Synthesis And Preliminary Activity Studies Of Boronic Acid Fluorescent Sensors For Selective Recognition Of Catecholamines

Catecholamines,including norepinephrine,dopamine,epinephrine,isoproterenol,serotonin,serotonin and its derivatives,could regulate the basic physiological function in the human body.These neurotransmitters are important signaling media in the physiological process,and consequently the corresponding changes of the content in the process.The content of catechins in medical research can be used to assist in the diagnosis of endocrine-related diseases such as hyperthyroidism,hypertension,neuroblastoma and pheochromocytoma.Catechins are widely present in tissues and organs,especially catecholamines(such as dopamine,levodopa,adrenaline,etc.)are associated with central nervous system diseases and cardiovascular disease in vivo.Therefore,the development of catecholamine selective detection tools have great significance on medical research and practical activty.Owing to high sensitivity,simple operation,fast detection,low detection limit and high selectivity,fluorescent sensors have been widely used in biochemical sensing,optical new material research,biomolecule detection and recognition aplications.In aqueous solution,boronic acid compounds can bind with cis-diols reversible to form five-or six-membered cyclic esters.Thus,boronic acid based sensors can be used to detect catecholamines,sugars,glycoproteins,bacteria,cells,etc.2-(4-dihydroxyborane)phenylquinoline-4-carboxylic acid(PBAQA)is a new type of water-soluable fluorescent sensor.The quinoline group,in the structure,is attached to the boronic acid through a benzene ring.The free carboxyl group in the structure,as the modification site,by which other functional groups or structures can be introduced in PBAQA.In order to overcome the problem of PBAQA in poor selectivity for structure similar catecholamines such as dopamine and levodopa,and to synthesize highly selective fluorescent sensors,we designed and synthesized a series of bis-boronic acid compounds,in which another boronic acid is a second binding site.Based on the structure of PBAQA,another boronic acid group was used to construct a fluorescent sensor with diboronic acid recognition site and the preliminary activity test was carried out.The effect of the spatial distance of the two recognition sites,the electron effect and the position of different substituents on the activity of fluorescence sensor was investigated.Considering the high cost of PBAQA as the starting material,it is not conducive to derivatization and in-depth research in the laboratory.Using bromoacetophenone and isatin as starting materials,we prepared PBAQA through Pfitzinger reaction,carboxyl esterification,palladium catalysis,hydrolysis,etc.The effects of solvent selection,reaction temperature and reaction time on the yield of 2-(4-bromophenyl)quinoline-4-carboxylic acid were investigated and the optimum synthesis conditions were obtained.PBAQA and another boronic acid were ligated to both ends of the Linker by amide condensation.A simple and efficient synthetic route was designed.The diamine compound was first protected with di-tert-butyl dicarbonate and then condensed with carboxyamides which was activated by SOCl2.The BOC group was removed by hydrochloric acid to form a hydrochloride salt and precipitated to a purity of 98 %.The carboxyphenylboronic acid compound is first dissolved in DMF and then a carbodiimide-based condensing agent is added.The carboxyl group was activated with DCC and HOBT,after that,the carboxyl group is activated with DCC and HOBT and then condensed with the PBAQA derivative amide.Compared to the acid chloride method,this step avoids the use of SOCl2,reducing the production of pungent gas and waste.It's green,easy to operate and efficient.The effect of solvent selection and reaction temperature ratio on the yield of the target compound was studied by using various methods to determine the optimum synthesis conditions.35 diboric acid compounds and 4 intermediate compounds were synthesized.All of the 39 new compounds were verified by TLC boronic color reaction and fluorescence experiments and confirmed by nuclear magnetic resonance spectroscopy,nuclear magnetic resonance spectroscopy or high resolution mass spectrometry.The binding constant of compound 12 h with dopamine hydrochloride is twice that of catechol and L-DOPA binding constant.The binding constant of compound 11 g with dopamine hydrochloride is about 6 times higher than that of PBAQA;the binding constant of compound 11 g with catechol is about 4 times higher than that of PBAQA;the binding constant of compound 11 g with levodopa is about 5 times higher than that of PBAQA.It indicates that the spatial structure of compound 11 g and the electron effect of chlorine atom are beneficial to improve the affinity of catecholamines.The fluorescence activity of the compounds with carbohydrate was also tested.The compounds with certain selectivity were screened out initially.