Solid Phase Synthesis And Antibacterial Activity Of Cyclohexapeptide Desotamide B And Its Derivatives

The problem of drug resistance of human pathogenic bacteria has become increasingly serious and it is urgent to find effective solutions.The discovery of natural antibiotics and the synthesis of their derivatives have played a pivotal role in the fight against bacterial infections and diseases.The current clinical use of peptide drugs,such as gramicidin,vancomycin,polymyxin E,and daptomycin have shown better efficacy,and their structure is mostly circular,with good membrane permeability and stability.Most of these drugs are derived from terrestrial organisms.However,recent studies have shown that there are also effective antibacterial ingredients in marine organisms.Recently,a hexacyclic peptide consisting of cyclo(-[Trp-Leu-D-Leu-Val-Asn-Gly]-)was extracted from the metabolites of Streptomyces scopuliridis SCSIO ZJ46 in the deep-sea of the South China Sea.Compared to several other cyclic peptides of the Desotamide family,Desotamide B has a broader antibacterial activity,has a good inhibitory effect against methicillin-resistant Staphylococcus epidermidis(MRSE),and has low cytotoxicity to mammals,and it is beneficial to the development of antibacterial drugs.Therefore,this project mainly carried out the following research work:(1)First,the synthesis of Desotamide B was optimized by solid phase synthesis of peptides,and a set of routes and methods that are more favorable for the synthesis of cyclic peptides were explored to increase the yield of solid phase synthesis,paving the way for further research.(2)Based on the above method,combined with the alanine scanning method,we continued to optimize the synthesis of 6 other derivatives of Desotamide B,in order to obtain special active sites in the Desotamide B structure for exploring its antibacterial mechanism.(3)Finally,for the above seven target compounds,the selection of gram-positive bacteria such as Streptococcus pneumoniae,Staphylococcus aureus,Bacillus subtilis,and other Gram-positive bacteria for their in vitro antibacterial activity test will provide a basis for the further development of new antibacterial drugs.The main results of this project's research work include the following:(1)In the synthesis of linear peptides,the desired amino acid was activated in DIPEA in dichloromethane for 1 h before the amino acid linkage,and the synthesis purity of the linear peptide was successfully improved.It can be purified without high-performance liquid chromatography and can be directly used in the next experiment.(2)When separating the cyclic peptide from the reacted cyclization system,the yield was 30.6% using high-performance liquid chromatography and the yield was 68.4% using a GE Healthcare Sephadex LH-20 Sephadex column.Comparing the two,it was proved that the separation efficiency from the cyclization system using dextran gel column was much higher than that of high performance liquid chromatography.(3)Four gram-positive bacteria,Streptococcus pneumoniae,Staphylococcus aureus,and two Bacillus subtilis strains,were selected for in vitro antibacterial activity against 7 target compounds.The results showed that the minimum inhibitory concentration(MIC50)of derivative Desotamide B-1 against the above four Gram-positive bacteria was 4.5±1,5±1,3±2,4±1.It was confirmed that the specific amino acid residue in the structure of Desotamide B was asparagine.In summary,this project successfully explored an optimization method for the synthesis of cyclic peptides,which provided a reference value for the synthesis of cyclic peptides in the future,and discovered the special position of the cyclic peptide Desotamide B,and laid the foundation for further transformation of it in search of new types of antibiotics.