EXCITED-STATE PROPERTIES OF DNA METHYLATED AT THE NV7 POSITION OF GUANINE AND ITS FREE FLUOROPHORE AT ROOM TEMPERATURE

The room-temperature optical properties of calf thymus DNA, with about 75% of its guanine residues methylated at position N-7, are compared with those of 7-methyl GMP which has the same fluorophore. The fluorescence spectrum of the methylated guanine residues depends strongly on the excitation wavelength, shifting to the blue as the wavelength increases. The fluorescence quantum yield, corrected for the contribution to absorption by the other virtually nonfluorescent residues, exhibits a pronounced drop at long excitation wavelengths relative to that for excitation at 265 nm. The degree of fluorescence polarization exhibits a weak dependence on the excitation and emission wavelengths. For 7-methyl GMP, the fluorescence spectrum is very weakly dependent on the excitation wavelength and its fluorescence quantum yield shows a moderate increase at long wavelengths. The degree of fluorescence polarization increases with increasing excitation wavelength particularly when monitoring the emission in the short wavelength region of the fluorescence spectrum. A pronounced drop of unknown origin is observed when exciting at 265 nm, which is not observed for methylated DNA. The methylated DNA data are interpreted in terms of a combination of (i) a heterogeneous environment of the methylated guanine residues, which results from sequence-dependent stacking interactions, and (ii) transfer of excitation energy from the other residues to the fluorescing methylated guanine residues. From the values of the quantum yields and those of the decay times, which we have recently reported (Georghiou et al., 1985), the following values are obtained for the radiative, k_t, and the sum of the nonradiative, σk₁, rate constants for deexcitation of the excited states of methylated DNA and its free fluorophore: 1.6 × 10⁸ s^-1 7 × 10⁷ s^-1 and 5 × 10¹⁰ s^-lvs 6 × 10⁹ s^-1. Because of energy transfer from the other residues. the k_f value for the methylated guanine residues is overestimated but their σk₁, value is not affected significantly and is by about an order of magnitude larger than that for 7-methyl GMP, apparently because of stacking interactions.