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propane or to give a final ratio of approximately 1:0.5:4. For the generation of 1,1-difluoroethyl radical from 1,1-difluoroethane, bistrifluoromethyl peroxide was used, since di-ferf-butyl peroxide under the same conditions did not afford sufficiently high concentrations of radicals for esr measurements. For the generation of 2-fluoropropyl radical from the photolytic abstraction of hydrogen from 2-fluoropropane, spectra of radicals 801 with good signal to noise ratios could only be observed at temperatures higher than -80°.Acknowledgment. We thank the National Science Foundation for financial support and Drs. P. Meakin and P. J. Krusic for communication of their work prior to publication.

Abstract. Evidence for an internal oscillatory Brownian motion in the DNA helix is obtained from the measurement of the decay of the fluorescence emission anisotropy of the ethidium bromide-DNA complex. The amplitude of the oscillation is found to be equal to 350 and the relaxation time equal to 28 nanoseconds.The dye ethidium bromide has been found to bind specifically to doublestranded nucleic acid helix. The interesting characteristics of this binding are:(1) The fluorescence quantum efficiency of the dye increases considerably on binding to double-stranded nucleic acids." 2 (2) Binding occurs by intercalation.2' Then ethidium bromide is rigidly bound to the double-stranded helix and the direction of the absorption and emission transition moments which are in the plane of the dye have a known direction with reference to the helix axis.2 Then, ethidium bromide is an ideal candidate as a fluorescence label for fluorescence depolarization measurements since the ethidium bromide lifetime of fluorescence is relatively large.4It was first observed5 that the polarization fluorescence coefficient for ethidium bromide bound to DNA was dependent on the viscosity of the solution. In other words, in aqueous solvent a depolarization of fluorescence was observed. Since the DNA is a large molecule, the extent of its rotation during the time of the ethidium bromide excited state can be assumed negligible. Then, two hypotheses had to be postulated, either the DNA was not rigid and was able to undergo some kind of internal motion, or ethidium bromide was not really rigidly bound as predicted by the intercalation model.6 In these conditions it was of interest to further study this problem by measuring the emission anisotropy during the fluorescence decay. This method has been already used with macromolecules bearing covalently bound fluorescent chromophores. Detailed information on the Brownian motion and the internal deformation of these macromolecules have been obtained.7'0 Methods. Let Ill(t), I1(t), be, respectively, the decay of fluorescence intensity of the horizontal and perpendicular components when fluorescence excitation is made by an infinitely short flash of natural light. The time of the flash is taken as the origin.

We study the apparatus response function g (t) which must be known in order to deconvolute the fluorescence decay experiments obtained by the single photon counting method. Due to the flash and the PM characteristics, g (t) depends on the excitation and emission wavelengths. The PM contribution is analyzed qualitatively and found to be in agreement with the properties of the photoelectric effect. Therefore, the accurate determination of the ``true fluorescence decay'' cannot be obtained by the process commonly used, which consists in replacing g (t) by the apparatus response obtained with the PM receiving the light of the exciting pulse. The function g (t) may be obtained in using a reference fluorescent solution with a known fluorescence decay-time.