Synthesis and characterization of a photosensitive interface for hydrogen generation: Chemically modified p-type semiconducting silicon photocathodes

p-Si photocathodes functionalized first with an N,N′-dialkyl-4,4′-bipyridinium redox reagent, (PQ^2+/+-)_surf, and then with a Pt precursor, PtCl₆^2-, give significant efficiency (up to 5%) for photoelectrochemical H₂ generation with 632.8-nm light. Naked p-Si photocathodes give nearly zero efficiency, owing to poor H₂ evolution kinetics that are improved by the (PQ^2+/+-)_surf/Pt modification. The mechanism of H₂ evolution from p-Si/(PQ^2+/+-)_surf/Pt is first photoexcitation of electrons to the conduction band of Si followed by (PQ²⁺)_surf → (PQ^+-)_surf reduction. The dispersion of Pt then catalyzes H₂O reduction to give H₂ and regeneration of (PQ²⁺)_surf. The overall energy conversion efficiency rivals the best direct optical to chemical conversion systems reported to date.     

Inactivation of calcium-activated chloride channels in smooth muscle by calcium/calmodulin-dependent protein kinase

To determine the mechanisms responsible for the termination of Ca²⁺-activated Cl⁻ currents (I_Cl(Ca)), simultaneous measurements of whole cell currents and intracellular Ca²⁺ concentration ([Ca²⁺]_i) were made in equine tracheal myocytes. In nondialyzed cells, or cells dialyzed with 1 mM ATP, I_Cl(Ca) decayed before the [Ca²⁺]_i decline, whereas the calcium-activated potassium current decayed at the same rate as [Ca²⁺]_i. Substitution of AMP-PNP or ADP for ATP markedly prolonged the decay of I_Cl(Ca), resulting in a rate of current decay similar to that of the fall in [Ca²⁺]_i. In the presence of ATP, dialysis of the calmodulin antagonist W7, the Ca²⁺/calmodulin-dependent kinase II (CaMKII) inhibitor KN93, or a CaMKII-specific peptide inhibitor the rate of I_Cl(Ca) decay was slowed and matched the [Ca²⁺]_i decline, whereas H7, a nonspecific kinase inhibitor with low affinity for CaMKII, was without effect. When a sustained increase in [Ca²⁺]_i was produced in ATP dialyzed cells, the current decayed completely, whereas in cells loaded with 5′-adenylylimidodiphosphate (AMP-PNP), KN93, or the CaMKII inhibitory peptide, I_Cl(Ca) did not decay. Slowly decaying currents were repeatedly evoked in ADP- or AMP-PNP-loaded cells, but dialysis of adenosine 5′-O-(3-thiotriphosphate) or okadaic acid resulted in a smaller initial I_Cl(Ca), and little or no current (despite a normal [Ca²⁺]_i transient) with a second stimulation. These data indicate that CaMKII phosphorylation results in the inactivation of calcium-activated chloride channels, and that transition from the inactivated state to the closed state requires protein dephosphorylation.     

Similarities of artificial photosystems by ruthenium oxo complexes and native water splitting systems

The nature of chemical bonds of ruthenium(Ru)–quinine(Q) complexes, mononuclear [Ru(trpy)(3,5-t-Bu₂Q)(OH₂)](ClO₄)₂ (trpy = 2,2^′:6^′,2^′′-terpyridine, 3,5-di-tert-butyl-1,2-benzoquinone) (1), and binuclear [Ru₂(btpyan)(3,6-di-Bu₂Q)₂(OH₂)]²⁺ (btpyan = 1,8-bis(2,2^′:6^′,2^′′-terpyrid-4^′-yl)anthracene, 3,6-t-Bu₂Q = 3,6-di-tert-butyl-1,2-benzoquinone) (2), has been investigated by broken-symmetry (BS) hybrid density functional (DFT) methods. BS DFT computations for the Ru complexes have elucidated that the closed-shell structure (2b) Ru(II)–Q complex is less stable than the open-shell structure (2bb) consisting of Ru(III) and semiquinone (SQ) radical fragments. These computations have also elucidated eight different electronic and spin structures of tetraradical intermediates that may be generated in the course of water splitting reaction. The Heisenberg spin Hamiltonian model for these species has been derived to elucidate six different effective exchange interactions (J) for four spin systems. Six J values have been determined using total energies of the eight (or seven) BS solutions for different spin configurations. The natural orbital analyses of these BS DFT solutions have also been performed in order to obtain natural orbitals and their occupation numbers, which are useful for the lucid understanding of the nature of chemical bonds of the Ru complexes. Implications of the computational results are discussed in relation to the proposed reaction mechanisms of water splitting reaction in artificial photosynthesis systems and the similarity between artificial and native water splitting systems.     

