Ovchinnikov et al. showed that K + binds EB as EB2:K + sandwich complexes in organic solvents. Based on these results it was suggested that Na + and Ca 2+ adopt similar structures, as similar peak positions of 1746-1744/1666-1665 cm -1 were observed with the ions for the ν(COester)/ν(COamide) modes. Comparing these previous FT-IR results obtained from acetonitrile solutions with the present SEIRA spectra of EB in tBLMs, we note similar peak positions of 1744/1666 cm -1 in presence of K + , but not for Na + and Ca 2+ (Table ). As discussed above and previously , amide carbonyls are thermodynamically preferred ligands, so that in a 2:1 stoichiometry their vibrational frequency is highly sensitive to the carbonyl-metal interaction, while the ester CO modes report on the long-range electrostatics originating from the metal ion and the local electrostatics due to solvation, according to the vibrational Stark effect. In this way, for K + , a similar ligation pattern in acetonitrile and in tBLMs can be deduced from the match of amide frequency, whereas the 3 cm -1 difference of the ester vibration can be rationalized based on a solvatochromic blue-shift due to the more polar environment of acetonitrile in contrast to the membrane core. As such, we can assign the major species observed in tBLMs in presence of K + to a membrane-incorporated EB2:K + sandwich complex (see summarizing Figure ). This is consistent with our DFT-based spectra, in which the EB2:K + showed the most blue-shifted peak position (amide mode at 1656 cm -1 ) of all complexesthe remaining 10 cm -1 offset between experiments and DFT is acceptable taking into account the level of theory used herein. Our assignment is further consistent with SEIRA signals of EB in presence of K + showed band shapes that were modelled with one dominating Gaussian peak (red peaks in Figure ). Consequently, the additional minor peaks (blue peaks in Figure ) can be assigned to small contributions of EB:K + out residing at the membrane interface (Figure ).