Cationic amphipathic peptoids containing guanidine and indole side chains: Synthesis and biological activity evaluation

Abstract

Natural antimicrobial peptides (AMPs) have potent and broad-spectrum activity against bacteria and fungi. Despite their activity, AMPs are prone to enzymatic degradation in the physiological conditions. Peptoids, N-substituted glycines, have been explored as alternatives to peptides due to their improved proteolytic stability. In this study, we designed and synthesized novel antibacterial peptoids by incorporating guanidine and indole side chains. This modification allowed us to modulate the net charge, helicity, and hydrophobicity of peptoids, resulting in increased selectivity. The guanidine and indole side chains enhanced the interaction between peptoids and bacterial membrane, thereby improved their antibacterial activity. Through biological assays, we found 4 and 6 showed the most potent antibacterial activity. Furthermore, we confirmed the ability of 4 and 6 to interact with lipid chains and predicted the permeability into the bacterial cell membrane. While microinjection and electroporation have been extensively studied for efficient delivery of drug candidates such as RNA and protein, they suffer from limitations including high toxicity and low efficiency. Cell-penetrating peptides (CPPs) such as TAT, octa-arginine, and penetratin are widely studied to overcome these limitations. Peptoids, however, offer advantages over peptides in term of reduced enzymatic degradation and have emerged as promising candidates for transporters. Mitochondria targeting peptoids (MTPs), previously invented cell-penetrating peptoids, have demonstrated excellent internalization and ability to target the mitochondria specifically. In this study, we investigated the internalization of peptoids inside the HeLa cells. The guanidine and indole side chains of peptoids facilitated their internalization by interacting with cell membrane. With fluorescein conjugated peptoids, we performed fluorometer and confocal microscopy analyses and confirmed CF-9 exhibited the most potent penetration into the HeLa cells. Our researches suggest a novel class of peptoids including indole guanidine side chains that have been showed excellent antibacterial activity with rare damage to bacterial membrane. For further application of novel peptoids, we proved the internalization ability into HeLa cells that will be a cargo for other drug candidates.