Anoplin-functionalized gold nanoparticles cause greater disorder in model bacterial membranes compared to individual components

Antimicrobial peptides are vital to the innate immune system and serve as a safe alternative to traditional antibiotics for bacterial infections. However, their effectiveness is limited by the need for high concentrations to disrupt bacterial membranes and the challenge of protecting them from proteases that degrade them before reaching their target. Intending to improve the delivery of anoplin (ANP) to bacterial cells, we prepared ANP-coated gold nanoparticles (Au NPs) with a diameter of about 50 nm, and investigated their effects on the properties of model bacterial membranes using Raman and surface-enhanced Raman spectroscopy (SERS). Prepared as a mixture of 1-palmitoyl-2-oleoyl- sn -glycero-3-phosphoethanolamine (PE) and 1-palmitoyl-2-oleoyl- sn -glycero-3-phospho(1′-rac-glycerol) sodium salt (PG) at a ratio PE/PG 3/1, lipid membranes were measured in the absence/presence of pure ANP, Au NPs (coated only with citrate), and Au NPs coated with ANP and at two different temperatures, 10 °C and 26 °C, at which the lipids are found in the gel/fluid and fluid/fluid phases, respectively. In addition to serving as indicators of the disorder caused by the phase change in pure PE/PG bilayers, the bands of skeletal vibrations (νC-C trans/gauche ) proved to be sensitive enough to detect the increased disorder resulting from the adsorption of ANP and of ANP-coated Au NPs. Since the disorder caused by the adsorption of the latter is significantly greater than that caused by ANP itself, we believe that this study could pave new ways in the research and use of Au NPs (as well as other metal NPs) as targeted and effective delivery of therapeutics.