Dynamics of helical worm-like chains. VI. Fluorescence depolarization

Abstract

Fluorescence depolarization of both flexible and stiff chain polymers in dilute solution is studied on the basis of the discrete helical worm-like chain such that a fluorescent probe is attached rigidly or with a rotational degree of freedom to one of the subbodies composing the chain. The emission anisotropy r(t) and the average anisotropy r̄ are formulated, considering in general the difference between the directions of absorption and emission dipoles. For flexible chains, proper account is also taken of the difference between the subbodies with and without the probe in size, and for stiff chains, e.g., DNA, effects of wobbling of the probe are considered. If the absorption and emission dipole moment vectors are parallel to each other, r(t) is equal to the magnetic autocorrelation function multiplied by a factor with the internuclear vector being regarded as the emission vector. Naturally, therefore, the results of analysis of experimental data for the present and magnetic cases are then consistent with each other. For flexible chains, the present theory can well explain the dependences of r(t) on the direction of the emission dipole and on the location of the probe, and there is also correlation between the static stiffness parameter λ−1 and the dynamic stiffness as defined as the ratio of a properly defined fluorescence correlation time τF to that of the isolated subbody. For DNA, there is rather good agreement between the estimates of the torsional constant from dynamic properties (magnetic relaxation and fluorescence depolarization) and from equilibrium properties (cyclization and topoisomer distribution).