Cationic Substituents Research Articles

Reversible inhibitors of .beta.-glucosidase

A variety of reversible inhibitors of sweet almond beta-glucosidase were examined. These included simple sugars and sugar derivatives, amines and phenols. With respect to the sugar inhibitors and, indeed, the various glycoside substrates, the enzyme has what can be considered a "relaxed specificity". No single substituent on glucose, for example, is essential for binding. Replacement of a hydroxyl group with an anionic substituent reduces the affinity while substitution with a cationic (amine) substituent enhances the affinity. Amines, in general, are good inhibitors, binding more tightly than the corresponding alcohols: pKiRNH3+ = 0.645pKiROH + 1.77 (n = 9, r = 0.97). The affinity of a series of 10 primary amines was found to be strongly influenced by substituent hydrophobicity: pKi = 0.52 pi + 1.32 (r = 0.95). The major binding determinant of the glycoside substrates is the aglycon moiety. Thus, the Ki values of phenols are similar in magnitude to the Ks values of the corresponding aryl beta-glucoside. The pH dependence for the inhibition by various phenols indicates that it is the un-ionized phenol which binds to the enzyme when an enzymic group of pKa = 6.8 (+/- 0.1) is protonated. The affinity of the phenol inhibitor is dependent on its basicity with a Brønsted coefficient for binding of beta = -0.26 (n = 14, r = 0.98). The pH dependence of the binding of two particularly potent beta-glucosidase inhibitors was also examined. 1-Deoxynojirimycin (1,5-dideoxy-1,5-imino-D-glucitol) has a pH-corrected Ki = 6.5 microM, and D-glucono-1,5-lactam has a pH-corrected Ki = 29 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

Derivatives of hydrogen uranyl phosphate: excited-state properties of a family of lamellar solids

The excited-state properties of the layered compound hydrogen uranyl phosphate (HUP), HUO 2PO 4·4H 2O and of solids derived therefrom by intercalative ion-exchange reactions have been examined. The reaction exploited are given in eqn. (1)–(3). HUO 2PO 4 + M + → MUO 2PO 4 + H + (1) M = K, Ag, NH 4, NC 5H 6 (pyridinium), n-C 4H 9NH 3, n-C 8H 17NH 3 HUO 2PO 4 + 1 2 M 2+ → M 1 2 OU 2PO 4 + H + (2) M = Ca, Zn, Cu, (∼0.4 equivalents incorporated) HUO 2PO 4 + 1 3 M 3+ → M 1 3 UO 2PO 4 + H + (3) M = Cr(urea) 6, Eu (∼0.07 equivalents incorporated) The products of these reactions have all been characterized by elemental analysis, IR spectroscopy, and X-ray powder diffraction. The latter reveals that all compounds retain the lamellar structure of HUP and can be indexed in tetragonal symmetry, using c lattice values derived from 001 data and a lattice values of ∼6.99 Å. Although the a values are roughly constants, the interlamellar spacings (distance from the middle of one layer to the middle of the adjacent layer) vary widely; typical values are 8.69, 9.01, 10.34 and 18.76 Å for HUP, NH 4UP, Ca 1 2 UP, and n-C 8H 17NH 3UP, respectively. All of the samples exhibit electronic absorption spectra characteristics of the UO 2+ 2 chromophore; for substituent cation processing visible absorption bands, these transition appear superimposed in each spectrum. Except for the n-C 8H 17NH + 3, Ag +, Cu 2+, and Cr(urea) 3+ 6 salts, the samples all exhibits yellow-green emission characteristic of the UO 2+ 2 moiety when excited with blue or near-UV light at 295 K. Emission decay curves are exponential for all of the emissive solids and yields lifetimes, τ, ranging from ∼ 1–450 μs. Samples having τ values of ∼ 10 2–10 3 μs include HUP and the NH + 4 pyridinium +, K +, Ca 2+, and Zn 2+ derivatives. These solids also have radiative quantum efficiencies. ø r, approaching unity at 295 K. Values of τ and ø r have been used to calculate radiative (k r) and nonradiative (k nr) are constants for excited-state decay. Values of k r are nearly constants at ∼(1–) × 10 3s −1 for the samples, whereas k nr values span several orders of magnitude. Possible quenching mechanisms for the weakly emissive and nonemissive samples include excited-state energy transfer (CU 2+, Eu 3+, and Cr(urea) 3+ 6 derivatives), electron transfer (Ag + and Cu 2+ derivatives) and H-atom abstraction (n-C 4H 9NH + 3 and n-C 8H 17NH + 3 derivatives). Partially Eu-substituted HUP provides evidence for excited-state energy transfer: both the UO 2+ 2-based emission and pink Eu 3+ luminescence are simultaneously observed excitation of of the UO 2+ 2 chromophore. The relative intensities of the two types of emission vary with Eu content in both this system and Eu-substituted Ca 1 2 UP samples. Concentration effects on luminescence have also been investigated with Ag-substituted KUP. The UO 2+ 2-based emission is partially quenched in samples prepared from solution in which the Ag:K ratio is as little as ∼ 1:10,000. Some evidence for an exciton mechanism has been obtained in this studies. Derivatives of HUP based on cationic transition metal complexes such as Cr(urea) 3+ 6 afford interesting comparisons of excited-state properties with solution environments. To illustrate, the Cr complex exhibits fluorescence and phosphorescence at 77 K in an EPA glass. Absorption bands due to this complex are relatively unaffected when the complex is incorporated into the HUP lattice (λ max ∼ 635 nm), but no emission is observed from the solid at 77 K. These observation illustrate the role of environment on excited-state and the versatility of HUP as a host lattice for evaluating environmental effects.

