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

Abstract

Binding of Pirkle’s acid (2,2,2-trifluoro-1-(9-anthryl)ethanol; TFAE) to different 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 following changes in the photophysical properties of each enantiomer of the chromophore using steady state as well as time resolved absorption and fluorescence methods. The observed effects on absorption, fluorescence quantum yield and anisotropy, excited state lifetimes as well as energy transfer experiments give evidence for the occurrence of different enantiospecific binding modes. The photophysical properties of (S)-TFAE in the presence of polynucleotides are indicative of an intercalative binding mode with a clear dependence on the adenine–thymine (A–T) content of the DNA. Furthermore, the fluorescence quenching of (S)-TFAE in the vicinity of A–T pairs correlated with an increase of the fluorescence lifetime suggests 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)-(dA-dT) and binds only by surface interactions with the natural DNA. Neither TFAE enantiomer binds to poly(dG-dC)-poly(dG-dC). This chiral discrimination for intercalative and base specific binding modes is explained in terms of the helical asymmetry and interactions with the A–T bases which is matched by the asymmetry of the S enantiomer but precludes intercalation by the R enantiomer.

The photophysical properties of TFAE have never been previously studied. The fluorescence quantum yield of the chromophore in cyclohexane and water was found to be 0.35 ± 0.04. The triplet state of free TFAE was characterized by its absorption spectrum (ε_max = 56000 M^–1 cm^–1 at 425 nm) and its formation quantum yield (ϕ_T = 0.7 ± 0.1). Biphotonic ionization occurred upon laser excitation of TFAE in water and the solvated electron and the radical cation were identified [ε (TFAE^˙+) = 8000 M^–1 cm^–1 at 720 nm]. The triplet and radical cation formation and decay kinetics of free TFAE were not altered by the presence of polynucleotides under the experimental conditions used.