Primary photochemistry of iron-depleted and zinc-reconstituted reaction centers from Rhodopseudomonas sphaeroides

The primary photochemistry of Fe-depleted and Zn-reconstituted reaction centers from Rhodopseudomonas sphaeroides R-26.1 was studied by transient absorption spectroscopy and compared with native, Fe²⁺-containing reaction centers. Excitation of metal-free reaction centers with 30-ps flashes produced the initial charge-separated state P⁺I^- (P⁺BPh^-, where P is the primary donor and BPh is bacteriopheophytin) with a yield and visible/near-infrared absorption difference spectrum indistinguishable from that observed in native reaction centers. However, the lifetime of P⁺I^- was found to increase approximately 20-fold to 4.2 ± 0.3 ns (compared to 205 ps in native reaction centers), and the yield of formation of the subsequent state P⁺Q_A^- (Q_A is the primary quinone acceptor) was reduced to 47 ± 5% (compared to essentially 100% in native reaction centers). The remaining 53% of the metal-free reaction centers were found to undergo charge recombination during the P⁺I^- lifetime to yield both the ground state (28 ± 5%) and the triplet state P^R (25 ± 5%). Reconstitution of Fe-depleted reaction centers with Zn²⁺ restored the “native” photochemistry. Possible mechanisms responsible for the reduced decay rate of P⁺I^- in metal-free reaction centers are discussed.     

Crystal engineering of lattice metrics of perhalometallate salts and MOFs

The synthesis of the salt 3 and metallo-organic framework (MOF) [{(4,4^′-bipy)CoBr₂}_n] 4 by a range of solid state (mechanochemical and thermochemical) and solution methods is reported; they are isostructural with their respective chloride analogues 1 and 2. 3 and 4 can be interconverted by means of HBr elimination and absorption. Single phases of controlled composition and general formula [4,4^′-H₂bipy][CoBr_4-xCl_x] 5_x may be prepared from 2 and 4 by solid—gas reactions involving HBr or HCl respectively. Crystalline single phase samples of 5_x and [{(4,4^′-bipy)CoBr_2-xCl_x}_n] 6_x were prepared by solid-state mechanochemical routes, allowing fine control over the composition and unit cell volume of the product. Collectively these methods enable continuous variation of the unit cell dimensions of the salts [4,4^′-H₂bipy][CoBr_4-xCl_x] (5_x) and the MOFs [{(4,4^′-bipy)CoBr_2-xCl_x}_n] (6_x) by varying the bromide to chloride ratio and establish a means of controlling MOF composition and the lattice metrics, and so the physical and chemical properties that derive from it.     

