A. Oligopeptide and lipid vesicle preparation

Lys₈ and Arg₈ were synthesized by and purchased from GenScript (Piscataway, NJ, purity ≥ 98%). Powders of Lys₈ and Arg₈ were stored in sealed plastic vials at –20 °C prior to use and were used without further purification. Powders (∼50 mg) were dissolved in 50–100 μL of ultrapure water (>18 MΩ cm; GenPure Pro or Millipore, Thermo Scientific) containing 0.001 M NaCl and vortexed, producing solutions with concentrations on the order of hundreds of mM. Lower concentrations were achieved through serial dilution. These stock solutions were then covered with Parafilm and stored in glass vials or microcentrifuge tubes at 4 °C. Immediately before use, the appropriate volume of oligomer solution was diluted to the desired concentration in 0.1 M NaCl buffered to pH 7.4 with 0.01 M Tris.

Small unilamellar vesicles were prepared at 9 : 1 molar ratios from 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC, Avanti Polar Lipids) and 1,2-dimyristoyl-sn-glycero-3-phospho-(1′-rac-glycerol) (sodium salt) (DMPG, Avanti Polar Lipids) as previously described.45,46 Here, we made the following modifications to our previously published method: lipid films were reconstituted with 0.001 M NaCl buffered to pH 7.4 with 0.01 M Tris, vortexed, sonicated for 30 min, and subjected to three freeze–thaw cycles by freezing in liquid N₂ and thawing in a bath sonicator. Lipids were mechanically extruded as previously described.45,46 Immediately before use, extruded lipids were diluted to 0.5 mg mL^–1 for SHG experiments or 0.125 mg mL^–1 for QCM-D/NPS experiments in solutions composed of 0.1 M or in 0.15 M NaCl containing 0.005 M CaCl₂, and buffered to pH 7.4 with 0.01 M Tris. Briefly, SLBs were formed via the vesicle fusion method. Specific experimental details are provided in the ESI.^† For reference, all experiments described in the remainder of the article employed solutions buffered to pH 7.4 with 0.01 M Tris in the presence of 0.1 M NaCl.