Stable monovalent aluminum(i) in a reduced phosphomolybdate cluster as an active acid catalyst

Low-valent aluminum Al(i) chemistry has attracted extensive research interest due to its unique chemical and catalytic properties but is limited by its low stability. Herein, a hourglass phosphomolybdate cluster with a metal-center sandwiched by two benzene-like planar subunits and large steric-hindrance is used as a scaffold to stabilize low-valent Al(i) species. Two hybrid structures, (H₃O)₂(H₂bpe)₁₁[Al^III(H₂O)₂]₃{[Al^I(P₄Mo^V₆O₃₁H₆)₂]₃·7H₂O (abbr. Al₆{P₄Mo₆}₆) and (H₃O)₃(H₂bpe)₃[Al^I(P₄Mo^V₆O₃₁H₇)₂]·3.5H₂O (abbr. Al{P₄Mo₆}₂) (bpe = trans-1,2-di-(4-pyridyl)-ethylene) were successfully synthesized with Al(i)-sandwiched polyoxoanionic clusters as the first inorganic-ferrocene analogues of a monovalent group 13 element with dual Lewis and Brønsted acid sites. As dual-acid catalysts, these hourglass structures efficiently catalyze a solvent-free four-component domino reaction to synthesize 1,5-benzodiazepines. This work provides a new strategy to stabilize low-valent Al(i) species using a polyoxometalate scaffold.

Monovalent aluminum(i) species was successfully stabilized using a reduced phosphomolybdate scaffold as a dual-acid catalyst for a four-component domino reaction.

