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The cationic ZnII part of aqua(1,4,7,10-tetraazacyclododecane)zinc(II) bis(perchlorate), [Zn(C8H20N4)(H2O)](ClO4)2, exhibits a slightly distorted square-pyramidal coordination environment with a water molecule in the apical position. In the crystal, the macrocyclic ring alternates between two conformations with equal occupancies. Two of the three perchlorate anions are situated about a twofold rotation axis, and one of them shows disorder of the O atoms with occupancies of 0.62 (7) and 0.38 (7). In the crystal, the complexes are connected by intermolecular hydrogen bonding via the perchlorate anions.

Thiolate coordination to zinc(II) ions occurs widely in such functional biomolecules as zinc enzymes or zinc finger proteins. Here, we introduce a simple method for determining the affinity of ligands toward the zinc-enzyme active-center model tetramethylrhodamine (TAMRA)-labeled 1,4,7,10-tetraazacyclododecane (cyclen)–zinc(II) complex (TAMRA–ZnL). The 1 : 1 complexation of TAMRA-labeled cysteine (TAMRA–Cys) with TAMRA–ZnL (each at 2.5 μM), in which the TAMRA moieties approach one another closely, induces remarkable changes in the visible absorption and fluorescence spectra at pH 7.4 and 25 °C. The 1 : 1 complex formation constant (K = [thiolate-bound zinc(II) complex]/[uncomplexed TAMRA–ZnL][uncomplexed TAMRA–Cys], M−1) was determined to be 106.7 M−1 from a Job's plot of the absorbances at 552 nm. By a ligand-competition method with the 1 : 1 complexation equilibrium, analogous K values for thiol-containing ligands, such as N-acetyl-L-cysteine, L-glutathione, and N-acetyl-L-cysteinamide, were evaluated to have similar values of about 104 M−1. As a result of the ligand affinities to TAMRA–ZnL, nonlabeled zinc(II)–cyclen induced remarkable stabilization of the reduced form of L-glutathione and a cysteine-containing enolase peptide to aerial oxidation in aqueous solution at pH 7.4 and 25 °C.

A simple and efficient method based on magnetic-bead technology has been developed for the enrichment of thiol-containing biomolecules, such as l-glutathione and cysteine-containing peptides. The thiol-binding site on the bead is a mononuclear complex of zinc(II) with 1,4,7,10-tetraazacyclododecane (cyclen); this is linked to a hydrophilic cross-linked agarose coating on a particle that has a magnetic core. All steps for the thiol-affinity separation are conducted in aqueous buffers with 0.10 mL of the magnetic beads in a 1.5 mL microtube. The entire separation protocol for thiol-containing compounds, from addition to elution, requires less than one hour per sample, provided the buffers and the zinc(II)-cyclen-functionalized magnetic beads have been prepared in advance. The thiol-affinity magnetic beads are reusable at least 15 times without a decrease in their thiol-binding ability, and they are stable for six months at room temperature.

Cysteine-containing biomolecules are attractive targets in the study of thiol biology. Here we introduce a novel method for the selective enrichment of thiol-containing molecules using a thiol-capture zinc(II) complex of 1,4,7,10-tetraazacyclododecane (Zn 2+ –cyclen). Recognition of N -acetylcysteine amide by Zn 2+ –cyclen has been studied by potentiometric pH titration, revealing formation of a 1:1 thiolate-bound Zn 2+ –cyclen complex with a large thiolate-affinity constant of 10 6.2 M −1 at 25 °C and I = 0.10 M (NaCl). The Zn 2+ -bound thiolate anion is unexpectedly stable in aqueous solution at pH 7.8 under atmospheric conditions for a few days. These findings have contributed to the development of a convenient method for separation of thiol compounds by using a micropipette tip. A 200 μL micropipette tip containing 10 μL of hydrophilic cross-linked agarose beads attached to Zn 2+ –cyclen moieties was prepared. All steps for thiol-affinity separation (binding, washing, and eluting) are conducted using aqueous buffers at room temperature. The entire separation protocol requires less than 15 min per sample. We demonstrate practical example separations of cysteine-containing molecules. This micropipette tip method would be used preferentially as an alternative to existing tools for reliable enrichment of thiol-containing molecules.

A dinuclear zinc(II) complex, m,m-bis(Zn2+–cyclen) was synthesized as a novel host molecule for a malonate dianion. The dizinc(II) complex has two zinc(II)–cyclen moieties (cyclen = 1,4,7,10-tetraa...

