Studies of the fluorescence light-up effect of amino-substituted benzo[b]quinolizinium derivatives in the presence of biomacromolecules

Abstract

A comparative study of the ability of amino-substituted benzo[b]quinolizinium derivatives to act as DNA- or protein-sensitive fluorescent probes is presented. Spectrophotometric titrations, DNA denaturation studies and viscometric titrations showed that all tested aminobenzo[b]quinolizinium derivatives intercalate into DNA with binding constants K_b = 10⁴–10⁵ M⁻¹. The intense fluorescence of the 9-aminobenzo[b]quinolizinium (Φ_fl = 0.41) as well as the intrinsically very weak emission of the 7-aminobenzo[b]quinolizinium (Φ_fl < 0.005) are quenched by the addition of DNA, most likely caused by a photoinduced electron transfer (PET) between the excited intercalated ligand and the DNA bases. The 6-aminobenzo[b]quinolizinium (1b) and the 6-amino-9-bromobenzo[b]quinolizinium (1c) exhibit very low fluorescence intensity in water (Φ_fl < 0.005). However, in water–glycerol mixtures the emission intensity increases by factors of 56 (1b) and 27 (1c) with increasing glycerol content of the solution (0–100 wt%), which indicates the radiationless deactivation of the excited state of 1b and 1c due to a torsional relaxation, i.e. rotation about the exocyclic C_ar–NH₂ bond. In the case of the bromo-substituted derivative 1c, a viscosity-independent heavy-atom-effect of the bromo substituent leads to additional quenching. The association of 1b and 1c with ds DNA leads to a restricted conformational flexibility of the intercalated ligand and results in an increase of fluorescence intensity. This effect is particularly strong in the presence of poly[dA-dT]-poly[dA-dT]. Upon association with ct DNA or poly[dG-dC]-poly[dG-dC] only very small enhancement of emission intensity (1b) or even a slight quenching (1c) of the fluorescence was observed because of the interfering PET reaction with the guanine residues. Preliminary experiments reveal that the 6-aminobenzo[b]quinolizinium derivatives 1b and 1c may also be employed as protein-sensitive probes, because their emission intensity increases upon association with selected albumins.