Resonance Energy Transfer Between Molecular Rotors SYBR Green Intercalated in DNA

The dependences of the fluorescence intensity and anisotropy of molecular rotors SYBR Green (SG) and double-stranded DNA with 10, 20, and 100 base pairs on their relative concentrations in solutions and on the viscosity of the medium were studied. It was shown that an increase in the fluorescence intensity with an increase in the SG concentration and a subsequent leveling off at a constant value was associated with an initial increase in the number of SG molecules intercalated in DNA and further saturation with the formation of nonfluorescent states. A generalized model that takes into account both internal rotations and rotational diffusion of the molecular complex as a whole was developed to explain the sharp drop in fluorescence anisotropy due to Forster intramolecular energy transfer between DNA-bound SG molecules. The proposed model made it possible to calculate universally the obtained experimental dependences of the fluorescence anisotropy on the viscosity of the medium at various dye/DNA ratios and to estimate the Forster energy transfer rates.