Room-temperature optical properties of methylated calf thymus DNA

In the present study the room-temperature optical properties of calf thymus DNA, methylated at N-7 of the guanine residue, were studied and compared with those of the free fluorophore, 7-methyl GMP. It has been found that the fluorescence spectrum of methylated DNA depends strongly on the excitation wavelength, shifting to the blue as the excitation wavelength increases. The degree of fluorescence polarization exhibits a weak dependence on the excitation and emission wavelengths. The curve for the fluorescence quantum yield, q, corrected for absorbance by the A, C, and T residues, peaks at 270 nm and then drops for longer excitation wavelengths. For the excitation wavelength of 290 nm, q = 0.0017. The fluorescence decay time for that excitation wavelength has been found to be 0.11 ns.

For 7-methyl GMP at ph 5 (conditions under which the proton at N-1 is not dissociated) the fluorescence spectrum is virtually independent of the excitation wavelength. The degree of polarization exhibits a dependence on both the excitation and emission wavelengths; it increases with increasing excitation wavelength, particularly when monitoring the emission at the short wavelength region of the fluorescence spectrum, with the exception of 265 nm for which it drops dramatically. The fluorescence quantum yield, q, is weakly dependent on the excitation wavelength below 270 nm, peaks at 285 nm, drops at 300 nm and peaks again at 305 nm. For 290 nm, q = 0.024. The fluorescence decay time for that wavelength has been found to be 0.20 ns.

The observed differences between the optical properties of methylated DNA and 7-methyl GMP imply that the former cannot be explained on the basis of emission solely from monomeric methylated G residues. A scheme involving both monomeric G residues as well as G residues that have an excimer-like conformation in their ground electronic state is proposed. The data appear to favor transfer of excitation energy from A, C, and T residues to G residues.