Fluorescence of the DNA double helix (dA)20 x (dT)20 studied by femtosecond spectroscopy--effect of the duplex size on the properties of the excited states.

The fluorescence of the DNA double-stranded oligomer (dA)(20)center dot(dT)(20) is studied at room temperature by fluorescence up-conversion at times shorter than 10 ps. The profile of the up-conversion spectra is similar to that of the steady-state fluorescence spectrum, showing that the majority of the photons are emitted within the probed time scale. At all the probed wavelengths, the fluorescence decays are slower than those of the monomeric chromophores dAMP and TMP. The fluorescence anisotropy decays show strong wavelength dependence. These data allow us to conclude that energy transfer takes place in this double helix and that this process involves exciton states. The spectral and dynamical properties of the oligomer are compared to those of the polymer poly(dA)center dot poly(dT), composed of about 2000 base pairs, reported previously. The oligomer absorption spectrum is characterized by a smaller hypsochromic shift and weaker hypochromism compared to the polymer. Moreover, the fluorescence decays of (dA)(20)center dot(dT)(20) are twice as fast as those of poly(dA)center dot poly(dT), and its fluorescence anisotropy decays more slowly. These differences are the fingerprints of a larger delocalization of the excited states induced by an increase in the size of the duplex.