Two important control experiments were done to exclude the possibility of inter-and intramolecular PT between the D and the A in the absence of a bridge. To exclude intermolecular PT, we performed a D vs A titration in MeOH, and no deprotonation/protonation of D/A was observed even at high concentrations of A (Figure ). To exclude intramolecular PT without the peptide bridge, we covalently connected the D and A UAAs (SI section 2.5 and Scheme S5) where no deprotonation/protonation of D/A was observed in MeOH or CHCl3 (Figure and). At this stage, it is essential to note that, while having both the D and A in the system, the fluorescence measurements are primarily indicative of the D and are 'blind' to the A. This is due to the overlap in peak positions of the D and A fluorescence spectra (Fig. and) and the much lower quantum yield of the quinoline A with respect to the coumarin D . Next, we followed PT across our two synthesized peptide bridges, (EA)4 and (A)8. To test our hypothesis and the role of the PMF in the PT process, we compared the results of a D-B-A system having the PMF to a system composed of only D-B. As discussed, water is an excellent proton acceptor/donor solvent. Accordingly, a plausible outcome is a fast PT between water and D/A, with no role of the bridge. However, we found a clear change in the fluorescence spectra even in water but only when having the D-B-A system (Figure ), indicating a larger proportion