Design And Synthesis Of Series Of Antibacterial β-Carboline Derivatives Including Harman,and Their Mechanisms

β-Carboline compounds are a class of alkaloids with broad biological activity,and nine drugs containing theβ-carboline skeleton have been marketed currently.In this work,we designed the drug molecules based on three naturalβ-carboline templates,harman,canthin-6-one and eudistomin U with significant antibacterial activity,through previous research of our group,the reported structure-activity relationships,pharmacophore splicing technology and molecular docking study.Four series of 126 compounds,including 102new compounds,were synthesized and their structures were confirmed by NMR and HRMS spectra.Thirteen potential antibacterial lead compounds which MIC values are lower than 10μg/mL,were obtained by in vitro,in vivo and theoretical calculations.Furthermore,we studied their antibacterial mechanism.The main results were found as follows:1,Fourty-one harman analogues,including thirty-three new quaternization compounds,were designed and synthesized.Their antibacterial activity against four Gram-positive and two Gram-negative bacteria were evaluated and the structure-activity relationships proposed which indicated that methoxy substitution at C⁶ position and the4-trifluoromethylbenzyl group at N² position were beneficial.Seven potential lead compounds B4f,B4i,B4l,B4u,B4w,B4x and B5c showed low cytotoxicity,good thermal stability and“drug-like”properties.Specifically,compound B4x displayed the excellent antibacterial activity which showed about 4-fold superiority against MRSA（MIC=4μg/mL）than the positive control ampicillin sodium.Scanning electron microscopy and molecular docking studies indicated that the quaternization harman analogues might exert their bactericidal effect by damaging bacterial cell membrane and wall,and disrupting the function of DNA gyrase.In addition,the in vivo antibacterial assay with a protective efficacy of 81.3%further demonstrated the potential of these derivatives as new agricultural lead bactericides.2,Thirty-one canthin-6-one analogues,mainly including C² or C⁵ or D ring modified canthin-6-ones,were designed and synthesized.Their antibacterial activity against four Gram-positive and two Gram-negative bacteria were evaluated and the structure-activity relationships proposed.The potential lead compound C28 showed high activity,low cytotoxicity and good“drug-like”properties.The antibacterial mechanism of C28 was characterized further by SEM,cytoplasmicβ-galactosidase leakage assay,molecular docking evaluation,UV analysis and ITC measurement.The results showed that C28 may exert its bactericidal effect by damaging bacterial cell membranes and disrupting the function of DNA gyrase,both of which could lead to rapid cell death.3,Twenty-one canthin-6-one analogues,mainly including C¹⁰ modified and ring-truncated canthin-6-ones,were designed and synthesized.Their antibacterial activity against five bacteria was evaluated and the structure-activity relationships proposed which indicated that the ring-truncated ABD-ring analogues were better.Two potential lead compounds D21 and D22 were obtained,and compound D22 displayed 8-fold superiority against MRSA（MIC=2μg/mL）than the positive control cefotaxime sodium.Moreover,D22 showed low cytotoxicity and good“drug-like”properties.The antibacterial mechanism was further studied by fluorescent microscopy,SEM,cytoplasmicβ-galactosidase leakage assay and the molecular docking evaluation,which indicated that D22 could exert its bactericidal effect by damaging bacterial cell membranes,increasing membrane permeability and influencing the membrane formation.In addition,the in vivo antibacterial assay with a protective efficacy of 68%further demonstrated the potential of D22 as new agricultural lead bactericides.4,Thirty-three 3,9-disubstituted eudistomin U derivatives,including thirty-two new compounds,were designed and synthesized based on CADD.Their antibacterial activity against four bacteria was evaluated and three potential lead compounds E6f,E6j and E6p were obtained.Specifically,compound E6p with a 4-bromobenzyl group at N⁹ position,showed about 4-fold superiority than ciprofloxacin and 16-fold superiority than fosfomycin sodium against MRSA.The antibacterial mechanism was further studied by fluorescent microscopy,SEM,TEM,the molecular docking evaluation and ITC measurement,which indicated that these compounds could exert bactericidal effect by damaging bacterial cell membrane and disrupting the function of DNA gyrase.In summary,this work confirmed the potential ofβ-carboline compounds as antibacterial drugs and enriched the types of candidate antibacterial drugs.Moreover,the propose of the systematical structure-activity relationships and the antibacterial mechanism study based on the cellular and molecular levels,provided the theoretical basis and support for the design and development of these compounds.