Excited-state properties of the alternating polynucleotide poly(dA-dT).poly(dA-dT)

Measurements of the steady-state fluorescence spectrum and anisotropy, r, of the alternating polynucleotide poly(dA-dT).poly(dA-dT) were carried out in order to characterize its photophysical properties at room temperature. The shape of the fluorescence spectrum depends on the excitation wavelength, namely, the relative fluorescence intensity of the short-wavelength peak decreases for excitation at short wavelengths. When monitoring the emission at short wavelengths, r is 0.18 and independent of the excitation wavelength. When monitoring the emission at long wavelengths, however, r is very low, about 0.03. These results suggest that: (i) the short-wavelength emission stems from thymine; and (ii) the long-wavelength emission stems from an excited-state complex (excimer), with the same one being formed regardless of whether thymine or adenine is excited. The corresponding fluorescence spectra have been resolved. The occurrence of transfer of electronic energy is discussed.