2.2.1. Materials

Materials were custom ordered with purity > 95% (Lipopharm, Gda ´nsk, Poland). Sequences were as follows: wt GLFGAIAGFIEGGWQGMVDGWYGSGKKKKD and its E11A and W14A mutants. In all cases, the N-terminus was unmodified and the C-terminus was an amide. The -SGKKKD sequence was introduced to increase peptide solubility. Stocks were prepared from weighted amounts dissolved in water as 300–500 μM solutions. Concentrations were checked by UV spectroscopy using the extinction coefficient at 280 nm of 12490 M−1cm−<sup>1</sup> for wt and E11A peptides and 6990 M−1cm−<sup>1</sup> for W14A. N-(7-Nitrobenz-2-Oxa-1,3-Diazol-4-yl)-1,2-Dihexadecanoyl-*sn*-Glycero-3-Phosphoethanolamine (NBD-PE) and Lissamine ™ Rhodamine B 1,2-Dihexadecanoyl-*sn*-Glycero-3-Phosphoethanolamine (N-Rh-PE) used in fusion assays were from ThermoFisher Scientific. POPC (1-palmitoyl-2- oleoyl-*sn*-glycero-3-phosphocholine), Br4,5 (1-palmitoyl-2-stearoyl-(4,5)dibromo-*sn*-glycero-3-phosphocholine), Br6,7 (1-palmitoyl-2-(6,7-di-bromo)stearoyl-*sn*-glycero-3-phosphocholine), Br9,10 (1-palmitoyl-2-(9,10-dibro-mo)stearoyl-*sn*-glycero-3-phosphocholine), Br11,12 (1-palmitoyl-2-(11-,12-di-bromo)ste-aroyl-*sn*-glycero-3-phos-phocholine), and all other chemicals were from Sigma Aldrich (Merck, St. Louis, MO, USA). All experiments were performed in buffer pH 5.0 (10 mM citric acid/NaOH, 150 mM NaCl).

## 2.2.2. Liposome Preparation

Desired amounts of lipids in chloroform were dried under a stream of nitrogen overnight under vacuum, followed by rehydration with appropriate buffer to 2–10 mg/mL concentration. For LUV preparation, the dispersion was frozen and thawed in liquid nitrogen and room temperature at least 5 times, followed by extrusion (15–21 times) through polycarbonate filters with 100 nm pores (Whatman) using Avanti Mini Extruder. For SUV preparation, the dispersion was sonicated with a tip sonicator (VibraCell VCX130) in 7–20 pulses lasting 10 s separated by 10 s breaks until the solution was clear.