Low- or sub-valence aluminum compounds are increasingly growing into a significant frontier subject in coordination and modern organic synthetic chemistry owing to their unique singlet carbene character, Lewis acid/base properties and catalytic reactivity.¹ However, low-valence aluminum(i) compounds have inherent electron deficiency and exhibit thermodynamic instability, making them prone to self-polymerization with metal–metal bonds² or disproportionation³ to metallic Al and Al(iii) species. Inspired by the special stabilizing effect of metallocene compounds, a ligand stabilization strategy has recently been undertaken to stabilize the low-valence aluminum center.^4,5 In this regard, the utilized ligand should satisfy two key criteria: (i) sufficient steric hindrance is required to inhibit monomer polymerization; and (ii) a suitable electronic effect is needed to stabilize the aluminum(i) center. A few organometallic Al(i) compounds protected by bulky organic groups have been prepared such as [(Cp*Al)₄] (Cp* = C₅Me₅),⁶ and [(CMe₃)₃SiAl₄].⁷ However, despite having the ligand effect, most of these Al(i) compounds still decompose in aqueous solutions or heating conditions. In contrast to organometallic Al(i) compounds, inorganic Al(i) structures, i.e. monomeric monohalides, only exist in gaseous form at high temperature⁸ and to the best of our knowledge, no stable inorganic Al(i) compound is known at room temperature due to thermodynamic instability. Therefore, exploring efficient strategies to synthesize stable inorganic Al(i) compounds remains highly desired but a great challenge.Polyoxometalates (POMs), a diverse family of inorganic molecular clusters based on early-transition metals (W, Mo, V, Nb, and Ta), have extensively attracted attention in research in various fields of materials science, coordination chemistry, medicinal chemistry and catalysis science.^9–11 Owing to their adjustable constituent elements and well-defined structures, POMs have been considered as promising inorganic ligands to stabilize high- and low-valent metal ions. For instance, Rompel et al.¹² reported one Keggin-type [α-CrW₁₂O₄₀]⁵⁻ anion in which a labile {Cr^IIIO₄} tetrahedral unit was assembled at the center of the cluster. Li and co-workers employed a monolacunary Keggin-type inorganic ligand to stabilize a high-valent Cu³⁺ ion.¹³ As a unique member of the POM family, the hourglass-type phosphomolybdate cluster {M[P₄Mo^V₆O₃₁]₂}ⁿ⁻ (abbr. M{P₄Mo₆}₂), consisting of two [P₄Mo^V₆O₃₁]¹²⁻ (abbr. {P₄Mo₆}₂) subunits bridged by one metal (M) center, represents a fully reduced metal-oxo cluster. With all Mo atoms in the oxidation state of (+5), a more negative charge is endowed to the cluster surface.^14,15 Such high electron density of {M[P₄Mo^V₆O₃₁]₂}ⁿ⁻ polyoxoanions provides an electron-rich local environment for the possible stabilization of unusual-valence metals. It is worth noting that the [P₄Mo^V₆O₃₁]¹²⁻ subunit presents near-planar triangular structures with the side sizes ranging from 7.50–7.92 Å (Fig. S1^†). The structural feature can supply sufficient steric hindrance to restrain the polymerization of low-valence metal species. Moreover, the six Mo atoms in each [P₄Mo^V₆O₃₁]¹²⁻ subunit arrange in a planar hexagonal-ring structure like a benzene ring, implying that such {M[P₄Mo^V₆O₃₁]₂}ⁿ⁻ clusters may have a similar delocalized electron structure to conjugated benzene or cyclopentadiene. These features make [P₄Mo^V₆O₃₁]¹²⁻ a promising candidate with respect to organic protecting groups to construct an inorganic ‘ferrocene’ analogue of Al(i) (Scheme 1). Therefore, we hypothesize that hourglass-type polyoxoanion clusters are promising to stabilize the labile Al(i) center and isolate inorganic Al(i) species.Open in a separate windowScheme 1Similar ferrocene-like sandwich structure features of an inorganic hourglass-type [Al^I(P₄Mo^V₆O₃₁)₂]²³⁻ polyanion to an organometallic [(η⁵-Cp*)₂Al^I]⁺ cation.Herein, we show a [P₄Mo^V₆O₃₁]¹²⁻ cluster as an inorganic scaffold to stabilize the Al(i) center in two hybrid compounds, (H₃O)₂(H₂bpe)₁₁[Al^III(H₂O)₂]₃{[Al^I(P₄Mo^V₆O₃₁H₆)₂]₃·7H₂O (abbr. Al₆{P₄Mo₆}₆) and (H₃O)₃(H₂bpe)₃[Al^I(P₄Mo^V₆O₃₁H₇)₂]·3.5H₂O (abbr. Al{P₄Mo₆}₂) (bpe = trans-1,2-di-(4-pyridyl)-ethylene), in which the labile Al(i) center is sandwiched by two [P₄Mo^V₆O₃₁]¹²⁻ sides, forming an inorganic moiety of a ‘ferrocene’ analogue. Both Al₆{P₄Mo₆}₆ and Al{P₄Mo₆}₂ are experimentally determined at room temperature for the first time, and prepared by hydrothermal reactions of Na₂MoO₄·2H₂O, H₃PO₄, AlCl₃·6H₂O, ethanol and N-containing bpe at 160 °C with slightly different pH values. Notably, the combination of ethanol, N-containing bpe and high hydrothermal temperature is a prerequisite to the isolation of Al(i) species. First, both ethanol and N-containing bpe were used to provide