Discovery,Structure Characterization And Mechanism Of Antibacterial Metal Complexes In Scutellaria Baicalensis Georgi Decoction

Antibacterial drug resistance has become a major public health issue in the 21st century.Metal-based antibacterial compounds are not susceptible to drug resistance due to their different antibacterial mechanisms compared with traditional antibiotics.On this basis,drug developers are attaching importance to the R&D strategy of new antibacterial drugs targeting metal complexes.Searching for lead compounds from traditional Chinese medicine is considered as an important approach in the R&D process of new drugs.However,the current research mainly focuses on the organic components.Traditional Chinese medicine is usually decocted in water,during which many organic compounds with functional groups（i.e.phenolic hydroxyl,carboxyl,carbon,amino,sulfhydryl,etc.）in traditional Chinese medicine can form metal complexes with metal ions.These metal complexes,may exhibit favourable biological activity compared with corresponding ligands.Therefore,potential new antibacterial drugs are expected to be discovered among these various metal complexes.Scutellaria baicalensis is a kind of traditional Chinese medicine characterized by anti-inflammatory,antibacterial and antiviral activities.In this paper,Scutellaria baicalensis was taken as the research object and antibacterial metal complexes in its decoction were systematically studied through acid methanol dissolution method and metal ion addition method.Moreover,the structural characterization,antibacterial activity and antibacterial mechanism of the selected baicalin-manganese complex were studied.The main research contents and results are as follows:（1）Screening of antibacterial metal complexes in Scutellaria baicalensis decoction:First,the existence of a large amount of metal complexes in Scutellaria baicalensis decoction was proved through the acidic methanol dissolution method.Thereinto,norhabetin-7-O-glucuronide and baicalin are considered to be good ligands due to their strong metal complex forming ability.Second,the metal ion addition method found that Mn²⁺could achieve a 4-fold antibacterial activity of Scutellaria baicalensis decoction.Therefore,we mainly conducted systematical research on the baicalin-manganese complex.（2）Structural Characterization of baicalin-manganese complex:Compared with the UV-vis absorption spectra of baicalin,new substances were produced after the reaction of baicalin with Mn²⁺.The existence of coordination bonds between oxygen atoms and manganese atoms in baicalin was determined by synchrotron radiation X-ray absorption spectra.Baicalin and Mn²⁺formed complexes in the ratio of 1:1 and2:1 through the ESI-TOF MS detection.（3）Study on antibacterial activity of the baicalin-manganese complex:Under the condition of the experimental dose,baicalin exhibited a certain inhibitory effect on Staphylococcus aureus,but not on Escherichia coli and Pseudomonas aeruginosa.While baicalin-manganese complex showed significantly enhanced antibacterial activity against three bacteria.Thereinto,baicalin-manganese complex showed the strongest antibacterial activity against Staphylococcus aureus,which was 8 times higher than that of baicalin.According to the study on the growth curve of Staphylococcus aureus,bactericidal kinetic curve,biofilm formation and scavenging effect,baicalin-manganese complex exerted its antibacterial role in a concentration and time-dependent manner.Moreover,baicalin-manganese complex had a certain scavenging effect on mature biofilm.Notably,baicalin-manganese complex are not prone to induce drug resistance of Staphylococcus aureus on the one hand,and can reverse the resistance of some bacteria.（4）Antibacterial mechanism of baicalin-manganese complex:By observing the changes in the morphology and structure,the integrity of cell wall and membrane structure and the content of ROS in Staphylococcus aureus after the treatment with baicalin-manganese complex,it was found that baicalin-manganese complex could induce bacterial apoptosis by destroying the basic structure of bacteria and producing a large amount of ROS.In addition,transcriptome analysis showed that the baicalin-manganese complex might induce bacterial death through a series of approaches disrupting the energy metabolism process of bacteria,including interfering with glycolysis,carbon metabolism and pentose phosphate pathway.In this paper,we have proved the the high feasibility of metal complexes with favourable antibacterial properties from Scutellaria baicalensis decoction,and carried out in-depth study on the antibacterial mechanism of the selected baicalin-manganese complexes,hoping to shed new light on the study on the material basis of the efficacy of traditional Chinese medicine.