Author(s): Carvalho, Cátia Diana Parente Caldeira
Date: 2010
Persistent ID: http://hdl.handle.net/10400.1/1681
Origin: Sapientia - Universidade do Algarve
Subject(s): Supramolecular chemistry; Fluorescence; Cucurbit[n]urils; Minor groove binder; Hoechst 33258; Polyamines
Dissertação de mest., Ciências Biomédicas, Faculdade de Ciências e Tecnologia, Univ. do Algarve, 2010
The supramolecular interaction between the well-known DNA-binder Hoechst 33258 dye (guest) and cucurbit[7]uril (CB[7], host) was investigated in detail. The formation of the 1:1 complexes was verified by various methodologies such as Job’s plot method and electrospray ionization mass spectrometry. The binding is characterized by a high association constant, K = 3.5 × 106 M-1, which was determined by UV-Vis and fluorescence titrations. The formation of supramolecular 1:1 complexes is accompanied by drastic changes in the fluorescence quantum yield (Φf) of the dye. While the free dye is barely fluorescent in water at pH 7 (Φf= 0.01), the emission is considerably increased in the host-guest assembly (Φf= 0.74). The binding characteristics at pH 4.5 are similar to the ones at pH 7. However, the fluorescence enhancement at the former pH is less dramatic, because the free dye has here already its highest emission (Φf= 0.29). Proton magnetic resonance spectroscopy clearly demonstrated that CB[7] is accommodated around the piperazine-benzimidazole unit. A mechanism that explains this peculiar fluorescence behavior, which involves intramolecular proton transfer and charge transfer, internal rotation of the free and complexed dye is proposed. The competitive binding to CB[7] of several polyamines (putrescine, cadaverine, spermidine, spermine, and histamine) towards Hoechst 33258 was evaluated as well. For some of these polyamines the CB[7] binding constant has been determined the first time to the best of my knowledge. Finally, the well-characterized binding of the dye with calf-thymus DNA was used in competition experiments. It was found that the known unspecific binding of the dye to DNA leads to strong fluorescence quenching, which becomes appreciable through the initially high fluorescence of the dye in its CB[7] complex. Hence, the strongly emissive Hoechst 33258/CB[7] platform may be further used for the characterization of unspecific DNA binding through fluorescence.