a reducing environment under hydrothermal conditions. By combining high temperature and pressure, sufficient energy is supplied to reduce Mo⁶⁺ and Al³⁺ ions to Mo⁵⁺ and Al⁺ species, respectively. Then, Mo⁵⁺ species and phosphoric acid molecules are assembled to form [P₄Mo₆O₃₁]¹²⁻ subunits, which are subsequently combined with Al⁺ ions to form hourglass-type [Al(P₄Mo₆O₃₁)₂]²³⁻, hence effectively stabilizing Al(i) species (Fig. 1). From the perspective of stereochemistry, two highly negative [P₄Mo₆O₃₁]¹²⁻ fragments, resembling the methyl cyclopentadiene organic group, sandwich one low-valent metal Al(i) center. Hence, the construction of a strong reducing hourglass-like skeleton makes it possible to stabilize the existing Al⁺ species.Open in a separate windowFig. 1Ball-and-stick diagram showing the assembly of the hourglass-type cluster {Al(P₄Mo₆)₂}.Single crystal X-ray diffraction revealed the hourglass-type {Al(P₄Mo₆)₂} cluster in Al₆{P₄Mo₆}₆ and Al{P₄Mo₆}₂ (Table S1^†), in which the [P₄Mo₆O₃₁]¹²⁻ subunits have a C₃ symmetry and display a near-planar structure formed by six edge-sharing {MoO₆} octahedra with alternating short Mo–Mo single bonds and long non-bonding Mo⋯Mo contacts. The side sizes of the {P₄Mo₆} subunit range from 7.50–7.92 Å, which supplies sufficient steric hindrance to restrain the polymerization or disproportionation of low-valence Al(i) species. All Mo atoms are in a reduced oxidation state of +5 and the central Al atoms are in the +1 oxidation state, as confirmed by bond valence calculations (Table S2^†). Thus, the synthesized Al{P₄Mo₆}₂ represents a fully reduced metal–oxygen cluster. Moreover, the six Mo atoms in each {P₄Mo₆} subunit present a benzene-like planar hexagonal-ring structure with a similar π-type delocalization electron interaction with Al(i) instead of organic bulky groups. Such π-type delocalization electron interaction constructs an inorganic ‘ferrocene’ analogue of Al(i) and produces sufficient delocalization energy to stabilize Al(i) species. Considering the formation mechanism of traditional metallocenes, {P₄Mo₆} subunits with a similar strong electron-donating ability and suitable steric-hindrance effect on Cp rings, augment the stability of Al(i) species. Al₆{P₄Mo₆}₆ and Al{P₄Mo₆}₂ compounds also present the first isolation of aluminum-sandwiched hourglass-type clusters in POM chemistry. Importantly, regarding the inherent and strong hydrolysis of aluminum species in water, these low-valent Al(i)-containing clusters represent the first example of stable solid-state inorganic sub-valent Al(i) compounds at room temperature.The asymmetric structure of Al₆{P₄Mo₆}₆ consists of two crystallographically independent {Al(P₄Mo₆)₂} clusters sandwiched by central Al(1) and Al(4) atoms, two bridging [Al(H₂O)₂]³⁺ (Al(2) and Al(3)) cations and six protonated bpe cations (Fig. S2^†). Aluminum centers involve two kinds of oxidation states: the central Al(1) and Al(4) are in the +1 state, while the bridging Al(2) and Al(3) are in the +3 state. Both Al(1) and Al(4) display the six-coordinated octahedral configuration and bridge two {P₄Mo₆} subunits to form two {Al^I(P₄Mo₆)₂} clusters. The average lengths of Al–O bonds are 2.318–2.324 Å for Al(1) and Al(4) (Table S3^†), which are slightly longer than those of classic Al–O bonds (1.90 Å) for Al(2) and Al(3), but close to that of the Al–O bond in silica-supported alkylaluminum(i) composites.^16–20 The long Al–O lengths for Al(1) and Al(4) centers may be ascribed to the lower electron cloud density located at the surface of the Al(i) cation, resulting in slightly longer bonds with the surrounding oxygen donors.^5,21 Moreover, the small distorted extents (sum((d_ij − d_ave)/d_ave)²/coordination number) of {Al(1)O₆} (3.86 × 10⁻⁴) and {Al(4)O₆} (1.89 × 10⁻³) indicate that they are in regular octahedral geometry. Moreover, another structural feature of Al₆{P₄Mo₆}₆ is that {Al^I(P₄Mo₆)₂} clusters are connected by bridging [Al(H₂O)₂]³⁺ cationic fragments (Al(2) and Al(3)), forming an unusual chain-like arrangement (Fig. 2a). It is worth noting that the 1-D chain contains a large repeating monomer with the maximum spacing of 81.69 Å, consisting of twelve Al-containing fragments ({–Al2–Al1–Al3–Al4–Al3–Al1–Al2–Al1–Al3–Al4–Al3–Al1–}). Such a long repeating monomer is rare. Each repeating monomer has two types of symmetric systems: Al(2) in the middle of the monomer plays a center of mirror symmetry and divides the whole repeating monomer into two equidistant half-units of {–Al1–Al3–Al4–Al3–Al1–}; Al(4) in each half-unit further acts as the reverse symmetric center of two {–Al3–Al1–Al2–} subunits. The two types of symmetrical systems form the infinitely extending chain-like structure in Al₆{P₄Mo₆}₆. Since bpe is a rigid and conjugated molecular structure, an effective π⋯π stacking interaction emerges and results in a honeycomb-like supramolecular organic moiety, which accommodates these 1-D inorganic chains and stabilizes the whole Al₆{P₄Mo₆}₆ framework (Fig. S3 and S4^†).Open in a separate windowFig. 