Measurement of sodium ion concentration in the unstirred layer of rat small intestine by polymer Na +-sensitive electrodes

The concentration of sodium ion at the surface of rat small intestine both in vitro and in vivo was measured with plastic polymer sodium ion sensitive electrodes. In vitro the surface sodium ion concentration [Na s +] was found to be significantly higher than bulk bathing solutions of 25 mM concentration. This value could be increased by the addition of glucose to the medium and was significantly higher in the jejunum than in the ileum. Ouabain, deoxycholic acid and dithiothreitol all reduced the [Na s +] under in vitro conditions. In vivo, very high values for [Na s +] were found in the jejunum (approx. 80 mM) when the bulk concentration was 25 mM, indicating a substantial local accumulation of sodium ion at or near the brush border. This could be reduced by omission of glucose from the buffer and further reduced when magnesium was the substituent cation rather than choline. Consequently, in vivo, an appropriately orientated sodium ion gradient persists in the face of adverse tissue to bulk solution concentration gradients and potentially explains why solute absorption occurs under these circumstances. The further reduction of the correctly aligned sodium ion gradient, as required by the gradient hypothesis, by magnesium-substituted buffers indicates that there is no real need to postulate additionally a sodium-independent magnesium-sensitive glucose transport system in vivo.

Mössbauer effect studies of alkali bis(citrato)ferrates(III)

Mossbauer spectra of alkali bis(citrato)ferrates(III) i.e. M3Fe(C 6H5O7)2. x H2O (M = Li, Na, Rb, Cs, NH4) at room temperature display a doublet, the isomer shift and quadrupole splitting values indicating these complexes to be high spin with octahedral symmetry. However, in case of K3Fe(C 6H5O7)2.4H2O a single broad absorption band has been observed. The isomer shift shows a decreasing trend with the increase in the electronegativity of the substituent cation. Isomer shift also increases with the increase in cationic size (Li+, Na+, K+, Rb+, Cs+, Na+4). A linear correlation between isomer shift and the separation C=O stretching frequencies (Δν) has also been observed.

