DNA torsional dynamics by multifrequency phase fluorometry

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Abstract
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.