2(a) One-dimensional (1D) inorganic structure in Al₆{P₄Mo₆}₆ with a length of repeating units of 81.69 Å, consisting of twelve Al-containing fragments ({–Al2–Al1–Al3–Al4–Al3–Al1–Al2–Al1–Al3–Al4–Al3–Al1–}). (b) Four kinds of coordination environments of {AlO₆} octahedra, respectively (i = 1 − x, y, 0.5 − z; ii = 0.5 − x, 1.5 − y, 1 − z).Al{P₄Mo₆}₂ has a similar structure to Al₆{P₄Mo₆}₆ (Table S4^†), wherein the most obvious difference is that {Al^I[P₄Mo₆]₂} clusters exist in isolated form and interact with the surrounding protonated bpe cations via hydrogen bonding to form into a 3-D supramolecular framework (Fig. S5 and S6^†). The different peripheral environment around the {Al^I[P₄Mo₆]₂} cluster can affect its acidity and catalytic activity.The solid-state ²⁷Al NMR spectrum of Al₆{P₄Mo₆}₆ depicts two distinct resonances at δ = −22.34 and 27.33 ppm due to the octahedrally coordinated Al^III and Al^I sites, respectively (Fig. 3a), indicating two types of Al local environments in Al₆{P₄Mo₆}₆. In contrast, Al{P₄Mo₆}₂ displays only one sharp signal at δ = 7.20 ppm due to the octahedrally coordinated Al^I sites (Fig. 3b). The observed narrow peak-width corresponds to the highly symmetric charge distribution at the aluminum nucleus, similar to the ferrocene analogue [(η⁵-Cp*)₂Al^I]⁺.⁵ Noticeably, Al^I resonance in Al₆{P₄Mo₆}₆ appears at a lower magnetic field compared to Al{P₄Mo₆}₂, due to the different peripheral environment around the hourglass {Al(P₄Mo₆)₂} cluster. XPS spectra of Al₆{P₄Mo₆}₆ and Al{P₄Mo₆}₂ further affirm the valence states of Al and Mo elements (Fig. S7 and Table S5^†). The Al 2p XPS profile of Al₆{P₄Mo₆}₆ reveals two peaks at 74.39 and 73.75 eV ascribed to Al^III and Al^I, respectively (Fig. 3c). The area ratio of the two peaks is close to 1 : 1, in consistence with the chemical structure of Al₆{P₄Mo₆}₆. The high-resolution Al 2p XPS spectrum of Al{P₄Mo₆}₂ displays a weaker broad peak attributed to the low amount of Al⁺ (Fig. 3d). Moreover, the structural stabilities of Al₆{P₄Mo₆}₆ and Al{P₄Mo₆}₂ were investigated by soaking them in water for 24 hours. Fig. S9–S11^† show the comparison of XRD, IR and XPS spectra of Al₆{P₄Mo₆}₆ and Al{P₄Mo₆}₂ before and after soaking in water. It can be found that the characteristic diffraction peaks in XRD after soaking for 24 hours still show good agreement with the simulated data (Fig. S9^†). The characterized absorption bands in IR spectra also exhibit good match with the original Al₆{P₄Mo₆}₆ and Al{P₄Mo₆}₂ (Fig. S10^†). The XPS spectra of Al₆{P₄Mo₆}₆ after soaking in water were also obtained. There is basically no change in the high-resolution spectra of Al 2p with the Al^I/Al^III atomic ratios of ca. 1 : 1 (Fig. S11^†). The spectroscopic and theoretical observations verify that the low valence Al(i) species can stably exist in the reduced phosphomolybdates in the solid state (Fig. S12 and Table S6^†). Moreover, the acidities of Al₆{P₄Mo₆}₆ and Al{P₄Mo₆}₂ were measured to be 0.27 and 0.442 mmol g⁻¹, respectively, demonstrating the promising potential of Al₆{P₄Mo₆}₆ and Al{P₄Mo₆}₂ as dual-acid catalysts.Open in a separate windowFig. 3(a and b) ²⁷Al NMR spectra of solid Al₆{P₄Mo₆}₆ and Al{P₄Mo₆}₂; (c and d) XPS spectra of Al in Al₆{P₄Mo₆}₆ and Al{P₄Mo₆}₂.The catalytic performance of Al₆{P₄Mo₆}₆ and Al{P₄Mo₆}₂ was evaluated via a solvent-free four-component domino reaction for the synthesis of pharmaceutical intermediate 1,5-benzodiazepine (Table 1). With Al₆{P₄Mo₆}₆ and Al{P₄Mo₆}₂ as catalysts, the yields of the final product 8aaa reach 83% and 75%, respectively (Table 1, entries 1 and 2). Almost no 8aaa is observed without the acid catalysts, even when the reaction is set for a long time (Table 1, entry 3). This clarifies the excellent catalytic performance of Al₆{P₄Mo₆}₆ and Al{P₄Mo₆}₂. Typical Brønsted acid p-TsOH and Lewis acid AlCl₃ as control samples yield only 43% and 29% 8aaa, respectively (Table 1, entries 4 and 5), much lower than those attained by Al₆{P₄Mo₆}₆ and Al{P₄Mo₆}₂ catalysts. Moreover, (H₂en)₁₂[{Na_0.8K_0.2(H₂O)}₂{Na[Mo₆O₁₂(OH)₃(HPO₄)₂(PO₄)₂]₂}₂]·7H₂O^22,23 (abbr. {Na[P₄Mo₆]₂}) in contrast achieved 72% yield of 8aaa in 30 min, slower than that of Al₆{P₄Mo₆}₆ and Al{P₄Mo₆}₂. This indicates the advantage of the unique dual-acid features of Al(i)-stabilized reduced phosphomolybdate clusters with multiple Lewis and Brønsted acid active centers, in which the synergistic effect between the Al species and reduced phosphomolybdate cluster contributes to the catalytic activity.Comparison tests of one-pot synthesis of 1,5-benzodiazepine 8aaavia a four-component domino reaction^a

