Kazuhiko Ichikawa

Neutron-diffraction investigation of the intramolecular structure of a water molecule in the liquid phase at high temperatures

Time-of-flight neutron diffraction measurements were carried out on pure liquid water in the high-Q region and at high temperatures between 25 and 200°C. The observed data at high scattering angle ...

Kinetic study of catalytic CO2 hydration by water-soluble model compound of carbonic anhydrase and anion inhibition effect on CO2 hydration

A kinetic study of CO(2) hydration was carried out using the water-soluble zinc model complex with water-soluble nitrilotris(2-benzimidazolylmethyl-6-sulfonate) L1S, [L1SZn(OH(2))](-), mimicking the active site of carbonic anhydrase, in the presence and absence of anion inhibitors NCS(-) and Cl(-). The obtained rate constants k(cat) for CO(2) hydration were 5.9x10(2), 1. 7x10(3), and 3.1x10(3) M(-1) s(-1) at 5, 10, and 15 degrees C, respectively: the k(cat)=ca. 10(4) M(-1) s(-1) extrapolated towards 25 degrees C has been the largest among the reported k(cat) using zinc model complexes for carbonic anhydrase. It was also revealed that NCS(-), Cl(-) and acetazolamide play a role of inhibitors by the decrease of k(cat): 7x10(2) and 2x10(3) M(-1) s(-1) for NCS(-) and Cl(-) at 15 degrees C, respectively. The sequence of their magnitudes in k(cat) is Cl(-) approximately acetazolamide>NCS(-), where the sequence Cl(-)>NCS(-) is confirmed for native carbonic anhydrase. The difference of k(cat) or k(obs) between NCS(-) and Cl(-) resulted from that between the stability constants K(st)=2x10(3) for [L1SZn(NCS)](2-) and 1x10(2) M(-1) for [L1SZnCl](2-) in D(2)O: for water-insoluble tris(2-benzimidazolylmethyl)amine L1, K(st)=1.8x10(4) for [L1Zn(NCS)](2-) and 1.5x10(3) M(-1) for [L1ZnCl](2-)in CD(3)CN/D(2)O (50% v/v). The crystal structure of anion-binding zinc model complexes [L1Zn(OH(2))](0.5)[L1ZnCl](0.5) (ClO(4))(1.5) 1(0.5)2(0.5)(ClO(4))(1.5) was revealed by X-ray crystallography. The geometry around Zn(2+) in 1 and 2 was tetrahedrally coordinated by three benzimidazolyl nitrogen atoms and one oxygen atom of H(2)O, or Cl(-).

Phosphoester hydrolysis using structural phosphatase models of tren based zinc(II) complexes and X-ray crystal structures of [Zn(tren)(H2O)](ClO4)2 and [Zn(tren)(BNPP)]ClO4

Abstract New tris(2-aminoethyl)amine (tren) L1 based ligands, namely N,N′,N″-tris(2-benzylaminoethyl)amine L2 and N,N′,N″-tris(im-benzyl- l -histidylethylaminoethyl) amine L3 have been synthesized and characterized. Complexation studies on Zn2+ complexes 1, 2 and 3 derived from L1, L2, and L3 showed that the presence of benzyl and benzyl–histidyl moieties attached to the tripodal ligand L1 side arms decrease the pKa of the Zn-bound water molecule: 10.72 for 1, 9.61 for 2 and 7.43 for 3, respectively. The zinc complex of 3 was a much more active catalyst for the hydrolysis of bis(p-nitrophenyl)phosphate (BNPP−) and tris(p-nitrophenyl)phosphate (TNPP) compared with 1 and 2. In the case of 1 and 2, the pH-dependence of their observed pseudo-first-order rate constants kobsd showed sigmoidal pH–rate profile, while 3 gave bell-shape curve with a maximum rate constant of around 1.0×10−5 s−1 at pH 8.5. The pH dependence of kobsd indicated that the Zn-bound hydroxo species is responsible for catalytic activity. The crystal structures of [L1Zn–H2O]2+ (1) and [L1Zn–BNPP]+ (4) have been determined and showed trigonal–bipyramidal configurations around the central Zn. The zinc complexes of 1 and 4 served as structural models for the binding mode of coordinated water as well as substrates in the active site of zinc enzyme.

Indirect and direct correlations between unlike ions in incompletely hydrated solution

The nature of hydration around Li+ in incompletely hydrated solution was investigated by the neutron first-order difference method (Soper et al., 1977), using a time-of-flight neutron diffractometer. The authors observed, for the first time, direct evidence of Li-Cl correlations, which reflect the short-range correlations between water molecules and Li+ as well as between Cl- and Li+ within approximately 4 AA. The angle between the Li-O axis and the plane of the water molecule and the hydration number were strongly concentration dependent, even for incompletely hydrated solutions at large than approximately 25 mol.% LiCl.

