Antimicrobial activity profiles of Amphiphilic Xanthone derivatives are a function of their molecular Oligomerization.

Currently, membrane-targeting small antimicrobial peptidomimetics (SAP) are important in antibiotic development because bacteria appear to develop resistance to these surface-active compounds less readily. However, the molecular membrane-targeting action of SAPs has received little attention. In this study, we investigated the effect of oligomerization of amphiphilic xanthone, a model SAP, on its antimicrobial properties against both Gram-positive and Gram-negative bacteria. First, oligomer formation by an amphiphilic xanthone, compound 2 (also coded as AM052), was investigated via solution-state nuclear magnetic resonance (NMR) spectroscopy. Then, the effects of oligomerization on membrane disruption were further studied via biophysical approaches. The results showed that the antimicrobial activities of SAPs develop in several stages: oligomer formation in aqueous solution, initial binding of oligomers to the membrane-water interface followed by insertion into the membrane bilayer, aggregation of antimicrobial oligomers in the membrane, and induced membrane leakage. Ultimately, the presence of the oligomers in the bacterial membrane leads to decreased membrane fluidity and bacterial cell death. Interestingly, the early formation of large oligomers leads to stronger membrane disruption and more rapid bacterial killing. However, reduced antimicrobial activities against Gram-negative bacteria were observed for compounds that formed larger oligomers because the LPS layer acts as a barrier to large complexes. Taken together, our results suggest that oligomerization of SAPs has a strong impact on their antimicrobial properties.