Hiroshi Tsukube

Recognition and sensing of chiral biological substrates via lanthanide coordination chemistry

A series of lanthanide tris(β-diketonates) and porphyrinates was developed as effective receptors which offered chiral recognition and chirality sensing of biological substrates. These lanthanide complexes were electrically neutralized by coordinating ligands but further formed highly coordinated complexes with various substrates. We optimized the combination of coordinating ligand and lanthanide center to realize specific recognition and sensing of chiral amino alcohols and amino acids. Two further approaches were successful: (1) substitution of lanthanide tris(β-diketonates) with chiral ligands provided efficient enantiomer-selective extraction of zwitterionic amino acids; and (2) functionalization of lanthanide porphyrinates with highly structured functions enhanced the sensitivity in CD probing of chiral amino acids. Since the lanthanide complexes have broad structural variations, the molecular recognition phenomena described here offer promising possibilities in developing a new class of chiral recognition and chirality sensing systems incorporating intelligent lanthanide complexes.

Mechanical Tuning of Molecular Recognition To Discriminate the Single-Methyl-Group Difference between Thymine and Uracil

Construction of enzyme-like artificial cavities is a complex and challenging subject. Rather than synthesizing complicated host molecules, we have proposed mechanical adaptation of relatively simple hosts within dynamic media to determine the optimum conformation for molecular recognition. Here we have applied this concept to one of the most challenging biomolecular recognition problems, i.e., that of discriminating thymine from uracil. We synthesized the novel cholesterol-armed triazacyclononane as a host molecule and subjected it to structural tuning by compression of its Langmuir monolayers in the absence and in the presence of Li(+) cations in the subphase. Experimental results confirm that the monolayer of triazacyclononane host selectively recognizes uracil over adenine (ca. 7 times based on the binding constant) and thymine (ca. 64 times) under optimized conditions ([LiCl] = 10 mM at surface pressure of 35 mN m(-1)). The concept of mechanical tuning of a host structure for optimization of molecular recognition offers a novel methodology in host-guest chemistry as an alternative to the more traditional molecular design strategies.

Anion receptor functions of lanthanide tris(β-diketonate) complexes: naked eye detection and ion-selective electrode determination of Cl− anion

Lanthanide tris(fluorinated beta-diketonates) acted as effective receptors of Cl- anion in luminescence sensing and ion-selective electrode systems via highly coordinated complexation.

THIACROWN ETHER AS REGULATOR OF LIPASE-CATALYZED TRANS-ESTERIFICATION IN ORGANIC MEDIA : PRACTICAL OPTICAL RESOLUTION OF ALLYL ALCOHOLS

Abstract Thiacrown ether additive enhanced enantioselectivity in lipase-catalyzed trans-esterification of allyl alcohols. Small amounts of thiacrown ether offered highly enantioselective reaction when 5-phenylpentene-3-ol was subjected to the reaction of Pseudomonas cepacia lipase.

A Chemical Device That Exhibits Dual Mode Motions: Dynamic Coupling of Amide Coordination Isomerism and Metal-Centered Helicity Inversion in a Chiral Cobalt(II) Complex

Dynamic and consecutive molecular motions such as stretching, winding, and rotation are observed in nature. The ATP-driven F1 part of ATP synthase and the bacterial flagellar motor are typical examples, in which some external stimuli kick-off such events through conformational changes of biopolymers. Several molecular machines such as molecular rotors, gears, and shuttles have recently been developed, in which metal-coordination linkage isomerizes dynamically to offer single mode motion. Since the planar amide linkage (-CO-NH-) has two preferred structures (cis– trans isomers) and two different metal coordination modes (O-coordination and N-coordination), its isomerism is often used to alter the three-dimensional structures of biological proteins. Herein, we develop a chemical device based on a chiral Co complex that exhibits dual mode motions. The ligand employed here (H2L1) includes 2,5-dimethoxy benzene moieties attached through amide linkages to both terminals of a helical tetradentate ligand. The acid– base reaction of the corresponding cobalt complex triggered the interconversion of coordinating atoms between amide nitrogen atoms and amide oxygen atoms, giving rise to a stretching (extension/contraction) molecular motion. Since we previously demonstrated that the helicity of the Co complex with H2L2 was dynamically inverted from the L cis-a form to the D cis-a form by adding achiral NO3 ions, the employed H2L1-Co II complex was designed to work as a novel type of molecular machine that exhibits coupled stretching and inverting motions. Several types of helical ligands have shown extension/contraction molecular motion on metal complexation/decomplexation and/or protonation/deprotonation, but the present type of kinetically labile Co complex allows a dual molecular motion in a highly dynamic fashion, as would be required for a sophisticated supramolecular switching device. As established for the H2L2-CoACHTUNGTRENNUNG(ClO4)2 complex, [4] X-ray analysis of the pink-colored H2L1-Co ACHTUNGTRENNUNG(CF3SO3)2 complex [9,10] and its solid-state CD studies revealed that the complex had a L cis-a coordinated structure, in which two amine nitrogen atoms, two amide oxygen atoms, and two donors from solvent molecules and/or counter anions coordinated (see extended L-form in Figure 1, middle). F NMR and IR studies in acetonitrile/chloroform (1/9) indicated that one CF3SO3 ion coordinated to the Co center, and one CF3SO3 ion remained noncoordinated, as observed in the solid state. Its diastereomeric excess (de) value in the solution was determined to be above 95% on the basis of paramagnetic H NMR spectra. A green-colored Co complex was isolated by mixing H2L1 and Co ACHTUNGTRENNUNG(ClO4)2·6H2O in the presence of two equivalents of (C2H5)3N, and showed a characteristic CD spectrum in CH3CN (see contracted L-form in Figure 1, left). Two amine nitrogen, two amido nitrogen, and two methoxy oxygen atoms from the ligand coordinated to the Co center, to give overall a distorted octahedral geometry with contracted left-handed helical structure (L4D2 absolute configuration between the skew chelate pairs). In the H NMR spectrum recorded in CD3CN/CDCl3 (1/9), all signals appeared in the region d= 70 to 110 ppm with C2-symmetric patterns (see Figure S5 in the Supporting Information). Since no significant signals for the minor diastereomeric isomer were observed, it appears that this complex retains the contracted helical structure in the solution. The [Co(L1)] complex exhibited positive CD signals at 433 and 918 nm and negative signals at 474, 607, and 1100 nm in CH3CN/CHCl3 (1/9; Figure 2, *), which has a pattern similar to that observed in the solid-state CD spectrum (see Fig[a] Prof. H. Miyake, M. Hikita, Dr. M. Itazaki, Prof. H. Nakazawa, Dr. H. Sugimoto, Prof. H. Tsukube Department of Chemistry, Graduate School of Science Osaka City University, 3-3-138, Sumiyoshi-ku Osaka 558-8585 (Japan) Fax: (+81)6-6605-2522 E-mail : miyake@sci.osaka-cu.ac.jp Supporting information for this article is available on the WWW under http://www.chemistry.org or from the author.

