FLUORESCENCE ANISOTROPY OF DNA DAPI COMPLEX - TORSIONAL DYNAMICS AND GEOMETRY OF THE COMPLEX/

Fluorescence depolarization of synthetic polydeoxynucleotide/4'-6-diam idino-2-phenylindole dihydrochloride complexes has been investigated a s a function of dye/polymer coverage. At low coverage, fluorescence de polarization is due to local torsional motions of the DNA segment wher e the dye resides. At relatively high coverage, fluorescence depolariz ation is dominated by energy transfer to other dye molecules along the DNA. The extent of the observed depolarization due to torsional motio n depends on the angle the dye molecule forms with the DNA helical axi s. A large torsional motion and a small angle produce the same depolar ization as a small torsional motion and a large projection angle. Furt hermore; the extent of transfer critically depends on the relative ori entation of dye molecules along the DNA. The effect of multiple transf er is examined using a Monte Carlo approach. The measurement of depola rization with transfer, at high coverage, allows determination of the dye orientation about the DNA helical axis. The value of the torsional spring constant is then determined, al very low coverage, for few sel ected polydeoxynucleotides.