We have used triplet anisotropy decay techniques to measure the internal flexibility and overall rotational motion of DNA, covering a time range from 15 ns to 200 Is.Nearly monodisperse DNA fragments 65-600 base pairs long were studied by using the intercalating dye methylene blue as a triplet probe. We found that the slow end-over-end tumbling of short DNA fragments (s165 base pairs) is as predicted for a rigid rod.As expected, a longer DNA fragment (600 base pairs) experiences slow segmental motion of its helix axis. We found that, at the earliest times, anisotropy decays more rapidly than expected for a rigid rod, suggesting that, when bound, methylene blue monitors fast internal motion of the helix. Since the rod-like end-over-end tumbling of short fragments rules out fast bending motions, we conclude that the fast components of DNA anisotropy decay are due to twisting motion of the helix, occurring with a time constant near 50 ns.Under physiological conditions, B-form DNA is an exceedingly stable structure. Recently, however, evidence has accumulated suggesting that the structure ofthe helix fluctuates substantially (1)(2)(3)(4)(5)(6)(7)(8). It has been shown by NMR techniques (1-4) that the helix experiences large fast motions in the nanosecond time range. Fast motions in the 1-to 100-ns time range have also been seen by fluorescence anisotropy methods (5-7), using as a probe ethidium bromide intercalated in DNA, and by measuring the ESR line shapes of spin-labeled intercalated dyes (8).Here we describe triplet anisotropy decay measurements of DNA internal motions. As in fluorescence anisotropy measurements, we have used an intercalating dye as a probe; however, we have monitored the anisotropy decay of the triplet rather than the singlet state by using methylene blue, a dye with a high triplet yield. Since the triplet lifetime of methylene blue is =100 pus, we can use triplet anisotropy decay techniques to measure motions over a time scale 1,000 times larger than fluorescence methods allow.MATERIALS AND METHODS In time-resolved triplet anisotropy measurements, chromophores bound rigidly to macromolecules are excited by a short pulse of linearly polarized light. For dyes with high triplet yields, a population of oriented triplet-state molecules results, oriented here meaning that the angular distribution of excited dipole moments is not spherically isotropic. Via rotational Brownian motion, the angular distribution will randomize in time, and the time dependence of the return to a spherically isotropic distribution of dipoles can be monitored either by observing the polarization of the triplet-singlet emission (phosphorescence) or by measuring the absorbance anisotropy associated with the triplet state.In the triplet-state anisotropy decay experiments described here, we have monitored the absorbance anisotropy of the depleted singlet state rather than the triplet state directly. The two techniques are interchangeable; however, singlet-state measurements are often more sensitive because of the high...

Yttrium iron garnet (YIG, Y3Fe5O12) was examined up to 74 GPa and 1800 K using synchrotron x-ray diffraction in a diamond anvil cell. At room temperature, YIG remained in the garnet phase until abrupt amorphization occurred at 51 GPa, consistent with earlier studies. Upon laser heating up to 1800 K, the material transformed to a single-phase orthorhombic GdFeO3-type perovskite of composition (Y(0.75)Fe(0.25))FeO3. No evidence of decomposition of the sample was observed. Both the room-temperature amorphization and high-temperature transformation to the perovskite structure are consistent with the behaviour of other rare earth oxide garnets. The perovskite sample was compressed between 28-74 GPa with annealing to 1450-1650 K every 3-5 GPa. Between 46 and 50 GPa, a 6.8% volume discontinuity was observed without any accompanying change in the number or intensity of diffraction peaks. This is indicative of a high-spin to low-spin electronic transition in Fe(3+), likely in the octahedrally coordinated B-site of the perovskite. The volume change of the inferred spin transition is consistent with those observed in other rare earth ferric iron perovskites at high pressures.