Theoretical Calculation Of The Recognition Mechanism Of Fluorescent Probes In Life Systems

Fluorescent probes play an important role in detection of active oxygen and special biological enzyme in living cells.The luminescence properties of fluorescent probes depend on the molecular structures and photophysical properties.Theoretical calculation can reveal the fluorescence sensing mechanism from microscopic level,which has theoretical guiding significance for design of fluorescent probe.In this paper,density functional theory?DFT?and time dependent DFT?TDDFT?were applied to study the interaction between fluorescent probe molecules and singlet oxygen?¹O₂?as well targets in biological enzymes and their photophysical properties.The mechanisms of the photoinduced electron transfer?PET?and the intramolecular charge transfer?ICT?were investigated.1.The luminescence mechanism of europium complex fluorescent probe for detecting¹O₂ was calculated.The results indicated that both of?-diketone and terpyridine ligands are used as fluorescent groups which induce Eu³⁺ion to luminescence by fluorescence resonance energy transfer.However,the terpyridine links with the anthryl group which HOMO energy level is higher than that of the terpyridine and it can transfers one electron to the single electron occupied HOMO of terpyridine?PET process?.This hinders return of the excited state electron and hinder the energy transfer to the Eu³⁺ion,resulting weak fluorescence of the probe.After the specific reaction of anthryl group with ¹O₂,the endoperoxide derivative is formed,and its HOMO energy level is lower than that the of terpyridine,blocking the PET process.The fluorescence emission of the terpyridine is restored and transmits energy to Eu³⁺ion,the fluorescence intensity of the probe is remarkably increased.2.The luminescence mechanism of the NANQ-IMC6 fluorescent probe in detecting the target in biological enzyme was studied by theoretical calculation.The results indicated that the stable structure of the free state NANQ-IMC6 fluorescent probe molecule exists in a folded structure,and the PET process between the fluorescent group and the recognition group makes the fluorescence“OFF”.After interaction with the biological enzyme target,the probe molecules are stretched in an unfolded structure,and due to the space effects,the PET process is blocked and the fluorescence emission is restored.When the fluorescent group is excited by light,the LE excitation occurs first,and the maximum absorption wavelength is455 nm.The excited state structure is a more stable ICT state,resulting in red shift of the emission wavelength.In a polar solution,the S₁ excited state structure is more stable,and the fluorescence emission wavelength is 634 nm;In the non-polar solution,the S₅ excited state structure is more stable,and the fluorescence emission wavelength is 555 nm.Both of emission processes belongs to the mechanism of intramolecular charge transfer.