time given in the hydrodynamic limit by the Stokes-Einstein-Debye equation. For nonspherical molecules, more complex time dependence may be detected. (For more on these topics, see the book by Cantor and Schimmel in Further Reading.)... 

Interesting applications of anisotropy decays for proteins often develop not from tumbling of the protein as a whole, but from other reorientational degrees of freedom. These motions may include protein domain motions or segmental motions in proteins and peptides. The anisotropy decay in this case is non-single-exponential (see Fig. 4c) and takes the form ... 

For example, 2-aminopurine (AP) has been used to probe the dynamics of mismatches in DNA [340]. AP can be excited at 320 nm, where the normal DNA bases do not absorb (much), and emits at 380 nm (Fig. 4.36). Time-resolved anisotropy decays of AP across from all four natural DNA bases were performed (Fig. 4.37). AP can hydrogen-bond to T nearly as well as the natural A. The data were fitted to sums of two exponential terms the long time corresponded to overall tumbling and the short time to local motions within the DNA base stack. It was found that the internal correlation time corresponding to local motions, at 4°C,... 

Quenching studies of protein fluorescence provide answers regarding the accessibility of certain internal or external groups to quencher molecules. Another application concerns the study of associative behavior and properties of proteins and membranes. The rationale is that the fluorescence transition is polarized and this polarization can be exploited in time-resolved analysis and interpreted in terms of the rotation or tumbling motion which in turn is determined by the viscosity and structure of the environment of the fluorescing group. In particular, anisotropy decay studies have yielded a great deal of information on the mobility of natural and artificial membranes and/or the dynamics of proteins as well as small molecules in membranes. For such studies fluorescence lifetime labels that can be attached to proteins or that dissolve in membranes have... 

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