Synthesis and Sequence-Activity Relationship Study of Novel Antimicrobial Peptoids

Abstract

Antimicrobial peptides (AMPs) exert bactericidal or bacteriostatic activity against broad spectrum pathogens, including multidrug-resistant (MDR) strains, and numerous AMPs are under clinical investigations. Antimicrobial peptoids (ampetoids) have emerged as a promising antibiotic platform which overcomes the drawbacks of AMPs such as short serum half-life and poor stability against proteolytic degradations. In this study, we aim to improve the efficacy of the ampetoids in the physiological conditions by modulating salt resistance and metabolic stability. Histidine (Nhis)-containing peptoids are designed on the basis of the histidine-rich salt resistant marine AMPs. The α-helical conformations are retained as the parent peptoid 1 did; however, antibacterial activities decreased in both standard and physiological salt conditions (150 mM NaCl). It indicated that the intermolecular hydrophobic interactions among the ampetoids in the early stage of the mechanism may be affected by the incorporation of the Nhis monomer and by the reduced side chain hydrophobicity. Furthermore, because pKa of the histidine is around 6~6.5, net charge of the peptoids was decreased by the displacement of Nlys monomer with Nhis monomer resulting in the adverse effect on the antimicrobial activity. According to the previous biodistribution study, parent peptoid 1 appears to be too stable in the systemic circulation (> 24 h), which may cause undesired toxicity. Changing the aromatic side chains of peptoid 1 could modulate the metabolic stability and lead to the shorter serum half-life. Hydrolysis of amide bonds or other oxidation mechanisms at the aromatic side chains may cause the successive degradations of the ampetoids. Taken together, our results suggest that the substitution of hydrophobic Nspe monomer with the polar Nhis monomer reduces antimicrobial activity showing that the critical role of the hydrophobic aromatic side chains of the ampetoids. In addition, displacement of the Nlys monomer to the Nhis monomer reduces net charge critical to electrostatic interaction with anionic bacterial membrane. Further position specific incorporation of histidine monomer into peptoids and subsequent sequence-activity relationship including salt resistance is currently undergoing. In addition, side chain substitutions render the peptoids moderately stable providing us a strategy to design ampetoids with more rapid elimination from the body with reduced toxicity.