Molecular basis for specific viral RNA recognition and 2′-O-ribose methylation by the dengue virus nonstructural protein 5 (NS5)

Dengue virus (DENV) causes several hundred million human infections and more than 20,000 deaths annually. Neither an efficacious vaccine conferring immunity against all four circulating serotypes nor specific drugs are currently available to treat this emerging global disease. Capping of the DENV RNA genome is an essential structural modification that protects the RNA from degradation by 5′ exoribonucleases, ensures efficient expression of viral proteins, and allows escape from the host innate immune response. The large flavivirus nonstructural protein 5 (NS5) (105 kDa) has RNA methyltransferase activities at its N-terminal region, which is responsible for capping the virus RNA genome. The methyl transfer reactions are thought to occur sequentially using the strictly conserved flavivirus 5′ RNA sequence as substrate (G_pppAG-RNA), leading to the formation of the 5′ RNA cap: G_0pppAG-RNA→^m7G_0pppAG-RNA (“cap-0”)→^m7G_0pppA_m2′-O-G-RNA (“cap-1”). To elucidate how viral RNA is specifically recognized and methylated, we determined the crystal structure of a ternary complex between the full-length NS5 protein from dengue virus, an octameric cap-0 viral RNA substrate bearing the authentic DENV genomic sequence (5′-^m7G_0pppA₁G₂U₃U₄G₅U₆U₇-3′), and S-adenosyl-l-homocysteine (SAH), the by-product of the methylation reaction. The structure provides for the first time, to our knowledge, a molecular basis for specific adenosine 2′-O-methylation, rationalizes mutagenesis studies targeting the K61-D146-K180-E216 enzymatic tetrad as well as residues lining the RNA binding groove, and offers previously unidentified mechanistic and evolutionary insights into cap-1 formation by NS5, which underlies innate immunity evasion by flaviviruses.Several members of the Flavivirus genus from the Flaviviridae family are major human pathogens, such as the four serotypes of dengue virus (DENV1–4), West Nile virus (WNV), Japanese encephalitis virus (JEV), and yellow fever virus (YFV). Recent large-scale DENV vaccine trials using a tetravalent formulation and three dose injections have shown only limited protection against the four DENV serotypes, and no specific antiviral drug has reached the market so far (1–3). The flavivirus genome consists of a (+)-sense single-stranded RNA of ∼11 kb with a type 1 cap structure, followed by the strictly conserved dinucleotide sequence “AG”: 5′-^m7G_pppA_m2′-O-G-3′ (4, 5). Addition of the cap moiety to the 5′ end of the viral genome is crucial for viral replication, because this structure ensures efficient production of viral polyproteins by the host translation machinery and protection against degradation by 5′-3′ exoribonucleases, and also because it conceals the triphosphorylated (or diphosphorylated) end from recognition by host cell innate immune sensors (6–9). Following (+)-strand RNA synthesis by the C-terminal RNA-dependent RNA polymerase (RdRp) domain of nonstructural protein 5 (NS5), cap formation in flaviviruses results from several sequential enzymatic reactions carried out by (i) the RNA triphosphatase activity of the NS3 protease-helicase that hydrolyzes the γ-phosphate group of the viral 5′ untranslated region (UTR), yielding a diphosphate RNA, (ii) a guanylyl-transferase activity proposed to reside in the methyltransferase (MTase) domain of NS5, which transfers a GMP molecule to the 5′-diphosphate RNA, and (iii) NS5-mediated sequential N-7- and 2′-O-methylations according to the following scheme: G_0pppAG-RNA→^m7G_0pppAG-RNA (“cap-0”)→^m7G_0pppA_m2′-O-G-RNA (“cap-1”) (5, 10–12).During flavivirus RNA replication, 5′-guanosine N-7-methylation is shown to be essential for translation of the viral polyprotein (13), whereas 2′-O-methylation on the penultimate A nucleotide conceals the viral genome from host immune sensors, notably RIG-I (14), MDA5 (15), and IFIT1 (16–18). Specifically, WNV carrying the E218A mutation in NS5 (E216A in DENV3 NS5) devoid of 2′-O (but not N-7) MTase activity was attenuated in wild-type but not Ifit1^−/− cells (16). Furthermore, the translation of JEV viral proteins was inhibited by IFIT1 through direct binding to the 5′-capped 2′-O-unmethylated mRNA (17). More recently, 2′-O-methylation at internal adenosines (but not at G, C, or U positions) by the flavivirus NS5 protein was demonstrated. The functional consequence of methylation at internal adenosines was an attenuation of viral RNA translation and replication (12). In vitro, the MTase domain of NS5 catalyzes these two enzymatic reactions with distinct requirements of RNA substrates and buffers: 5′-guanosine N-7 methyl transfer is optimal on a 211-nt segment of the 5′UTR at pH 6 and is inhibited by MgCl₂, whereas adenosine ribose 2′-O-methylation only requires a short RNA with “AG” as the first two RNA nucleotides and is maximal at pH 9–10 in the presence of Mg²⁺ ions. Thus, NS5 plays a crucial role both in virus replication and evasion of the host innate immune response, and therefore constitutes an attractive therapeutic target for antiviral drug and vaccine development (2, 19).Several crystal structures of flavivirus MTases have been reported either as free enzymes or bound to GTP (20), to the broad antiviral nucleoside analog ribavirin (21), to short cap analogs (22, 23), and to a capped-RNA octamer (24). Collectively, these structures uncovered a GTP binding site, the S-adenosyl-methionine (SAM) methyl-donor binding pocket, and a basic cleft at the protein surface that was proposed to accommodate the incoming RNA substrate. However, in the absence of a viral RNA in a catalytically meaningful position, the mechanism accounting for specific viral RNA methylation, including the structural basis for specific adenosine 2′-O-methylation, remains elusive. Moreover, the size of the RNA substrate that can be accommodated by the putative RNA binding cleft is also unknown, as well as any requirement for a specific RNA conformation. Determination of the structure of NS5 bound to a viral RNA would give valuable information to guide the design of specific NS5 inhibitors.To elucidate how the flavivirus RNA is recognized and methylated, we determined the crystal structure of a ternary complex between the full-length NS5 protein (DENV serotype 3), an authentic cap-0 viral RNA substrate (5′-^m7G_0pppA₁G₂U₃U₄G₅U₆U₇-3′), and S-adenosyl-l-homocysteine (SAH), the by-product of the methylation reaction. Together with mutagenesis data informed by the present structure, this work reveals a unique and specific interaction between the protein and viral RNA and provides the molecular basis for the methyl transfer reaction. Furthermore, despite a low sequence identity between the two proteins, the RNA recognition mode is reminiscent of how the human 2′-O-ribose methyltransferase CMTr1 binds mRNA for cap formation, suggesting that the viral methyltransferase might derive from its eukaryotic homolog.     

