Artificial Cells
Porous molybdenum oxide capsules serve as models for biological ion-transport processes
A typical example of biological signaling processes in living cells is a controlled ion flow through special channel proteins in the cell membrane. This can be controlled through the binding of a suitable ligand or by the electrochemical potential across the cell membrane, so ultimately by the difference in concentration of ions inside and outside the cell. Calcium ions (Ca2+) play an important role in many biological functions. For this reason, Müller et al. chose to use Ca2+ for their further experiments. They added Ca2+ ions to an aqueous solution of the molybdate capsules and examined the resulting crystals by X-ray crystal structure analysis, which revealed that not only did the calcium ions wander into capsules but that the urea stoppers were also back in place inside the Mo 9O9 pores.
This behavior of the artificial cell mirrors events that unfold in a voltage-gated ion channel in a living cell. Initially, the pores are closed. When an excess of Ca2+ ions is added, their positive charges cancel out the negative charges on the surface of the sphere which changes the electrochemical gradient across the artificial cell membrane. The lids on the pores open, allowing Ca2+ ions to flow into the capsule. This possibly changes the charge distribution across the artificial cell membrane again such that the pores close up.
Original publication: A. Müller; "Porous Capsules Allow Pore Opening and Closing That Results in Cation Uptake"; Angewandte Chemie International Edition 2005.
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