Enniatins alkali metal complexes

The mechanistic problems associated with the transport of cations (especially Na and K+) through membranes have continued to receive attention. The structures of macrocyclic alkali-metal complexes and the thermodynamics of their formation have been reviewed, as has the general question of the selectivity of the carriers towards the metals. The role of hydration energy has been considered and it has been suggested that, at least in the case of the enniatin ionophores, sandwich... 

The alkali metal cation complexes of compounds of the valino-mycin group (valinomycin, enniatins, macrotetrolides, beauveridn, antamanide) are positively charged. 

The hexapeptide, cycb( L-Pro-Gly)3, has been shown to form complexes with a number of metal ions175 The compound exhibits ion selectivity for Li+ and Na+ over K+ and larger alkali metal ions. It also forms a Ca++ complex which has a stability constant in acetonitrile of stab = 1.1 x 10s M 1. With Mg++ three different complexes with cyclopeptide-cation stoichiometries of 2 1,1 1, and 1 2 are formed. Hypothetical structures of these complexes (Fig. 45) have been proposed which are reminiscent of the enniatin sandwich complexes. 

The compounds form 1 1 complexes with all alkali ions and with a large number of other metal ions in organic solvents267,268,307. Among the alkali ions, K+ is bound most strongly, but K+-complex stability as well as K+ > Na+ selectivity are much less pronounced than with valinomycin. In concentrated enniatin solutions, non-equimolar complexes are formed. NMR- und CD-spectroscopic titrations have demonstrated that 2 1 enniatin-cation complexes are formed with a stability order of K+ > Cs+ > Na+. Evidence for formation of a 3 2 complex with Cs+ was also obtained133,216 ... 

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