水/硅胶吸附体系的热化学研究

我们用精密自动绝热量热计测定了几种不同吸附水含量的水/硅胶吸附体系在200～320 K温度范围内的热容. 结果表明, 当吸附水含量使表面复盖度(θ)大于1时, 在相应的C_p～T曲线上会出现吸附水的相变峰. 这说明吸附在硅胶表面上的水分子已经形成了聚集态; 而当θ<1时, 由于尚未形成聚集态水, 故没有相变过程出现, 其C_p～T曲线呈光滑状. 这些现象与H_2O/γ-Al_2O_3吸附体系是一致的. 又由于硅胶表面对水分子的吸附力较γ-Al_2O_3的要小, 故在同样的吸附量的C_p～T曲线上, 水/硅胶的峰要比H_2O/γ-Al_2O_3的尖锐, 且蜂温增高的速度要快. 这些都表明, 吸附在硅胶表面上的二维表相水会随吸附量的增加而以较快的趋势接近于体相水. 此外, 由不同含水量的C_p～T曲线外推, 求出了不含吸附水的硅胶在200～300 K范围内的热容.     

Enantioselective separation in capillary electrophoresis using a novel mono-6-propylammonium-β-cyclodextrin as chiral selector

The chiral resolving ability of a novel single-isomer cationic β-cyclodextrin (CD), mono-6^A-propylammonium-6^A-deoxy-β-cyclodextrin chloride (PrAMCD), as a chiral selector in capillary electrophoresis (CE) is reported in this work for the enantioseparation of hydroxy, carboxylic acids and amphoteric analytes. The effect of chiral selector concentration on the resolution was studied. Good resolutions were achieved for hydroxy acids. Optimum resolutions were obtained even at 3.5 mM CD concentration for carboxylic acids. The electrophoretic method showed good linearity and reproducibility in terms of migration times and peak areas, which should make it suitable for routine analysis. In addition, baseline chiral separation of a six-acid mixture was achieved within 20 min. PrAMCD proved to be an effective chiral selector for acidic analytes.     