Structural basis for efficient phosphorylation of 3′-azidothymidine monophosphate by Escherichia coli thymidylate kinase

The crystal structures of Escherichia coli thymidylate kinase (TmpK) in complex with P¹-(5′-adenosyl)-P⁵-(5′-thymidyl)pentaphosphate and P¹-(5′-adenosyl)P⁵-[5′-(3′-azido-3′-deoxythymidine)] pentaphosphate have been solved to 2.0-Å and 2.2-Å resolution, respectively. The overall structure of the bacterial TmpK is very similar to that of yeast TmpK. In contrast to the human and yeast TmpKs, which phosphorylate 3′-azido-3′-deoxythymidine 5′-monophosphate (AZT-MP) at a 200-fold reduced turnover number (k_cat) in comparison to the physiological substrate dTMP, reduction of k_cat is only 2-fold for the bacterial enzyme. The different kinetic properties toward AZT-MP between the eukaryotic TmpKs and E. coli TmpK can be rationalized by the different ways in which these enzymes stabilize the presumed transition state and the different manner in which a carboxylic acid side chain in the P loop interacts with the deoxyribose of the monophosphate. Yeast TmpK interacts with the 3′-hydroxyl of dTMP through Asp-14 of the P loop in a bidentate manner: binding of AZT-MP results in a shift of the P loop to accommodate the larger substituent. In E. coli TmpK, the corresponding residue is Glu-12, and it interacts in a side-on fashion with the 3′-hydroxyl of dTMP. This different mode of interaction between the P loop carboxylic acid with the 3′ substituent of the monophosphate deoxyribose allows the accommodation of an azido group in the case of the E. coli enzyme without significant P loop movement. In addition, although the yeast enzyme uses Arg-15 (a glycine in E. coli) to stabilize the transition state, E. coli seems to use Arg-153 from a region termed Lid instead. Thus, the binding of AZT-MP to the yeast TmpK results in the shift of a catalytic residue, which is not the case for the bacterial kinase.     

