Synthesis And Evaluation Of AIE Fluorescent Probe Based On Quinoline Nitrile Derivatives

Alzheimer’s disease（AD）is a progressive central nervous system degenerative disease characterized by persistent memory decline and cognitive dysfunction.Before a definite diagnosis of AD,there is an insidious period of cognitive decline such as memory for more than 10 years,which makes the early diagnosis of AD very challenging.At present,the clinical diagnosis of AD mainly relies on medical history,brain imaging and neuropsychological detection.The sensitivity and specificity of diagnosis are not high,and there is still a lack of early diagnosis of AD occurrence.In addition,the diagnosis of AD depends on brain biopsy,but the practical operation is difficult.Therefore,it is urgent to develop simple and non-invasive diagnostic tools for early diagnosis and intervention of AD.There are many hypotheses about the pathogenesis of AD,but it can be confirmed that different forms of solubleβ-protein（Aβ）oligomers have been proved to be highly related to the pathogenesis of AD,which is also one of the key pathogenesis mechanisms of AD.The development of NIRF(Near-infrared fluorescence（NIRF）probe technology offers the possibility for early diagnosis of AD.Non-invasive NIRF imaging has become a hot topic in vivo imaging because of its high sensitivity,strong tissue penetration,high spatial resolution,and low background fluorescence interference.Based on the complexity of brain and neuropsychiatric disorders,and the high limitation of the blood-brain barrier on imaging contrast agents,Fluorescence imaging studies on Aβpeptide aggregates have been developed from traditional NIRF probes to the current NIRF probes with Aggregation-induced luminescence（AIE）effect.Overcoming the defect of Aggregation-caused quenching（ACQ）of the former provides a greater possibility for the development of a new NIRF probe for the early diagnosis of AD.Based on previous research results,by comparing luminescence properties,molecular structural characteristics of compounds that cross the blood-brain barrier and synthesis difficulty,we selected the AIE fluorescence groups that are easier to synthesize.The structure of ethyl-substituted Quinoline-malononitrile（QM）with high yield was used as the basic structure for molecular design:The aromatic ring structure was replaced by water-soluble sulfonyl at the end of the side chain,and the conjugate chain length of QM was extended by substituting p-hydroxybenzoxy at the Linker part connecting the basic structure with the side chain,which made the emission wavelength of the compound redshifted to the near-infrared wavelength range.Using Aβoligomer Aβ_1-40as the target,19 target compounds were selected for laboratory synthesis by molecular docking technique.Finally,18 intermediates were synthesized and 16 target compounds that have not been reported in the literature were obtained.All target structures were confirmed by mass spectrometry and hydrogen NMR spectroscopy.The AIE effect of the target compounds was determined.Eight compounds with AIE luminescence characteristics were selected according to the gradient fluorescence performance under 365 nm UV lamp,and their maximum ultraviolet absorption wavelength and maximum fluorescence emission wavelength were determined.The Aβ_1-40oligomer was synthesized according to the literature,and its structure was confirmed by ultrahigh resolution transmission electron microscopy.The results showed that the structure of Aβ_1-40oligomer was clear and complete,with spherical structure and uniform particle size,respectively.Ultraviolet-visible spectrophotometer and fluorescence spectrometer were used to detect the in vitro binding of AIE compounds to A_β1-40oligomers.The results showed that（1）All the eight compounds had good affinity for Aβ_1-40oligomers,and the maximum emission wavelength was in the near infrared region（650～900 nm）.The compound GDC-18 has typical AIE characteristics,showing a maximum absorption wavelength at 417 nm and a maximum emission wavelength at 840 nm.（2）The fluorescence spectra of 8 compounds acting with Aβ_1-40oligomer showed A positive correlation between fluorescence intensity and concentration gradient of the compounds,among which the fluorescence intensity of GDC-18 and Aβ1-40oligomer was nearly 3 times higher than that of the simple compound.（3）The preliminary structure-activity relationship analysis showed that:1,the compounds with ethyl substitution in QM structure had AIE luminous effect,but the methyl substitution did not have this effect;2.Aromatic rings with electron-donating groups and/or sulfonyl groups substituted on the benzene ring of Linker part have power supply effect or electron-donating groups can cause the fluorescence emission spectrum of compounds to redshift to near infrared range.These results indicate that the target compounds basically meet the fluorescence requirements of AIE fluorescent probes,and have the potential to be used for the targeted detection of soluble Aβoligomers,which needs further validation in vivo imaging experiments.