Concerning the relative non-toxicity of silacrown ionophores

Silacrowns, ionophoric materials in which an ethylene bridge of a normal crown ether has been replaced by a R2Si group, facilitate ion transport across liquid membranes. A major aspect of their properties is their nontoxicity when compared to these crown ethers. This is probably due to their ready hydrolysis to the corresponding polyethylene glycols. The glycols also exhibit ion transport properties, even with an efficiency close to the silacrowns but the silacrowns stay intact long enough to partition into the hydrophobic phase of a liquid membrane whereas their hydrolysis products preferentially partition into the aqueous phases. Ready removal from the hydrophobic phase of a membrane will reduce the long term transporting effectiveness of the glycols. Thus, in vivo hydrolysis of the silacrowns will result in the loss of transmembrane carriers. Similar removal will not occur for normal crown ionophores which are both hydrolytically stable and partition into the hydrophobic phase, and remain there for long time periods.