Electrocatalytic Properties of Ni(II) Schiff Base Complex Polymer Films

Platinum electrodes were modified with polymers of the (±)-trans-N,N′-bis(salicylidene)-1,2-cyclohexanediaminenickel(II) ([Ni(salcn)]) and (±)-trans-N,N′-bis(3,3′-tert-Bu-salicylidene)-1,2-cyclohexanediaminenickel(II) ([Ni(salcn(Bu))]) complexes to study their electrocatalytic and electroanalytical properties. Poly[Ni(salcn)] and poly[Ni(salcn(Bu))]) modified electrodes catalyze the oxidation of catechol, aspartic acid and NO₂⁻. In the case of poly[Ni(salcn)] modified electrodes, the electrocatalysis process depends on the electroactive surface coverage. The films with low electroactive surface coverage are only a barrier in the path of the reducer to the electrode surface. The films with more electroactive surface coverage ensure both electrocatalysis inside the film and oxidation of the reducer directly on the electrode surface. In the films with the most electroactive surface coverage, electrocatalysis occurs only at the polymer–solution interface. The analysis was based on cyclic voltammetry, EQCM (electrochemical quartz crystal microbalance) and rotating disc electrode method.     

Targeted inactivation of αi2 or αi3 disrupts activation of the cardiac muscarinic K+ channel, IK+Ach, in intact cells

Cardiac muscarinic receptors activate an inwardly rectifying K⁺ channel, I_K^+Ach, via pertussis toxin (PT)-sensitive heterotrimeric G proteins (in heart G_i2, G_i3, or G_o). We have used embryonic stem cell (ES cell)-derived cardiocytes with targeted inactivations of specific PT-sensitive α subunits to determine which G proteins are required for receptor-mediated regulation of I_K^+Ach in intact cells. The muscarinic agonist carbachol increased I_K^+Ach activity in ES cell-derived cardiocytes from wild-type cells, in cells lacking α_o, and in cells lacking the PT-insensitive G protein α_q. In cells with targeted inactivation of α_i2 or α_i3, channel activation by both carbachol and adenosine was blocked. Carbachol-induced channel activation was restored in the α_i2- and α_i3-null cells by reexpressing the previously targeted gene and guanosine 5′-[γ-thio] triphosphate was able to fully activate I_K^+Ach in excised membranes patches from these mutants. In contrast, negative chronotropic responses to both carbachol and adenosine were preserved in cells lacking α_i2 or α_i3. Our results show that expression of two specific PT-sensitive α subunits (α_i2 and α_i3 but not α_o) is required for normal agonist-dependent activation of I_K^+Ach and suggest that both α_i2- and α_i3-containing heterotrimeric G proteins may be involved in the signaling process. Also the generation of negative chronotropic responses to muscarinic or adenosine receptor agonists do not require activation of I_K^+Ach or the expression of α_i2 or α_i3.     

Average time until fixation of a mutant allele in a finite population under continued mutation pressure: Studies by analytical, numerical, and pseudo-sampling methods

