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 2.B.5 The Macrocyclic Polyether (MP) Family
Macrocyclic polyethers, or crown compounds, are neutral synthetic compounds that form stable complexes with alkali metal ions. Because of their hydrophobic (or amphipathic) character, they can transport these ions across biological membranes, thus dissipating ion gradients. Selectivity toward different cations varies with polyether ring size, the optimum being such that the cation just fits into the hole. 

The generalized transport reaction catalyzed by MP family members is:

M+ (in) M+ (out)

References associated with 2.B.5 family:

Eisenman, G., G. Szabo, S.G.A. McLaughlin and S.M. Ciani (1973). Molecular basis for the action of macrocyclic carriers on passive ionic translocation across lipid bilayer membranes. Bioenergetics 4: 93-148. 4717529