Fluorescence depolarization and temperature dependence of the torsion elastic constant of linear .vphi.29 deoxyribonucleic acid

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Abstract
The torsion elastic constant .alpha. of linear .vphi.29 DNA was determined as a function of temperature from 10.degree.-78.degree. C by studying the decay of the fluorescence polarization anisotropy (FPA) of intercalated ethidium dye. The time-dependent FPA was measured by using a picosecond dye laser for excitation and time-correlated single photon counting detection. Over the region 10.degree.-74.degree. C, .alpha. was effectively constant within experimental error, varying from (3.5 .+-. 0.4) .times. 10-12 dyn cm at 10.degree. C to (3.7 .+-. 0.3) .times. 10-12 dyn cm at 74.degree. C. At 78.degree. C, which is just above the melting temperature (Tm) = 76.degree. C, .alpha. decreased to (3.3 .+-. 0.3) .times. 10-12 dyn cm and at 90.degree. C, where the DNA is completely denatured, the fluorescence lifetime and the decay time of the FPA are characteristic of unbound ethidium bromide. The weak temperature dependence of .alpha. implies that DNA torsional deformations do not occur primarily at sites of high enthalpy perturbed structures such as open base pairs.