REGULATION OF ION RECOGNITION BY UTILIZING INFORMATION AT THE MOLECULAR-LEVEL

The regulation of molecular functions utilizing information at the mol ecular level is important and necessary to elucidate the mechanisms of allostery, cooperativity, feed-back, etc, in biological systems and t o regulate biological molecular systems by artificial functionalized m olecules responsive to such information, In particular, ion recognitio n among molecular functions has drawn much attention, because many enz ymatic activities and the transport of stimuli in nervous systems are controlled by ion binding. Thus, in this article, we describe very eff icient strategies and examples to regulate the ion recognition of arti ficial systems utilizing a heavy metal ion, an electron, and a small o rganic molecule as an external effector, The first strategy is to prod uce a pseudocrown ether. A heavy metal ion is used as an effector. Com plexation of a linear polyether bearing the metal. binding sites at th e two termini gives the corresponding cyclic compound (pseudocrown eth er). This methodology is very effective to control alkali metal recogn ition. This concept is applied to a pseudocryptand and a pseudothiacro wn ether, which is a nice double recognition system for heavy metal io ns. In addition, complexation with a heavy metal ion is used for regul ation of molecular recognition. Secondly, redox reactions between thio l and disulfide are employed for regulation of ion recognition. Confor mational change and/or change of spatial arrangement of binding site a re useful for the regulation. However, these methods are not sufficien t to construct perfect ah-or-none type control. The ideal regulation i s successfully performed by crown ethers with a redox gate in the bind ing site for metal ions. The gate responds to redox reactions between thiol and disulfide to afford an open and a closed state. The open sta te provides a remarkably selective binding site for Ag(I). The Ag(I) s electivity is considered to result from synergistic coordination of su lfur and oxygen atoms. This is a general binding mode for the high Ag( I) selectivity of crown ethers containing sulfur atom(s). The third st rategy to modulate ion recognition is molecular assembly using a recep tor which has hydrogen bonding sites. A new binding site of the molecu lar assembly for alkali metal ion is formed from several polyether cha ins each of which does not exhibit binding ability toward metal ions.