EXCITED STATE PROPERTIES OF THE N‐(DEOXYGUANOSIN‐8‐YL)‐2‐AMINOFLUORENE ADDUCTS

Abstract— The spectroscopic characteristics of adducts derived from the covalent binding of the carcinogen 2‐aminofluorene to the C8 position of deoxyguanosine [N‐(deoxyguanosin‐8‐yl)‐2‐amino‐fluorene, dGuo‐C8‐AF], and from an adduct of similar structure formed with the synthetic polynucleotide poly(dG‐dC)‐poly(dG‐dC), were investigated. At 77 K both adducts are characterized by well‐defined and rather narrow fluorescence emission spectra with maxima at 370 and 390 nm characteristic of the aromatic, monomolecular 2‐aminofluorene (AF) residue. In contrast, at room temperature, the fluorescence is characterized by a broad, structureless emission band with a maximum at 460 nm in aqueous mixtures, shifting to 415 nm in solvents of lower polarity (100% propanol); the maxima are located at intermediate wavelengths in solutions of different propanol/water compositions, and this emission is attributed to an excited state complex (exciplex). The fluorescence quantum yield decreases when either the solvent polarity or the temperature are increased, varying from 5.4% (100% propanol) to 0.04–0.05% (100% H₂O). The fluorescence decay profiles of dGuo‐C8‐AF adducts (measured at the National Synchrotron Light Source facility at the Brookhaven National Laboratory) can be roughly, but not exactly, modeled in terms of two exponential decay components in the range of about 0.3–1.0 ns with the propanol concentration > 60%; at lower propanol concentrations, a single short lifetime is observed and in 100% water solutions its value is 0.08 ns. The shorter lifetime, favored in solvent mixtures of higher polarities, is attributed to an exciplex with significant charge‐transfer character. The disappearance of the exciplex emission at low temperatures suggests that the excited state interactions in these dGuo‐C8‐AF adducts at room temperature involve some conformational mobility of the two covalently linked moieties.