## 2 Results and Discussion
### Interactions of P13#1 with SDS, LPSs and Bacterial Cells  
We recently developed a peptide-labelling strategy based on an environment-dependent luciferin-like fluorophore [70]. The amino and hydroxy versions of this fluorophore are strongly sensitive to environmental polarity and proton acceptor availability and/or local pH, respectively. This labelling strategy allows one to study the interactions of peptides with detergents, lipids, micelles, liposomes and even whole cells. For this reason, we designed CG-P13#1, a P13#1 variant with two additional residues at the N-terminus: cysteine and glycine. The N-terminal cysteine was needed to perform the cyclization reaction that generates the fluorophore, whereas the glycine residue was added to provide a flexible joint between the fluorophore and the peptoid (Figure S3A in Supplementary Materials). CG-P13#1 was labeled to obtain Luc-P13#1 and aLuc-P13#1, carrying the hydroxy and the amino versions of the fluorophore, respectively (Figure S3A in Supplementary Materials). Hence, we studied the interactions of the labeled P13#1 variants with sodium dodecyl sulfate (SDS) micelles, LPSs (free and micellar) and whole bacterial cells. The previously characterized labeled variants of GKY20, Luc- and aLuc-GKY20 [70], were used as controls. The excitation and emission spectra of Luc- and aLuc-P13#1 in the presence of SDS, LPSs and bacterial cells (_E. coli_ ATCC 25922, _P. aeruginosa_ PAO1 and _S. aureus_ ATCC 6538P) were very similar to those of Luc- and aLuc-GKY20, respectively, thus demonstrating that the binding properties of GKY20 and P13#1 are comparable. The excitation spectra in the water buffer of the Luc-labeled species show a main peak at 330 nm due to the neutral (phenolic) form of luciferin and a remarkable shoulder at 400 nm due to the phenolate form (Figure S3B in Supplementary Materials and Figure 4). The shoulder disappears when Luc-P13#1 and Luc-GKY20 bind to SDS or LPSs because binding suppresses the ionization of the phenol group (Figure S3B in Supplementary Materials). The suppression of luciferin ionization is also confirmed via the emission spectra after excitation at 430 nm, a wavelength that allows for the selective excitation of the phenolate form. The presence of SDS, LPSs or bacterial cells causes an almost complete disappearance of the fluorescence emission at 540 nm (Figure 5). The interpretation of the emission spectra after excitation of the neutral form at 330 nm (Figure S3 in Supplementary Materials) is more complicated. In an aqueous buffer, Luc-GKY20 and Luc-P13#1 show only the emission at 540 nm caused by the phenolate form (Figure 6). Figure 4: Excitation spectra of Luc-GKY20 and Luc-P13#1 in the presence of SDS or LPS micelles. (**A**) Excitation spectra of Luc-GKY20. (**B**) Excitation spectra of Luc-P13#1. Blue line—spectra recorded in sodium phosphate (NaP); red line—spectra recorded in the presence of SDS; green line—spectra recorded in the presence of LPS. Figure 5: Emission spectra after excitation at 430 nm of Luc-GKY20 and Luc-P13#1 in the presence of SDS, micellar LPSs or bacterial cells. (**A**,**B**) Emission spectra of Luc-GKY20 and Luc-P13#1, respectively, in NaP (blue line), SDS (red line) and LPS (green line). (**C**,**D**) Emission spectra of Luc-GKY20 and Luc-P13#1, respectively, in NaP (blue line) or in the presence of _E. coli_ (red line) _P. aeruginosa_ (green line) and _S. aureus_ (yellow line) cells. In each panel, the emissions in the presence of SDS, LPS and bacterial cells were normalized to the emission in NaP. This is typical of luciferin and is due to a well-known photoinduced deprotonation process (Figure S3 in Supplementary Materials). In the presence of SDS micelles, in addition to the major peak in the green region due to the phenolate, the emission spectra also show a broad peak in the blue region (430-450 nm).