Reactivity of organolanthanide and organolithium complexes containing the guanidinate ligands toward isocyanate or carbodiimide: synthesis and crystal structures

The direct reactions of (C5H5)2LnCl with LiN=C(NMe2)2 proceeded at room temperature in THF under pure nitrogen to yield the lanthanocene guanidinate complexes [(C5H5)2Ln(mu-eta1:eta2-N=C(NMe2)2)]2 (Ln = Gd (1), Er (2)). Treatment of phenyl isocyanate with complexes 1 and 2 results in monoinsertion of phenyl isocyanate into the Ln-N(mu-Gua) bond to yield the corresponding insertion products [(C5H5)2Ln(mu-eta1:eta2-OC(N=C(NMe2)2)NPh)]2 (Ln = Gd (3), Er (4)), presenting the first example of unsaturated organic small molecule insertion into the metal-guanidinate ligand bond. Further investigations indicate that N,N'-diisopropylcarbodiimide does not react with complexes 1 and 2 under the same conditions; however, it readily inserts into the lithium-guanidinate ligand bond of LiN=C(NMe2)2. As a synthon of the insertion product Li[(iPrN)2C(N=C(NMe2)2)], its reaction with (C5H5)2LnCl gives the novel organolanthanide complexes containing the guanidinoacetamidinate ligand, (C5H5)2Ln[(iPrN)2C(N=C(NMe2)2)] (Ln = Yb (5), Er (6), Dy (7)). All complexes were characterized by elemental analysis and spectroscopic properties. The structures of complexes 1, 3, 5 and 7 were determined through X-ray single-crystal diffraction analysis.     

SARS-CoV protease inhibitors design using virtual screening method from natural products libraries

Two natural products databases, the marine natural products database (MNPD) and the traditional Chinese medicines database (TCMD), were used to find novel structures of potent SARS-CoV protease inhibitors through virtual screening. Before the procedure, the databases were filtered by Lipinski's ROF and Xu's extension rules. The results were analyzed by statistic methods to eliminate the bias in target-based database screening toward higher molecular weight compounds for enhancing the hit rate. Eighteen lead compounds were recommended by the screening procedure. They were useful for experimental scientists in prioritizing drug candidates and studying the interaction mechanism. The binding mechanism was also analyzed between the best screening compound and the SARS protein.     

Human immunoglobulin adsorption investigated by means of quartz crystal microbalance dissipation, atomic force microscopy, surface acoustic wave, and surface plasmon resonance techniques

Time-resolved adsorption behavior of a human immunoglobin G (hIgG) protein on a hydrophobized gold surface is investigated using multitechniques: quartz crystal microbalance/dissipation (QCM-D) technique; combined surface plasmon resonance (SPR) and Love mode surface acoustic wave (SAW) technique; combined QCM-D and atomic force microscopy (AFM) technique. The adsorbed hIgG forms interfacial structures varying in organization from a submonolayer to a multilayer. An "end-on" IgG orientation in the monolayer film, associated with the surface coverage results, does not corroborate with the effective protein thickness determined from SPR/SAW measurements. This inconsistence is interpreted by a deformation effect induced by conformation change. This conformation change is confirmed by QCM-D measurement. Combined SPR/SAW measurements suggest that the adsorbed protein barely contains water after extended contact with the hydrophobic surface. This limited interfacial hydration also contributed to a continuous conformation change in the adsorbed protein layer. The viscoelastic variation associated with interfacial conformation changes induces about 1.5 times overestimation of the mass uptake in the QCM-D measurements. The merit of combined multitechnique measurements is demonstrated.     