We consider a single locus, and denote by A the wild-type allele and by A′ the mutant allele that is produced irreversibly in each generation from A at the rate v. Let 1 + s, 1 + h, and 1 be, respectively, the relative fitnesses of mutant homozygote A′A′, mutant heterozygote A′A, and wild-type homozygote AA. Then, it is shown, on the basis of the diffusion equation method, that the average time until fixation of the mutant allele (A′) in a randomly mating population of effective size N_e, given that the initial frequency is p, is [Formula: see text] in which B(x) = (S/2)x² + Hx(1 - x), S = 4N_es, H = 4N_eh, and V = 4N_ev. Of particular interest are the cases in which the mutant allele is deleterious (s = -s′, s′ > 0). Three cases are considered; the mutant is: (i) completely dominant s = h = -s′, (ii) completely recessive s = -s′, h = 0, and (iii) semidominant s = -s′, h = -s′/2, in which s′ is the selection coefficient against the mutant homozygote. It is shown that the average time until fixation is shorter when the deleterious mutant allele is dominant than when it is recessive if 4N_ev is larger than 1. On the other hand, the situation is reversed if 4N_ev is smaller than 1. It is also shown that for a mutant allele for which N_es′ > 10, it takes such a long time until fixation that we can practically ignore the occurrence of random fixation of a deleterious allele under continued mutation pressure. To supplement the analytical treatment, extensive simulation experiments were performed by using a device called the pseudo-sampling variable, which can enormously accelerate the process of simulation by a computer. This method simulates the diffusion process itself rather than the binominal sampling process (in population genetics the diffusion model is usually regarded as an approximation of the discrete binomial sampling process).     

Highly efficient and robust molecular ruthenium catalysts for water oxidation

Water oxidation catalysts are essential components of light-driven water splitting systems, which could convert water to H₂ driven by solar radiation (H₂O + hν → 1/2O₂ + H₂). The oxidation of water (H₂O → 1/2O₂ + 2H⁺ + 2e^-) provides protons and electrons for the production of dihydrogen (2H⁺ + 2e^- → H₂), a clean-burning and high-capacity energy carrier. One of the obstacles now is the lack of effective and robust water oxidation catalysts. Aiming at developing robust molecular Ru-bda (H₂bda = 2,2^′-bipyridine-6,6′-dicarboxylic acid) water oxidation catalysts, we carried out density functional theory studies, correlated the robustness of catalysts against hydration with the highest occupied molecular orbital levels of a set of ligands, and successfully directed the synthesis of robust Ru-bda water oxidation catalysts. A series of mononuclear ruthenium complexes [Ru(bda)L₂] (L = pyridazine, pyrimidine, and phthalazine) were subsequently synthesized and shown to effectively catalyze Ce^IV-driven [Ce^IV = Ce(NH₄)₂(NO₃)₆] water oxidation with high oxygen production rates up to 286 s^-1 and high turnover numbers up to 55,400.     

Synthesis and Characterization of a Crystalline Imine-Based Covalent Organic Framework with Triazine Node and Biphenyl Linker and Its Fluorinated Derivate for CO2/CH4 Separation

A catalyst-free Schiff base reaction was applied to synthesize two imine-linked covalent organic frameworks (COFs). The condensation reaction of 1,3,5-tris-(4-aminophenyl)triazine (TAPT) with 4,4′-biphenyldicarboxaldehyde led to the structure of HHU-COF-1 (HHU = Heinrich-Heine University). The fluorinated analog HHU-COF-2 was obtained with 2,2′,3,3′,5,5′,6,6′-octafluoro-4,4′-biphenyldicarboxaldehyde. Solid-state NMR, infrared spectroscopy, X-ray photoelectron spectroscopy, and elemental analysis confirmed the successful formation of the two network structures. The crystalline materials are characterized by high Brunauer–Emmett–Teller surface areas of 2352 m²/g for HHU-COF-1 and 1356 m²/g for HHU-COF-2. The products of a larger-scale synthesis were applied to prepare mixed-matrix membranes (MMMs) with the polymer Matrimid. CO₂/CH₄ permeation tests revealed a moderate increase in CO₂ permeability at constant selectivity for HHU-COF-1 as a dispersed phase, whereas application of the fluorinated COF led to a CO₂/CH₄ selectivity increase from 42 for the pure Matrimid membrane to 51 for 8 wt% of HHU-COF-2 and a permeability increase from 6.8 to 13.0 Barrer for the 24 wt% MMM.     

