DNA torsional dynamics by multifrequency phase fluorometry

The time decay of the fluorescence polarization anisotropy of calf thymus DNA‐ethidium complexes is obtained from measurements with sine‐modulated excitation employing the so‐called multifrequency phase fluorometry. A torsional dynamics model developed by J. M. Schurr [(1984) Chemical Physics , Vol. 84 , pp. 71–96] and translated into the frequency domain is found here to describe accurately DNA‐ethidium fluorescence data collected under modulated excitation. At a low dye/DNA ratio (1 : 400) the value of the DNA torsional constant (α = 4.63 ± 0.2 10 −12 dyne cm) fitting the data agrees very well with the known values of α. When the measurements are extended to a higher ethidium/DNA ratio, energy transfer effects between intercalated dyes are observed. A theoretical prediction of the donor and acceptor dye contributions to the fluorescence polarization anisotropy is made here, taking into account also dye‐dye distance distributions. © 1992 John Wiley & Sons, Inc.