English abstract
Up to date, the design of highly selective probes for amino acid cysteine is still a challenge and very few examples have been reported so far. This research is principally focused on the synthesis, characterization and optical properties investigation of a new fluorescent probe based on flavone skeleton and bearing a nitroolefin moiety in different media. In this work, the probe, 4' nitroolefinylflavone (4'-NOF) 9 was synthesized through an Aldol type of reaction between nitromethane and an aldehyde group of corresponding 4'-formylflavone. The formed probe consists of flavone as the fluorophore, linked via C-4' to nitroolefin moiety which contain a double bond as a functional trigger moiety. The chemical structure of the probe was confirmed by IR, 'H NMR, 13C NMR and MALDI-TOF spectral data. In order to expand its application potential, its photophysical properties have been investigated using absorption and emission spectroscopic techniques. Indeed, the choice of nitroolefin group not only offers us the desired functionality for participating in the Michael-type reaction in specific detection of cysteine, but also helps in achieving a favorable intramolecular charge transfer (ICT) transition. The probe exhibited absorption maximum at 340 nm and emission maximum at 454 nm (ex= 330 nm). Upon the addition of Cys to the solution of 4'-NOF, remarkable shifts in its spectra can be observed from 340 nm to 300 nm in absorption spectrum (hypsochromic shift) and from 454 nm to 456 nm in emission spectrum (bathochromic shift). The emission intensity of 4'-NOF was greatly enhanced while adding 0.05 mM Cys. This fluorescence enhancement and absorption change are attributed to the Michael addition of SH group of cysteine to the double bond of nitroolefin moiety in the sensor, resulting in switching off the intramolecular charge transfer (ICT) from flavone moiety to nitroolefin moiety. The Michael addition mechanism of the reaction between 9 and cysteine was confirmed by 'H-NMR spectroscopy and mass spectrometry. The detection limit of 9 to Cys was evaluated to be 1.633 UM, which is lower than the normal human intracellular cysteine concentration 30-200 UM.