Protonophores induce plastoquinol oxidation and quench chloroplast fluorescence: Evidence for a cyclic, proton-conducting pathway in oxygenic photosynthesis

The photosynthetic apparatus converts light into chemical energy by a series of reactions that give rise to a coupled flow of electrons and protons that generate reducing power and ATP, respectively. A key intermediate in these reactions is plastoquinone (PQ), the most abundant electron and proton (hydrogen) carrier in photosynthetic membranes (thylakoids). PQ ultimately transfers electrons to a terminal electron acceptor by way of the Rieske Fe-S center of the cytochrome bf complex. In the absence of a terminal acceptor, electrons accumulate in the PQ pool, which is reduced to plastoquinol (PQH₂), and also on a specialized PQ, Q_A, which is reduced to an unprotonated semiquinone anion (Q_A^-). The accumulation of Q_A^- is measured by a rise in fluorescence yield and the accumulation of PQH₂ is measured by absorption difference spectrometry. We have found that in the absence of a terminal electron acceptor, two chemically diverse proton-conducting ionophores (protonophores), 2,6-di-t-butyl-4-(2′,2′-dicyanovinyl)phenol (SF 6847) and carbonylcyanide p-trifluoromethoxyphenylhydrazone (FCCP), induced oxidation of PQH₂ and quenching of chloroplast fluorescence, signifying oxidation of Q_A^-. The two protonophores produced the same effects even when the only recognized pathway of PQH₂ oxidation by way of the cytochrome bf complex was inhibited by dibromothymoquinone. Two other uncouplers, gramicidin and nigericin, which are not protonophores but facilitate proton movement across membranes by other mechanisms, were ineffective. These findings are consistent with the operation in the oxygen-generating photosystem (photosystem II) of a cyclic, proton-conducting pathway.     

Effects of ethanol on sodium, 3-O-methyl glucose,andl-alanine transport in the jejunum

Effects of ethanol on Na⁺, Cl^–, 3-O-methyl glucose (3-O-MG), andl-alanine fluxes were studied in the isolated rabbit jejunal mucosa. Ethanol (3% v/v present on both sides of the mucosa) decreased electrical potential difference (PD), short-circuit current (I_sc) and inhibited active transport of Na⁺, 3-O-MG, andl-alanine. This concentration also increased the permeability of the mucosa for Cl^–, 3-O-MG, andl-alanine. Ethanol at 5.4% potentiated the effects on PD, I_sc, and the permeability for electrolytes and organic substances. These effects of ethanol could not be fully explained by an osmotic action.This research was supported by funds from NIAAA grant 2R01 AA-00194-05.     

Structure of a Fe4O6-Heteraadamantane-Type Hexacation Stabilized by Chelating Organophosphine Oxide Ligands

Metal-containing heteraadamantanes are compounds of interest due to their spectroscopic and magnetic properties, which make them promising materials for non-linear optics and semiconductors. Herein we report the comprehensive structural characterization of a new coordination compound of the formula [(µ-OH′)₂(µ-OH″)₄(O = P(Ph₂)CH₂CH₂(Ph₂)P = O)₄{Fe(t-BuOH)}₄](PF₆)₄(Cl)₂ with the chelating ligand Ph₂P(O)-CH₂CH₂-P(O)Ph₂. The compound crystallizes as a polynuclear metal complex with the adamantane-like core [Fe₄O₆] in the space group I-43d of a cubic system. The single-crystal XRD analysis showed that the crystal contains one symmetrically independent octahedrally coordinated Fe atom in the oxidation state +3. The adamantine-like scaffold of the Fe complex is formed by hydroxy bridging oxygen atoms only. Hirshfeld surface analysis of the bridging oxygen atoms revealed two types of µ-OH groups, which differ in the degree of exposure and participation in long-range interactions. Additionally, the Hirshfeld surface analysis supported by the enrichment ratio calculations demonstrated the high propensity of the title complex to form C-H^…Cl, C-H^…F and C-H^…O interactions.     

