Artificial Cells Allow Ion Entry

Supramolecular Chemistry

Chemists in germany have constructed artificial cells with stoppered pores that open for the uptake of calcium ions and then close. The process resembles the functioning of gated calcium channels in biological cell membranes.

The cells-spherical nanoscale polyoxomolybdate clusters-were constructed by Achim Mller and coworkers at the University of Bielefeld (Angew. Chem. Int. Ed., published online Nov. 8, dx.doi.org/10.1002/anie.200502202). Each cell has 20 Mo₉O₉ pores that are noncovalently bonded to protonated urea ions that act as stoppers.

The Bielefeld team added Ca²⁺ ions to an aqueous solution of the molybdate capsules and examined the precipitated product by X-ray crystallography. The analysis revealed not only that Ca²⁺ had entered the capsules but also that the urea stoppers had closed the pores after ion entry.

The presence of a large number of added cations, like Ca²⁺, in the vicinity or at the surface of the highly charged capsules decreases the electrochemical gradient across the artificial cell membrane, Mller explains. As a result, the affinity for the positively charged urea guests decreases and the guests are released. The uptake of ions also results in a cell response in the sense that the structure of the encapsulated water/electrolyte assembly changes, he adds.

We now intend to study systematically the interaction of our artificial cells with their environments and investigate how the properties of the cells can be tuned by changing their charges and internal clothes,' as well as the strength of the interactions of the stoppers in the pores, Mller says. This type of investigation could yield information about the functioning of cell membrane calcium channels in physiological processes and also provide insights into counterion transport of pathological relevance.