Single electron emission from the closed-tips of single-walled carbon nanotubes

The single electron emission behaviors and characteristics from the well-defined quantized energy levels, corresponding to localized electronic states at the dome-structure tips, in single-walled carbon nanotubes (SWNTs) are investigated and illuminated by use of the energy level emission model in combination with the first-principles calculations on the electronic structures. Under the external electric field, the confined electrons are emitted simultaneously from each quantized energy level by virtue of the resonant tunneling effects. With increasing applied voltage, the emission current increases monotonically and exponentially up to the first peak value, and then steps into the increasing and decreasing "sawtoothlike" variations in sequence. The negative differential resistance or conductivity and the maximum current for SWNTs are simulated. The influences of localized electronic states and curvatures of the different closed tips on the single electron emission behaviors of SWNTs are evaluated and discussed. Also a few issues and applications relevant to electron emission of carbon nanotubes are addressed.     

Dehydrogenation activity and structure of polyaniline supported heteropolyacid catalysts

Polyaniline(PAN) supported H₆P₂W₁₈O₆₂(PW) , H₃PMo₁₂O₄₀ (PMo) and H₄PMo₁₁VO₄₀(PMoV) catalysts were prepared and their activities for hexanol conversion were tested. IR, XRD, ICP and SEM measurements proved that the heteropolyacids (HPA) could be supported on this type of polymer. The PAN supported HPA catalysts exhibit higher redox activities and low acid-base activities for the hexanol conversion. The redox activities increase with increasing amount of the heteropolyacid. Substitution of Mo ion by V ion results in an increase of redox activities of the catalysts. This revised version was published online in June 2006 with corrections to the Cover Date.     

Transition metal substituted tungstophosphoric compound catalyzed oxidation of hexanol to hexanal with hydrogen peroxide

Summary The oxidation of hexanol in the presence of the Keggin-type heteropoly compounds (HPCs) H₃PMo_nW_12-nO₄₀ (denoted as PMo_nW_12-n, n=0,1) and Na₅PW₁₁ZO₃₉ (denoted as PW₁₁Z, Z = Mn, Fe, Co, Ni, Cu and Zn) was carried out to produce hexanal and hexanoic acid. The reaction was conducted in tert-butanol (t-BuOH), using cetylpyridinium bromide (CPB) salts of HPA and 15% aqueous H₂O₂ as oxidant under mild condition. The PMoW₁₁ catalyst showed higher hexanol conversion of 25%, the lowest selectivity to hexanal of 64.4% and an efficient utilization of H₂O₂ of 34%. Over the transition metal substituted PW₁₁Z catalysts decomposition of H₂O₂ was rapid. For these PW₁₁Z catalysts, the efficient utilization of H₂O₂ decreased to 9% or even lower. By means of IR, UV-visible and GC-MS techniques the catalysts were characterized.     

Electrostatic recognition and induced fit in the kappa-PVIIA toxin binding to Shaker potassium channel

Brownian dynamics (BD) and molecular dynamics (MD) simulations and electrostatic calculations were performed to study the binding process of kappa-PVIIA to the Shaker potassium channel and the structure of the resulting complex. BD simulations, guided by electrostatic interactions, led to an initial alignment between the toxin and the channel protein. MD simulations were then carried out to allow for rearrangements from this initial structure. After approximately 4 ns, a critical "induced fit" process was observed to last for approximately 2 ns. In this process, the interface was reorganized, and side chains were moved so that favorable atomic contacts were formed or strengthened, while unfavorable contacts were eliminated. The final complex structure was stabilized through electrostatic interactions with the positively charged side chain of Lys7 of kappa-PVIIA deeply inserted into the channel pore and other hydrogen bonds and by hydrophobic interactions involving Phe9 and Phe23 of the toxin. The validity of the predicted structure for the complex was assessed by calculating the effects of mutating charged and polar residues of both the toxin and the channel protein, with the calculated effects correlating reasonably well with experimental data. The present study suggests a general binding mechanism, whereby proteins are pre-aligned in their diffusional encounter by long-range electrostatic attraction, and nanosecond-scale rearrangements within the initial complex then lead to a specifically bound complex.