Energy transfer from thiacyanine to acridine orange facilitated by DNA

The drastic enhancement of the fluorescence of 3,3'-diethyl thiacyanine iod ide (THIA) in the presence of DNA is mainly due to the reduction of skeleto n vibration of the dye on its binding to DNA, as inferred from the enhancem ent of fluorescence of THIA with increasing the solvent viscosity by sucros e, glycerol or poly-(vinyl alcohol), or by freezing a methanolic solution o f the dye. Energy transfer from THIA to acridine orange (AO), both bound to DNA has been observed and characterised by the enhancement of AO fluoresce nce at 524 nm with congruent reduction of DNA-THIA fluorescence. Energy tra nsfer between the two dyes in the absence of polymer has also been observed but to a minor extent. Keeping DNA and AO concentrations fixed when the co ncentration of THIA is increased, the fluorescence peak of AO shifts from 5 24 to 560 nm indicating the formation of an exciplex. Time resolved fluores cence spectroscopy gives characteristic evidence for the energy transfer an d also for the exciplex formation. (C) 1999 Elsevier Science B.V. All right s reserved.