New anode materials for photoelectrolysis

The design strategy for a successful oxide anode for use in photoelectrolysis cells is reviewed. Conduction bands based on Ti4+ and Nb5+ can be made to possess sufficiently small electron affinities to allow the cell to operate without external bias. For example, data on SrTi0.75Zr0.25O3 are presented to show that the Ti4+ conduction band can be fine-tuned to the required electron affinity using Zr4+ as dopant. The Zr4+ dopant also enhances remarkably oxygen evolution in the dark, and a mechanism for this observation is advanced. In order to reduce the bandgap in oxide titanates, incorporation of a second metal-ion sublattice appears to be essential. Three types of substituent cation that can, in principle, give rise to a valence band at the right energy are discussed: (i) transition metals giving rise to ndm bands, (ii) lanthanides giving rise to 4fm bands and (iii) reduced B-metals giving rise to ns2 bands. Optical absorption studies of MnTiO3 and NaCeTi2O6 indicate that the bulk Mn2+ 3d5 and Ce3+ 4f1 levels are at about the right energy, but holes photogenerated in these levels become trapped in surface–polaron sites, as they only oxidise water slowly. Analysis of the transient photocurrents for MnTiO3 shows that holes arrive at the surface in both the O2–:2p6 band and the Mn2+:3d5 levels and the former oxidise water rapidly.

Performance of Ga1−xAlxAs light emitting diodes in radiation environments

Addition of aluminum as a cation substituent in GaAs improves the relative radiation hardness of the GaAs light emitting diode, as deduced from both gamma and neutron irradiation experiments to 108 rads (Si) and 3.5t1013 neutrons/cm2, respectively. The gamma damage coefficient, K y , shows a marked and unexpected decrease from 52 (rads (Si) .s)−1 to ∼ 2 (rads(Si) .s)−1 in the composition range from GaAs to ∼ Ga .90 Al 10 AS. Above 10 percent Al content, the damage coefficient changes only slightly. On the other hand, the neutron damage coefficient K n shows a gradual change over the entire compositional range from 0 to 34 percent aluminum. Annealing of the gamma-irradiated samples indicates that the 240° C stage noted for GaAs and attributed to an arsenic vacancy is reduced with addition of aluminum. On the other hand the annealing of the Ga 1−x Al x As samples following neutron irradiation indicates that the annealing characteristics are virtually independent of the aluminum composition. For both types of irradiation there is a shift of the peak emission wavelength to shorter wavelengths following irradiation.

Electrostatic catalysis and inhibition in aqueous solution. Rate effects on the reactions of charged esters with a cationic steroid bearing an imidazolyl substituent

The steroid (1) with a 17β-imidazoyl-and an 11β-ammonio-substituent catalyses the hydrolysis of aryl acetates; esters with an anionic substituent on the phenolic unit show rate enhancements, an ester with a cationic substituent shows a rate retardation, and the rate enhancement for a meta-anionic substituent is much larger than for para-anionic substituents.

Mössbauer effect studies of alkali metal ferricyanides and hydrogen-bonded ferricyanic acid

Mössbauer spectra of anhydrous ferricyanic acid, H 3Fe(CN) 6, and alkali metal ferricyanides, M 3[Fe(CN) 6]. xH 2O (where M  Li, Na, Rb, Cs, NH 4 and Ag), have been studied at room temperature. These compounds show a two line spectrum indicating definite quadrupole splitting ( ΔE Q ). The values of ΔE Q range from 0·20 to 0·76 mm/sec. The isomer shifts (δ) vary from 0·08 to 0·26 mm/sec. (relative to sodium nitroprusside dihydrate). The isomer shift shows generally a decreasing trend with the increase in the electronegativity of the substituent cation. It is suggested that the cations influence the π-acceptor property of the cyanide ligands. The s-electron density at the iron nucleus increases as the π-acceptor nature of the ligand increases. The quadrupole splitting data is also consistent with this interpretation. A linear relationship is observed between δ and ΔE Q . The isomer shift for H 3Fe(CN) 6 is comparatively high presumably due to the presence of intermolecular hydrogen bonding in this compound.