Chemical potentials, concentration fluctuations, etc., in binary metal alloys

Abstract Relationships among transport parameters and the mean square composition fluctuations as determined by measurements of the chemical potentials are described for a variety of binary liquid metal alloys. The measurement of the chemical potential is described and the thermodynamic basis for the analysis given in terms of the concentration correlation functions as well as more traditional thermodynamic parameters. Alloys involving group IIIA metals with Te are used as an example

Electronic Excitations in Homopolyatomic Bismuth Cations: Spectroscopic Measurements in Molten Salts and an ab initio CI-Singles Study

The electronic excitations of the low-valence bismuth cluster cations Bi5(3+), Bi8(2+), and Bi9(5+) have been studied with experimental and theoretical techniques. The UV-visible spectra of the bismuth ions were measured in acidic chloroaluminate melts (mixture of 1-methyl-3-benzyl imidazolium chloride and AlCl3). The spectra of the Bi5(3+) and Bi8(2+) ions agree fairly well with previous reports, but also revealed additional low-energy absorptions. Ab initio methods were employed to assign the experimentally observed electronic transitions of these homopolyatomic bismuth cations. Structures were optimized at the RHF, MP2, and B3LYP levels of theory by using split-valence LANL2DZ basis sets that were augmented with one and two sets of pure d functions. The computed structures agree well with the results of neutron diffraction analyses of melts. Electronically excited states of the three clusters were treated by using the CI-Singles theory. The results of these calculations were used to explain the observed UV-visible spectra. The observed electronic excitations in the UV-visible range are all found to result from transitions involving the molecular orbitals formed by 6p-atomic-orbital overlap. This leads to the necessity of using basis sets that include d-type functions, which allow for an adequate description of the bonding that results from such p-orbital overlap. Spin-orbit coupling becomes increasingly important with increasing atomic number and its consideration is necessary when describing the electronic transitions in clusters of heavy atoms. The calculations show that singlet-triplet transitions, which are made accessible by strong spin-orbit coupling, are responsible for some of the observed absorptions.

A novel macrotricyclic receptor for the inclusion of fluoride ion

Abstract A novel macrotricyclic receptor 1 with hydrocarbon chain of (CH 2 ) 5 has been synthesized and the 19 F NMR study on the aqueous solution of 1 shows the single coordination geometry of the encapsulated fluoride ion.

Intermolecular structure around lithium monovalent cations and nitrogen atoms in molten LiNO3

Time of flight neutron diffraction measurements have been carried out for five LiNO3 melts by changing the isotopic composition of either the Li or the N nucleus. We have determined the intramolecular distances and root mean square displacements of NO- 3 in the molten state: r NO = 1·251 + 0·004 A, r OO = 2·15 + 0·01 A, l NO = 0·045 + 0·003 A, and l OO = 0·073 ± 0·006 A. Here we used a least squares fit to the intramolecular interference terms for 6,7Li14NO3 and 6,7Li15NO3 associated with a zero coherent scattering length of Li. The isotopic difference method was used to determine the distribution functions [Gbar] Li(r) and [Gbar] N(r) around Li+ and N, and intermolecular partial pair distribution functions g LiN(r), g LiO(r), g NN(r), g NO(r) and g OO(r). The nearest neighbour distances were: r(LiO) = 2·1 ± 0·1 A, r(LiN) = 2·7 ± 0·1 A, and r(NN) = 4·8 ± 0·2 A, r(NO) = 3·9 ± 0·2 A, and r(OO) = 3·2 ± 0·2 A. We have proposed a perspective view of most probable local structure around Li+: a lithium monovalen...

The intra-molecular structure of a water molecule in hydrated and incompletely hydrated LiCl solutions

Time-of-flight neutron diffraction measurements were carried out on the hydrated and incompletely hydrated solutions (LiCl)x(D2O)1-x for x=0.20, 0.27, 0.32 and 0.35. The data for the high-Q region observed at the high scattering angle 2 theta =91 degrees have been analysed in terms of intra-molecular interference, including an inelasticity correction, to determine the intra-molecular distances of a water molecule, rOD and RDD. The former shows no composition dependence within experimental error; the distance rOD=0.970+or-0.006 AA is equal to that in pure D2O. On the other hand, the distance rDD=1.59+or-0.02 AA in the solutions with above 20 mol.% LiCl is 0.04 AA longer than that of liquid D2O because of the strong interaction between the D2O molecule and the ions. The conclusion reached in this work suggests that the results of the molecular dynamics simulation are not reasonable.

Biochemical CO2 fixation by mimicking zinc(II) complex for active site of carbonic anhydrase

The complex coordinated by three benzimidazolyl moieties and a single water molecule was syntesized as a model complex of the active site in carbonic anhydrase. A catalytic reaction of CO2 fixation was simulated by using the model complex.