Enniatins are cyclohexadepsipeptides that are composed of an alternating structure of three hydrophobic Nmethylated amino acids (such as valine, leucine or isoleucine) and three hydroxyl acids (typically hydroxyvaleric acid) forming a six-unit ring structure. To date, approximately 29 variants have been found and further biosynthesized to investigate their biomedical applications, with the valine-containing enniatin B (EB) being one of the most prevalent, active and best studied example (Figure ). Due to the hydrophobicity of the sidechains, EB and other enniatins can incorporate into cell membranes, whereas demonstrated by electrophysiological experimentsthey increase the transmembrane flux of monoand divalent cations similar to beauvericin and valinomycin. Furthermore, it was found that EB has a preference of mono-over divalent cations and a slightly pronounced specificity for K + over Na + ions according to the conductivity series K + > Ca 2+ ≥ Na + > Mg 2+ > Li + . Interestingly, it has been suggested that this ion flux is accomplished by the existence of several distinct EBn:M z+ complexes in the lipid membrane with different stoichiometries. Based on the possible coordination of cations via the three amide carbonyl groups and the three ester carbonyl groups several distorted octahedral EBn:M z+ complexes have been proposed: (a) a 1:1 complex where the metal ion is positioned in the middle of the EB ring (Figure ), (b) a "sandwich" 2:1 complex with the metal cation between two EB rings (Figure ), and (c) a much less likely "double-sandwich" 3:2 complex in which one EB ring is located between two metal ions that are enclosed by two EBs. While in the 1:1 complex all carbonyls of one EB molecule are involved in coordination, in 2:1 and 3:2 complexes either amides or esters (all amide and ester carbonyls are pointing towards opposite sides of the ring, see Figure ) can contribute to coordination. It was discussed previously that amide carbonyls are preferable ligands, so that, e.g., amide-amide-coordinated 2:1 complexes are predominant, which, in principle, makes the existence of 3:2 complexes in general less likely. Experimental studies of these potential complexes in organic solvents supported such a situation while crystallography of an admixture of EB and K + SCN -suggests in general that both coordinations (ester and amide) are viable in a crystal. However, even though these studies provided initial knowledge on the structure of EBn:M z+ complexes in various environments, much knowledge is missing regarding EB's cation-dependent mechanism of incorporation and its structure-function relationship in lipid membranes, where it exerts its natural bioactive function. This understanding is essential to guide the design of more active derivatives.