Regulation of ion recognition by utilizing information at the molecular level

The regulation of molecular functions utilizing information at the molecular level is important and necessary to elucidate the mechanisms of allostery, cooperativity, feed-back, etc, in biological systems and to regulate biological molecular systems by artificial functionalized molecules responsive to such information, In particular, ion recognition among molecular functions has drawn much attention, because many enzymatic activities and the transport of stimuli in nervous systems are controlled by ion binding. Thus, in this article, we describe very efficient strategies and examples to regulate the ion recognition of artificial systems utilizing a heavy metal ion, an electron, and a small organic molecule as an external effector, The first strategy is to produce a pseudocrown ether. A heavy metal ion is used as an effector. Complexation of a linear polyether bearing the metal. binding sites at the two termini gives the corresponding cyclic compound (pseudocrown ether). This methodology is very effective to control alkali metal recognition. This concept is applied to a pseudocryptand and a pseudothiacrown ether, which is a nice double recognition system for heavy metal ions. In addition, complexation with a heavy metal ion is used for regulation of molecular recognition. Secondly, redox reactions between thiol and disulfide are employed for regulation of ion recognition. Conformational change and/or change of spatial arrangement of binding site are useful for the regulation. However, these methods are not sufficient to construct perfect ah-or-none type control. The ideal regulation is successfully performed by crown ethers with a redox gate in the binding site for metal ions. The gate responds to redox reactions between thiol and disulfide to afford an open and a closed state. The open state provides a remarkably selective binding site for Ag(I). The Ag(I) selectivity is considered to result from synergistic coordination of sulfur and oxygen atoms. This is a general binding mode for the high Ag(I) selectivity of crown ethers containing sulfur atom(s). The third strategy to modulate ion recognition is molecular assembly using a receptor which has hydrogen bonding sites. A new binding site of the molecular assembly for alkali metal ion is formed from several polyether chains each of which does not exhibit binding ability toward metal ions.