A transient outward-rectifying K+ channel current down-regulated by cytosolic Ca2+ in Arabidopsis thaliana guard cells

Sustained (noninactivating) outward-rectifying K⁺ channel currents have been identified in a variety of plant cell types and species. Here, in Arabidopsis thaliana guard cells, in addition to these sustained K⁺ currents, an inactivating outward-rectifying K⁺ current was characterized (plant A-type current: I_AP). I_AP activated rapidly with a time constant of 165 ms and inactivated slowly with a time constant of 7.2 sec at +40 mV. I_AP was enhanced by increasing the duration (from 0 to 20 sec) and degree (from +20 to −100 mV) of prepulse hyperpolarization. Ionic substitution and relaxation (tail) current recordings showed that outward I_AP was mainly carried by K⁺ ions. In contrast to the sustained outward-rectifying K⁺ currents, cytosolic alkaline pH was found to inhibit I_AP and extracellular K⁺ was required for I_AP activity. Furthermore, increasing cytosolic free Ca²⁺ in the physiological range strongly inhibited I_AP activity with a half inhibitory concentration of ≈ 94 nM. We present a detailed characterization of an inactivating K⁺ current in a higher plant cell. Regulation of I_AP by diverse factors including membrane potential, cytosolic Ca²⁺ and pH, and extracellular K⁺ and Ca²⁺ implies that the inactivating I_AP described here may have important functions during transient depolarizations found in guard cells, and in integrated signal transduction processes during stomatal movements.     

Interconversion of Phospho- and Dephospho- Forms of Pig Heart Pyruvate Dehydrogenase

Pyruvate dehydrogenase from pig heart exists in active and inactive forms. Interconversion from the active (dephospho) form into the inactive (phospho) form is catalyzed by an ATP-dependent kinase. Conversely the enzyme is reactivated by a phosphatase which removes the phosphate group from the protein. By gradient centrifugation pyruvate dehydrogenase was prepared free of phosphatase but still containing the kinase. Reactivation of pyruvate dehydrogenase is stimulated by adenosine 3′,5′-cyclic phosphate. There is incorporation of ³²P from γ-³²P-ATP into the protein fraction containing the phosphatase and this phosphorylation reaction is also stimulated by adenosine 3′,5′-cyclic phosphate. The participation of this phosphate in the pyruvate dehydrogenase interconversion system suggests that, in heart muscle, pyruvate oxidation may be under hormonal control by a mechanism similar to that involved in the regulation of glycogen synthesis and breakdown.     

Comparative Evaluation of Cu(acac)2 and {[Cu(μ-O,O′-NO3) (L-arg) (2,2′-bpy)]·NO3}n as Potential Precursors of Electroless Metallization of Laser-Activated Polymer Materials

This paper presents a comparative assessment of Cu(acac)₂ and {[Cu(μ-O,O′-NO₃) (L-arg)(2,2′-bpy)]·NO₃}_n as potential precursors for the electroless metallization of laser activated polymer materials. Coatings consisting of polyurethane resin, one of the two mentioned precursor compounds, and antimony oxide (Sb₂O₃), as a compound strongly absorbing infrared radiation, were applied on the polycarbonate substrate. The coatings were activated with infrared Nd: YAG laser radiation (λ = 1064 nm) and electroless metallized. It was found that after laser irradiation, a micro-rough surface structure of the coatings was formed, on which copper was present in various oxidation states, as well as in its metallic form. For selected parameters of laser irradiation, it was possible to deposit a copper layer on the coating containing Cu(acac)₂ and Sb₂O₃, which is characterized by high adhesion strength. It was also found that the {[Cu(μ-O,O′-NO₃) (L-arg)(2,2′-bpy)]·NO₃}_n complex was not an effective precursor for the electroless metallization of Nd:YAG laser activated coatings. An attempt was made to determine the influence of the precursor chemical structure on the obtained metallization effects.