A new bis-zinc(II) receptor (Zn2L), which has two macrocyclic 12-membered tetraamine (cyclen) ZnII complexes connected through a p-xylene bridge, has been synthesized as a novel host molecule to recognize barbiturates (such as barbital (bar)) in aqueous solution. Each of the zinc(II) ions in the biszinc(II) receptor was originally intended to match the dianionic barbital anion (bar2−) with supplementary hydrogen bonds between the cyclen NH's and the three carbonyl oxygens in complementary positions to yield a 1:1 complex, Zn2L–bar2−. From an aqueous solution of equimolar Zn2L and barbital at pH 8, however, a cyclic 2:2 complex, (Zn2L-bar2−)2, was isolated and characterized by X-ray crystal analysis. The NMR study in 10% (v/v) D2O/H2O has revealed dissociation of (Zn2L–bar2−)2 solely into the original target 1:1 complex Zn2L–bar2− and established the dimerization constant for 2Zn2L–bar2− → (Zn2L–bar2−)2, Kd ( = [(Zn2L–bar2−)2]/[Zn2L–bar2−]2) to be 103.4 M−1. The thermodynamic parameters were evaluated from the NMR measurements at 25, 35, 45, and 55°C: ΔG = –1.9 × 104 J mol−1, ΔH = – 3.3 × 104 J mol−1, ΔS = – 49 J mol−1 K−1 at 25°C. Potentiometric pH titration of Zn2L (1 mM) and barbital (1 mM) disclosed extremely facile deprotonation of the two imido groups of barbital at pH less than 7 to form the dianionic barbital-bound ZnII complexes Zn2L–bar2− and (Zn2L–bar2−)2, where-by the barbital binding affinity for Zn2L was estimated to be Kbar ( = [Zn2L–bar2−]/[uncomplexed Zn2L][uncomplexed barbital]) = 105.8 M−1 at pH 8 and 25°C with I = 0.10 (NaNO3). The significance of the bis-zinc(II) receptor in stabilizing the dianionic barbital is evident by comparison with the interaction of ZnII–cyclen complex (ZnL) with barbital, which yields only a 1:1 monoanionic barbital complex, ZnL–bar− (Kbar = [ZnL–bar−]/[uncomplexed ZnL][uncomplexed barbital] = 104.2M−1 at pH 8 and 25°C with I = 0.10 (NaNO3)).

1, 1'-(m-Xylenediyl)-bis (1, 4, 7, 10-tetraazacyclododecane)-ZnII2 complex (m-xylenediyl-bicyclen-ZnII2), a potent inhibitor of human immunodeficiency virus (HIV), was obtained from cyclen by a combination of dimerization and metal complexation. The ratio of median cytotoxic concentration against host cells (CC50) and median effective concentration against HIV cytopathogenicity (EC50), referred to as the selectivity index (SI), was regarded to be a measure of anti-HIV activity. These two chemical modifications contributed to a potent, in vitro anti-HIV activity of m-xylenediyl-bicyclen-ZnII2 by respectively increasing the CC50 and decreasing the EC50 in comparison with those of cyclen.

A new 2-(9-anthrylmethylamino)ethyl-appended cyclen, L(3) (1-(2-(9-anthrylmethylamino)ethyl)-1,4,7,10-tetraazacyclododecane) (cyclen = 1,4,7,10-tetraazacyclododecane), was synthesized and characterized for a new Zn(2+) chelation-enhanced fluorophore, in comparison with previously reported 9-anthrylmethylcyclen L(1) (1-(9-anthrylmethyl)-1,4,7,10-tetraazacyclododecane) and dansylamide cyclen L(2). L(3) showed protonation constants log K(a)(i)() of 10.57 +/- 0.02, 9.10 +/- 0.02, 7.15 +/- 0.02,2, and2. The log K(a3) value of 7.15 was assigned to the pendant 2-(9-anthrylmethylamino)ethyl on the basis of the pH-dependent (1)H NMR and fluorescence spectroscopic measurements. The potentiometric pH titration study indicated extremely stable 1:1 Zn(2+)-L(3) complexation with a stability constant log K(s)(ZnL(3)) (where K(s)(ZnL(3)) = [ZnL(3)]/[Zn(2+)][L(3)] (M(-)(1))) of 17.6 at 25 degrees C with I = 0.1 (NaNO(3)), which is translated into the much smaller apparent dissociation constant K(d) (=[Zn(2+)](free)[L(3)](free)/[ZnL(3)]) of 2 x 10(-)(11) M with respect to 5 x 10(-)(8) M for L(1) at pH 7.4. The quantum yield (Phi = 0.14) in the fluorescent emission of L(3) increased to Phi = 0.44 upon complexation with zinc(II) ion at pH 7.4 (excitation at 368 nm). The fluorescence of 5 microM L(3) at pH 7.4 linearly increased with a 0.1-5 microM concentration of zinc(II). By comparison, the fluorescent emission of the free ligand L(1) decreased upon binding to Zn(2+) (from Phi = 0.27 to Phi = 0.19) at pH 7.4 (excitation at 368 nm). The Zn(2+) complexation with L(3) occurred more rapidly (the second-order rate constant k(2) is 4.6 x 10(2) M(-)(1) s(-)(1)) at pH 7.4 than that with L(1) (k(2) = 5.6 x 10 M(-)(1) s(-)(1)) and L(2) (k(2) = 1.4 x 10(2) M(-)(1) s(-)(1)). With an additionally inserted ethylamine in the pendant group, the macrocyclic ligand L(3) is a more effective and practical zinc(II) fluorophore than L(1).