We report that cation-selective transmembrane pores formed by synthetic p-octiphenyl ß barrels with internal aspartate residues can be transformed into anion-permeable metallopores with internal Mg2+-aspartate complexes. These metallopores are shown to be useful for fluorimetric sensing of a broad variety of organic anions of biological relevance such as phytate, heparin, thiamine phosphates, and adenosine triphosphate. The negligible flippase activity measurable for Mg2+-free pores indicates that transmembrane p-octiphenyl barrels do not disturb the lipid bilayer suprastructure, in other words, they form barrel-stave rather than toroidal pores.