(17), and an E m ϭ -790 mV (18) was estimated for the 1ϩ͞0 couple, which is in line with E m estimates of all-ferrous Fe-S clusters by using discrete Fourier transform calculations (19). Indeed, this latter study raised the question of whether [Fe 4 S 4 ] 0 Av2 can even be made in vivo (1). A value of -790 mV is not consistent with turnover potential measured by Ti(III) and other reductants and with reported potentials for the Ti(IV)͞Ti(III) couple (7,20). If an E m ϭ -460 mV for the 1ϩ͞0 couple (16)

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Macrocyclic ethers are used extensively for their ability to form strong and selective interactions with charged species. It has been shown that the incorporation of heterocyclic groups into macrocylclic ethers increase rigidity and interaction with certain ions. Alkylation of these azacrowns with benzylic halides can attach fluorionophoric substituents onto the ring. One of the molecules used for this alkylation is 5-chloro-8-methoxy-2-͑bromomethyl͒quinoline. Azacrowns with this alkyl ligand have displayed interesting ion complexation properties.