Masako Takani

Metal–amino acid chemistry. Weak interactions and related functions of side chain groups

α-Amino acids are highly functional small molecules whose side chain groups are like prototypes of metal coordination and weak interactions in proteins. This perspective focuses on non-covalent or weak interactions of the side chain groups of amino acids, such as the charged groups of arginine and aspartic acid and the aromatic rings of tyrosine and tryptophan, in metal complexes in solution and in the solid state. The structure and function of small complexes exhibiting weak interactions and their biological relevance are described.

The Utility of Cyclodextrin in Mobile Phase for High-Performance Liquid Chromatographic Separation of Isomeric Estrogens

Abstract Cyclodextrin was used as a component of mobile phase for the separation of isomeric estrogens by reversed-phase high-performance liquid chromatography. The positional isomers of catechol and guaiacol estrogens were distinctly resolved by the addition of β-cyclodextrin in the mobile phase. The separation of estriol 16- and 17-glucuronides requires usually a prolonged time. The use of β-cyclodextrin in the mobile phase, however, reduced the retention time considerably. The effect of β-cyclodextrin concentration in the mobile phase on the detector response was also investigated. The response of a fluorescence detector was raised with an increasing concentration of β-cyclodextrin, while that of an electrochemical detector was significantly depressed.

Palladium(II) complex formation by indole-3-acetate. Mixed ligand complexes involving a unique spiro-ring formed by cyclopalladation☆

Formation of mixed ligand palladium(II) complexes involving indole-3-acetate (IA) has been studied by synthetic, spectroscopic and X-ray crystallographic methods. Reaction of IA with Na2PdCl4 in methanol gave NaPd(IAH−1)Cl (1) (IAH−1=IA deprotonated from the indole ring), which reacted with pyridine (py) to give [Pd(IAH−1)(py)] (2) as orange crystals. Similar reactions carried out in the presence of water gave Pd(IAH−1)·1.5H2O (1′), which further gave Pd(IAH−1)(py)·0.5H2O (2′) by the reaction with py. X-ray crystal structure analysis of 2 revealed a unique dimeric structure, where IAH−1 coordinates to Pd(II) through the carboxylate oxygen atom and the tetrahedral C3 atom of the indole nucleus, forming a unique spiro-ring. The two complex units are bridged by the indole nitrogens in the 3H-indole form, and there is a close contact (2.75 A) around the nitrogen-C2 bonds of the five-membered rings of the indole nuclei positioned in parallel with each other. IA in the neutral form was liberated upon refluxing 1 in methanol containing 10% acetic acid, showing that IAH−1 and IA are interconvertible under proper conditions. The 1H and 13C NMR spectra in CDCl3CD3OD indicated that the C3 atom of IA in 1 and 2 is tetrahedral. Large shift differences of the C2 proton signals were observed between 1 and 1′ and between 2 and 2′, which indicates that 1 and 2 are dimers in solution whereas 1′ and 2′ are monomers and that the differences are due to the close contact between the two indole rings in 2 as detected in the solid state and probably in 1.

CH⋯Metal(II) axial interaction in planar complexes (metal = Cu, Pd) and implications for possible environmental effects of alkyl groups at biological copper sites

Intramolecular M(II)...H-C interactions (M(II)=Cu(II), Pd(II)) involving a side chain alkyl group of planar d8 and d9 metal complexes of the N-alkyl (R) derivatives of N,N-bis(2-pyridylmethyl)amine with an N3Cl donor set were established by structural and spectroscopic methods. The methyl group from the branched alkyl group (R=2,2-dimethylpropyl and 2-methylbutyl) axially interacts with the metal ion with the M...C and M...H distances of 3.056(3)-3.352(9) and 2.317(1)-2.606(1) A, respectively, and the M-H-C angles of 122.4-162.3 degrees . The Cu(II) complexes showing the interaction have a higher redox potential as compared with those without it, and the (1)H NMR signals of the interacting methyl group in Pd(II) complexes shifted downfield relative to the ligand signals. Dependence of the downshift values on the dielectric constants of the solvents used indicated that the M(II)...H-C interaction is mainly electrostatic in nature and may be regarded as a weak hydrogen bond. Implications for possible environmental effects of the leucine alkyl group at the type 1 Cu site of fungal laccase are also discussed.

Substituent Effect on the Cationic Monomer-Isomerization Polymerization of (Cyclic Imide)-Substituted Oxetanes with Two Different Ring-Opening Routes

This study reports the effect of substituent on the cationic monomer-isomerization ring-opening polymerization of 3-(R 1 -methyl)-substituted 3-R 2 -oxetanes (1), in which R 1 is phthalimide, maleimide, succinimide, or glutarimide and R 2 is ethyl, benzyl, phenyl, or isopropyl. The acid-catalyzed polymerization of 1 gave polyacetal (3) or polyether (4), together with an isomeric bicyclic acetal (2). The isomerization of 1 to 2 took place prior to polymerization. Subsequently the polymerization of 2 occured by either single or double ring opening depending on temperature. The polymerization mechanism is discussed in detail based on the coordination of 1 to Lewis acid and the substituent effect on the polymerization manner. In the double ring-opening polymerization of 2 at 130°c, a carbon-carbon double bond of the lactam ring was indispensable for stabilizing the carboxonium-propagating end. Therefore, 2 carrying a saturated lactam ring did not polymerize in such a manner. Phenyl-substitued oxetane phthalimide was unique in undergoing an unusual cyclodimerization at 130°C, primarily because of the high susceptibility of the neophyl-type carbon skeleton to a cation transfer. On the other hand, most 2 brought about the single ring-opening polymerization below room temperature, regardless of the lactam substituent and the R 2 group. This polymerization was an equilibrium polymerization through a bicyclic oxonium-propragating end, and the thermodynamic parameters of polymerization were determined. Thus, 3 was transformed into 4 in one pot, by a combination of the depolymerization of 3 and the repolymerization of 2 above the ceiling temperature. From the structure analysis of 2 it was inferred that the single ring-opening polymerizability arises from dipole-dipole repulsion between the parallel standing lone pairs of two acetalic oxygen atoms in a nearly symmetric bicycle. There fore, 2 having a somewhat twisted bicyle showed no single ring-opening polymerizability.