Large-amplitude picosecond anisotropy decay of the intrinsic fluorescence of double-stranded DNA.

The conformational flexibility of the DNA double helix is of great interest because of its potential role in protein recognition, packaging into chromosomes, formation of photodefects, and interaction with drugs. Theory finds that DNA is very flexible; however, there is a scarcity of experimental results that examine intrinsic properties of the DNA bases for the inherent flexibility in solution. We have studied the dynamics of poly(dA).poly(dT) and (dA)20.(dT)20 in a 50 mM cacodylate, 0.1 M NaCl, pH 7 buffer by using the time-correlated picosecond fluorescence anisotropy of thymine selectively excited at 293 nm. For both nucleic acids, a large-amplitude biphasic decrease in the anisotropy is observed that has a very fast, large-amplitude component on the picosecond time scale and a slower, smaller-amplitude component on the nanosecond time scale. These modes are sensitive to sucrose concentration, and are greatly attenuated at 77% sucrose by volume. This observation suggests that motions of the bases make a significant contribution to the observed fluorescence depolarization (in the absence of sucrose). Measurements on the single-stranded systems poly(dT) and (dT)20 reveal a much smaller amplitude of the very fast depolarization mode. These observations are consistent with a mechanism that involves concerted motions in the interior of the double-stranded systems.