Dendrimer container for anion-responsive lanthanide complexation and “on–off” switchable near-infrared luminescence

A new dendrimer-type ligand dynamically switched the lanthanide complexation and luminescence profiles in response to external anions.

Combinatorial screening of a lanthanide complex library for luminescence sensing of amino acids

A lanthanide complex library including 196 combinations of N-heteroaromatic ligands, luminescent lanthanide centers and amino acid substrates was prepared to develop visible and near-infrared luminescent sensory systems for a series of amino acids.

Asymmetric Twisting and Chirality Probing Properties of Quadruple-Stranded Helicates: Coordination Versatility and Chirality Response of Na+, Ca2+, and La3+ Complexes with Octadentate Cyclen Ligand

A series of Na(+), Ca(2+), and La(3+) complexes with octadentate cyclen ligand were prepared and structurally characterized in the crystal and solution states. The employed cyclen ligand formed 6-, 7-, and 9-coordinated, crystalline complexes with Na(+), Ca(2+), and La(3+) cations, respectively, in which the parent cyclen ring and quinoline-functionalized side arms were cooperatively coordinated. These three metal cations provided the quadruple-stranded helicates in CH(3)CN-C(2)H(5)OH solutions. In each helicate, four quinoline-functionalized side arms were arranged in a propeller-like fashion to yield an enantiomer-pair of Delta- and Lambda- forms. Addition of a chiral anion to the cyclen-Ca(2+) complex solution induced circular dichroism (CD) signals around the quinoline chromophore, which indicated that 1:1 diastereomeric complexation between the Ca(2+) complex and the chiral anion imposed the stereoisomeric equilibrium. The intensity and sign of the observed CD signal were significantly dependent on both the absolute configuration and the enantiomeric purity of the added anion. The corresponding cyclen-Na(+) complex rarely induced a CD signal, while the La(3+) complex exhibited complicated anion-induced spectral changes. Thus, the octadentate cyclen ligand employed was demonstrated to form the quadruple-stranded helicate with the Ca(2+) cation in the solution state, which functioned as an effective CD probe for the determination of enantiomer excess (ee%) of the chiral anions.

Helix Architecture and Helicity Switching via Dynamic Metal Coordination Chemistry

A variety of metal complex helicates have been developed for helix architecture and helicity switching via ligand optimization. Among them, kinetically labile metal helicates offered dynamic inversion of complex helicity induced by external stimuli. They are promising candidates for excellent supramolecular devices based on dynamic metal coordination chemistry.

Experimental and Theoretical Approaches Toward Anion‐Responsive Tripod–Lanthanide Complexes: Mixed‐Donor Ligand Effects on Lanthanide Complexation and Luminescence Sensing Profiles

A new series of tripods were designed to form anion-responsive, luminescent lanthanide complexes. These tripods contain pyridine, thiazole, pyrazine, or quinoline chromophores combined with amide carbonyl oxygen and tertiary nitrogen atoms. Crystallographic and EXAFS studies of the 10-coordinated tripod-La(NO(3))(3) complexes revealed that each La(3+) cation was cooperatively coordinated by one tetradentate tripod and three bidentate NO(3)(-) anions in the crystal and in CH(3)CN. Quantum chemical calculations indicated that the aromatic nitrogen plays a significant role in lanthanide complexation. The experimentally determined stability constants of complexes of the tripod with La(NO(3))(3), Eu(NO(3))(3), and Tb(NO(3))(3) were in good agreement with the theoretically calculated interaction energies. Complexation of each tripod with lanthanide triflate gave a mixture of several lanthanide complex species. Interestingly, the addition of a coordinative NO(3)(-) or Cl(-) anion to the mixture significantly influenced the lanthanide complexation profiles. The particular combination of tripod and a luminescent Eu(3+) center gave anion-selective luminescence enhancements. Pyridine-containing tripods exhibited the highest NO(3)(-) anion-selective luminescence and thus permit naked-eye detection of the NO(3)(-) anion.