13 December 2005: Molecular guests stay at the gates

Researchers have simulated the gating of ion channels in the cell with purely inorganic porous nano-capsules built from molybdenum oxide modules. 

Achim M?ller and colleagues at the University of Bielefeld, Germany, have created a variety of nanosized constructs from molybdenum oxide building blocks. Recently, they demonstrated a hollow sphere with a chemically well-defined set of 20 entry channels.¹ Now they show that the import of cations into the sphere can be controlled by the charge gradient across the shell of the construct, making the system comparable to a voltage-gated ion channel.² 

’We were very much interested in pore-gating processes for a long time,’ says M?ller, but success came only after his group crystallised the porous capsule in the presence of high concentrations of praseodymium salts. As they had lined the inside of the capsule with negative charges, the researchers expected the Pr³⁺ions - chosen because of their high charge and high electron density - to enter through the pores and occupy most of the 30 binding sites inside the capsule. 

However, the crystal structure of the complex revealed that only five Pr³⁺ions enter the capsule. Further ions remain just outside the pores, binding to them in a specific coordination with water molecules. M?ller thinks he knows why the guests stay at the gates: ’With sufficient uptake of the highly positively charged Pr³⁺ions, the electrochemical gradient approaches zero, and the pores are closed,’ he suggested.  

He hopes to establish this system as a ’tunable artificial cell’ for studies of processes like ion transport. ’This type of investigation could definitely give information about fundamental principles of transport through nanochannels in general, and in biological systems,’ M?ller concluded. Michael Gross