Reactivity of ruthenium carbonyls on metal oxide surfaces: effects of the surface acid-base chemistry

The reactivity of tetraruthenium carbonyl clusters supported on metal oxides (SiO₂, TiO₂, γ-Al₂O₃, and MgO) under various atmospheres has been investigated by infrared and ultraviolet-visible spectroscopies and electron microscopy. [H₄Ru₄(CO)₁₂] physisorbed on SiO₂ easily decomposes and aggregates to form metal particles. The tetraruthenium clusters supported on TiO₂ are oxidized by surface hydroxyl groups, giving mononuclear complexes; virtually the same chemistry occurs, but less readily, on γ-Al₂O₃. The tetramthenium clusters supported on MgO, in contrast, are highly resistant to oxidation. The strong basicity of this support leads to an increased stability of small polynuclear complexes, hindering the formation of oxidized complexes and aggregated metallic structures. The unique character of the basic support appears to be related to the tendency of the support to form and stabilize cluster anions.     

Structural aspects of anthocyanin-flavonoid complex formation and its role in plant color

The complex formation of flavonoids with anthocyanins, resulting in increase in both absorbance and in a bathochromic shift of the visible absorption maximum of the latter, is based mainly on hydrogen bond formation between the carbonyl group of the anthocyanin anhydrobase and aromatic hydroxyl groups of the complex-forming flavonoids. The larger the number of hydroxyl groups in the flavonoid molecule, the stronger the complex formation. The presence of a 3-hydroxyl group in the flavonoid molecule has little effect on the complex-forming ability. The nature of the sugar substituent of the complex-forming flavonoid compound has no influence on the reaction. The 5-hydroxyl group of flavonoids is strongly bound by intramolecular hydrogen bond to the 4-carbonyl and does not participate in the complex formation. The most important hydroxyl group in the flavonoid molecule is the one in the 7-position. Unsaturation at C₂C₃ in the heterocyclic ring is an important factor for complex formation. Aromatic hydroxyl groups in the flavonoid system alone cannot account for all the complex-forming ability, suggesting additional involvement by electrostatic forces and configurational or steric effects.     

Electron exchange coupling in a naturally occurring tetramangano cluster in the mineral helvite, (Mn4S)(SiBeO4)3

The mineral helvite, (Mn₄S)(BeSiO₄)₃, contains discrete tetrahedral Mn₄S⁺⁶ clusters in which the S^?2 is tetrahedrally coordinated and each Mn(II) is in a distorted tetrahedron of one S^?2 and three oxygens; the cluster is situated within an encompassing lattice of SiO₄^?4 and BeO₄^?6 tetrahedra. Mn₄S⁺⁶ centers provide an interesting model for comparison to the polynuclear manganese center that is associated with photosynthetic water oxidation. Magnetic susceptibility data between 77 and 298 K have been measured for a natural helvite sample containing principally Mn₄S⁺⁶ centers but with significant contamination from Mn₃FeS⁺⁶ and Mn₃CaS⁺⁶. The data exhibited Curie-Weiss behavior with μ_eff = 5.969 B.M. and θ = 178.3 K. An analysis of the magnetic susceptibility, based on Van Vleck's formalism, demonstrated the presence of antiferromagnetic coupling, with a coupling constant J = ?5.83 cm^?1. Mössbauer spectra of Mn₃FeS centers in helvite and of Fe₄S centers in the related mineral danalite have also been recorded. Isomer shifts show little temperature dependence and lie in the range 1.23–1.43 $\text{mm}\text{sec.}$. This range is typical of tetrahedrally coordinated Fe(II) in several ionic crystals but is significantly above that of Fe(II) in ferredoxins and below that in the [quinone-Fe(II)-quinone] complex of the photosynthetic bacterium,Rhodopseudomonas sphaeroides. Quadrupole splittings are highly temperature dependent, ranging from 2.4 $\text{mm}\text{sec}$ at 4.2 K to less than 0.5 $\text{mm}\text{sec}$ at 248 K.     

Modulating weak intramolecular interactions through the formation of beryllium bonds: complexes between squaric acid and BeH2

The electronic structure of the two most stable isomers of squaric acid and their complexes with BeH₂ were investigated at the B3LYP/6-311?+?G(3df,2p)// B3LYP/6-31?+?G(d,p) level of theory. Squaric acid forms rather strong beryllium bonds with BeH₂, with binding energies of the order of 60 kJ?mol^?1. The preferential sites for BeH₂ attachment are the carbonyl oxygen atoms, but the global minima of the potential energy surfaces of both EZ and ZZ isomers are extra-stabilized through the formation of a BeH···HO dihydrogen bond. More importantly, analysis of the electron density of these complexes shows the existence of significant cooperative effects between the beryllium bond and the dihydrogen bond, with both becoming significantly reinforced. The charge transfer involved in the formation of the beryllium bond induces a significant electron density redistribution within the squaric acid subunit, affecting not only the carbonyl group interacting with the BeH₂ moiety but significantly increasing the electron delocalization within the four membered ring. Accordingly the intrinsic properties of squaric acid become perturbed, as reflected in its ability to self-associate.

