Spectroscopic and photophysical study of an anthryl probe: DNA binding andchiral recognition

Binding of Pirkle's acid (2,2,2-trifluoro-1-(9-anthryl)ethanol;TFA to diffe rent double stranded polynucleotides namely poly(dA-dT)-(dA-dT), poly(dG-dC )-poly(dG-dC) and calf thymus DNA, was examined for the first time by follo wing changes in the photophysical properties of each enantiomer of the chro mophore using steady state as well as time resolved absorption and fluoresc ence methods. The observed effects-on absorption, fluorescence quantum yiel d and anisotropy, excited state lifetimes as well as energy transfer experi ments give evidence for the occurrence of different enantiospecific binding modes. The photophysical properties of (S)-TFAE in the presence of polynuc leotides are indicative of-an intercalative binding mode with a clear depen dence on the adenine-thymine (A-T) content of the DNA. Furthermore, the flu orescence quenching of(S)-TFAE in the vicinity of A-T pairs correlated with an increase of the fluorescence lifetime suggest's that there are at least two different intercalative binding sites for this enantiomer. In contrast , (R)-TFAE does not recognize the synthetic polynucleotide poly- (dA-dT)-(d A-dT) and binds only by surface interactions with the natural DNA. Neither TFAE enantiomer binds to poly(dG-dC)-poly(dG-dC). This chiral discriminatio n for intercalative and base specific binding modes is explained in terms-o f the helical asymmetry and interactions with the A-T bases which is matche d by the asymmetry of the S enantiomer but precludes intercalation by the R enantiomer. The photophysical properties of TFAE have never been previously studied. Th e fluorescence quantum yield of the chromophore in cyclohexane and water-wa s found to be 0.35 +/- 0.04. The triplet state of free TFAE was characteriz ed by its absorption spectrum (epsilon(max) = 56000 M-1 cm(-1) at 425 nm) a nd its formation quantum yield (phi(T) = 0.7 +/- 0.1). Biphotonic ionizatio n occurred upon laser excitation of TFAE in water and the solvated electron and the radical cation were identified [epsilon (TFAE(.+)) = 8000 M-1 cm(- 1) at 720 nm]. The triplet and radical cation formation and decay kinetics of fi-ee TFAE were not altered by the presence of polynucleotides under the experimental conditions used.