The Design,Synthesis And Biological Activities Of Novel Antimicrobial Peptides And Antimicrobial Peptide Mimics

Infectious diseases pose a serious threat to human health,with bacterial infections are contributing substantially to this trend. The abuse of antibiotics causes uncontrolled spread of multidrug resistant bacteria leading to widespread fatalities and economic disruption.Nowadays, antimicrobial peptides (AMPs) have attracted more considerable attention and been implied as potential therapeutic alternatives to conventional antibiotics because of their specific features of broad-spectrum antimicrobial activities,rapid action, lower risk of antimicrobial resistance and distinct mode of action.Although the advantages of AMPs,their intrinsic drawbacks,such as susceptibility to enzyme degradation, systemic toxicity and high-cost production still hampered the development of AMPs. The development of novel AMPs and antimicrobial peptide mimics to overcome the major drawbacks of AMPs is therefore in urgent need.Herein, we present results of a study aimed to design a novel group of AMPs and AMP-mechanism based antimicrobial peptide mimics. It will be divided into the following two sections:1. The design of novel antimicrobial peptide mimics based on conjugation of sapogenin and polyarginineAs previously reported, cationicity and amphipathicity are characteristics understandably conserved among many AMPs and play a key role in their antimicrobial activity. Cationicity is important for the initial electrostatic attraction of antimicrobial peptides to negatively charged phospholipid membranes of bacteria, and amphipathic structures facilitate self-association or multimerization of AMPs and promote them integration into and disruption of the bacterial membrane.Pentacyclic triterpenes are a group of biologically active compounds with antibacterial activity. However,the limited solubility and poor bioavailability of pentacyclic triterpenes restrict their therapeutic application. Polyarginine peptides are small cationic peptides with high affinities for multiple negatively charged cell membranes and possess moderate antibacterial activities. In the first part of our study,we designed and synthesized a series of sapogenin-polyarginine conjugates in which the triterpene sapogenin moiety was covalently appended to the positively charged polyarginine via click chemistry. A clear synergistic effect was found, and the conjugates exhibited potent and selective antibacterial activity against Gram-positive strains. Among them, BAc-R3 was the most promising compound, which was also proven to be nontoxic toward mammalian cells as well as stable in plasma. The mechanism of BAc-R3 primarily involves an interaction with the bacterial membrane,similar to that of AMPs.2. The design of novel AMPs with peptide staplingChicken cathelicidin-2 (CATH-2), a potent cathelicidin produced by chicken bone marrow cells, possess antimicrobial and immunomodulatory activities. NMR spectroscopy has been revealed that CATH-2 consists of two a-helices connected by a hinge region. Among them, the first 15 N-terminal amino acids of CATH-2 (C1-15),containing N-terminal ?-helix and hinge region, is the core element of CATH-2 required for antibacterial activity. Then, E. M. Molhoek and co-workers substituted the three phenylalanine residues to tryptophan in C1-15 and resulted in peptide F2,5,12W. F2,5,12W possessed improved antibacterial activity and lipopolysaccharide neutralizing activity compare to C1-15. Despite the promising potencies of F2,5,12W,it is susceptible to proteolytic degradation and poor in bioavailability.Peptide stapling is an important tool within peptide medicinal chemistry for stabilizing and pre-organising peptides in a desired conformation. Through peptide stapling, peptide can be enhanced the selectivity with specific target and therefore improved the biological activities. On the other hand, peptide stapling may enhance protease stability by restricting the peptide conformation and thereby making recognition by the protease and subsequent hydrolysis, more difficult. In the second part of our study, we were aimed to improve the resistance of F2,5,12W against proteolytic degradation by different peptide stabling techniques. We selected S-alkylation stapling, triazole stapling and hydrocarbon stapling to synthesize a series of F2,5,12W stapled peptides. It was revealed that minimal inhibitory concentrations(MICs) of the stapled peptides, which were cyclized by hydrocarbon stapling and S-alkylation stapling with 1,3-bis (bromomethyl) linker were comparable to that of F2,5,12W. Of note, the stapled peptide F2,5,12W-2,6A*-st showed a significant enhanced stability in plasma and improved antibacterial activity against S. aureus, B. subtilis and E. coli compared to F2,5,12W.In conclusion, we designed and synthesized a total of 30 cationic and amphiphilic triterpene sapogenin-polyarginine conjugates and 16 novel F2,5,12W stapled peptides. The innovativeness of this study would be listed as follows: (1) the conjugation of hydrophobic pentacyclic triterpene and cationic polyarginine form a cationic and amphiphilic structure that was necessary for antimicrobial peptide activity. While, the fusion of two moieties would possibly change the physicochemical properties and enhance the antibacterial activities of bioactive troterpenes and polyarginine peptides. With the encouraging results from these triterpene sapogenin-polyarginine conjugates and the urgent need to develop a new generation of antibiotics, this study provides useful insights for the rational design of novel antibiotics comprised of the combination of natural products and peptides. (2)based on the peptide stapling, we found a novel stapled peptide with enhanced stability and maintained antibacterial activity.