A Simple Method for the Size Controlled Synthesis of Stable Oligomeric Clusters of Gold Nanoparticles under Ambient Conditions

Reducing dilute aqueous HAuCl₄ with sodium thiocyanate (NaSCN) under alkaline conditions produces 2 to 3 nm diameter nanoparticles. Stable grape-like oligomeric clusters of these yellow nanoparticles of narrow size distribution are synthesized under ambient conditions via two methods. The delay-time method controls the number of subunits in the oligoclusters by varying the time between the addition of HAuCl₄ to alkaline solution and the subsequent addition of reducing agent, NaSCN. The yellow oligoclusters produced range in size from ~3 to ~25 nm. This size range can be further extended by an add-on method utilizing hydroxylated gold chloride (Na⁺[Au(OH_4-x)Cl_x]^-) to auto-catalytically increase the number of subunits in the as-synthesized oligocluster nanoparticles, providing a total range of 3 nm to 70 nm. The crude oligocluster preparations display narrow size distributions and do not require further fractionation for most purposes. The oligoclusters formed can be concentrated >300 fold without aggregation and the crude reaction mixtures remain stable for weeks without further processing. Because these oligomeric clusters can be concentrated before derivatization they allow expensive derivatizing agents to be used economically. In addition, we present two models by which predictions of particle size can be made with great accuracy.     

Flavone Derivatives as Inhibitors of Insulin Amyloid-Like Fibril Formation

Several natural and synthetic flavone derivatives have been reported to inhibit formation of amyloid fibrils or to remodel existing fibrils. These studies suggest that the numbers and positions of hydroxyl groups on the flavone rings determine their effectiveness as amyloid inhibitors. In many studies the primary method for determining the effectiveness of inhibition is measuring Thioflavin T (ThT) fluorescence. This method demonstrably results in a number of false positives for inhibition. We studied the effects of 265 commercially available flavone derivatives on insulin fibril formation. We enhanced the effectiveness of ThT fluorescence measurements by fitting kinetic curves to obtain halftime of aggregation (t ₅₀). Maximal values of ThT fluorescence varied two fold or more in one third of all cases, but this did not correlate with changes in t ₅₀. Changes in t ₅₀ values were more accurate measures of inhibition of amyloid formation. We showed that without a change in an assay, but just by observing complete kinetic curves it is possible to eliminate numbers of false positive and sometimes even false negative results. Examining the data from all 265 flavones we confirmed previous observations that identified the importance of hydroxyl groups for inhibition. Our evidence suggests the importance of hydroxyl groups at locations 5, 6, 7, and 4’, and the absence of a hydroxyl group at location 3, for inhibiting amyloid formation. However, the main conclusion is that the positions are not additive. The structures and their effects must be thought of in the context of the whole molecule.     

Synthesis, characterization and luminescence property of heterobimetallic trinuclear Mo(W)-Ag-S clusters containing 1,2-bis(diphenylphosphino)-1,2-dicarba-closo-dodecaborane

Reactions of (NH₄)₂MS₄ or (NH₄)₂MOS₃ (M = Mo, W) with AgSCN and closo carborane diphosphine ligand 1,2-(PPh₂)₂-1,2-C₂B₁₀H₁₀ (L) in CH₂Cl₂ yielded four heterobimetallic trinuclear Mo(W)-Ag-S clusters: [Ag₂MoS₄L₂] (1), [Ag₂WS₄L₂] (2), [Ag₂MoOS₃L₂] (3) and [Ag₂WS₄L₂] (4), respectively. All the new clusters have been characterized by elemental analysis, FT-IR, UV-Vis, ¹H and ¹³C NMR spectroscopy and their molecular structures (except for 3) were further confirmed by single-crystal X-ray diffraction. X-ray crystal structure analysis showed that the closo carborane diphosphine ligand was coordinated bidentately to Ag(I) atom through its two phosphorus atoms, resulting in a stable five-member chelating ring between the diphosphine ligand and the metal. The coordination sphere of the central M atom, as well as all the Ag atoms, was tetrahedron. The skeletons of these clusters could be classified into two types: with (NH₄)₂MS₄, the three metal atoms (two Ag atoms and one M atom) are in a linear conformation, while with (NH₄)₂MOS₃, the conformation of the heterobimetallic trinuclear cluster is butterfly shaped. The luminescence properties of the clusters were investigated in CH₂Cl₂ solution at room temperature and for the first time the butterfly-shaped Ag-W-S cluster containing the Ag₂WS₄ core has been proved to show luminescence property.     

Superoxide Dismutase Enhanced the Formation of Hydroxyl Radicals in a Reaction Mixture Containing Xanthone under UVA Irradiation

To clarify the effect of superoxide dismutase (SOD) on the formation of hydroxyl radical in a standard reaction mixture containing 15 μM of xanthone, 0.1 M of 5,5-dimethyl-1-pyrroline N-oxide (DMPO), and 45 mM of phosphate buffer (pH 7.4) under UVA irradiation, electron paramagnetic resonance (EPR) measurements were performed. SOD enhanced the formation of hydroxyl radicals. The formation of hydroxyl radicals was inhibited on the addition of catalase. The rate of hydroxyl radical formation also slowed down under a reduced oxygen concentration, whereas it was stimulated by disodium ethylenediaminetetraacetate (EDTA) and diethyleneaminepentaacetic acid (DETAPAC). