Geometry optimization and vibrational analysis using normal mode analysis of the EBn:M z+ complexes was performed with the functional BP86 in Gaussian16 . Based on related studies on complexes of (alkali) metals and organic molecules, C, N, O and H atoms were treated using the 6-31g* basis set, and the alkaline and alkaline earth metal ions were described using def2-TZVP basis set together with its core potential for Cs + . In addition n-pentadecane was specified as solvent in a polarizable continuum model to mimic the hydrophobic parts of the membrane. Initial structures were based on crystal structures of belted 1:1 and 2:1 sandwich complexes with Na + and K + from previous studies (QQQGMD and IHECUT in the Cambridge Structure Databank ). Infrared spectra were determined through normal mode analysis and are displayed without corrections using scaling factors; ester and amide C=O stretching modes are shown using a band width (FWHM) of 8 and 16 cm -1 to match the experimental spectral shapes. Optimized structures are deposited under DFT.