Total Synthesis and Functional Evaluation of Fourteen Derivatives of Lysocin E: Importance of Cationic,Hydrophobic, and Aromatic Moieties for Antibacterial Activity

Lysocin E ( 1 ) is a structurally complex 37‐membered depsipeptide comprising 12 amino‐acid residues with an N‐methylated amide and an ester linkage. Compound 1 binds to menaquinone (MK) in the bacterial membrane to exert its potent bactericidal activity. To decipher the biologically important functionalities within this unique antibiotic, we performed a comprehensive structure‐activity relationship (SAR) study by systematically changing the side‐chain structures of l ‐Thr‐1, d ‐Arg‐2, N‐Me‐d ‐Phe‐5, d ‐Arg‐7, l ‐Glu‐8, and d ‐Trp‐10. First, we achieved total synthesis of the 14 new side‐chain analogues of 1 by employing a solid‐phase strategy. We then evaluated the MK‐dependent liposomal disruption and antimicrobial activity against Staphylococcus aureus by 1 and its analogues. Correlating data between the liposome and bacteria experiments revealed that membrane lysis was mainly responsible for the antibacterial functions. Altering the cationic guanidine moiety of d ‐Arg‐2/7 to a neutral amide, and the C7‐acyl group of l ‐Thr‐1 to the C2 or C11 counterpart decreased the antimicrobial activities four‐ or eight‐fold. More drastically, chemical mutation of d ‐Trp‐10 to d ‐Ala‐10 totally abolished the bioactivities. These important findings led us to propose the biological roles of the side‐chain functionalities.