Facilitating Flip-Flop: How Small-Molecule Structure Influences Interactions with Living Bacterial Membranes

The first barrier a small molecule must overcome before trespassing into a living cell is the lipid bilayer surrounding the intracellular content. It is imperative, therefore, to understand how the structure of a small molecule influences its fate in this region. Through the use of second harmonic generation (SHG), we show how the differing degrees of ionic headgroups, conjugated system, and branched hydrocarbon tail disparities of a series of four styryl dye molecules influence the propensity to `flip-flop' or to be further organized in the outer leaflet by the membrane. We show here that initial adsorption experiments match previous studies on model systems, however, more complex dynamics are observed over time. Aside from probe molecule structure, these dynamics also vary between cell species and can deviate from trends reported based on model membranes. Specifically, we show here that the membrane composition is revealed to be an important factor to consider for headgroup-mediated dynamics. Overall, the findings presented here on how structural variability of small molecules impacts their initial adsorption and eventual destinations within membranes in the context of living cells could have practical applications in antibiotic and drug adjuvant design.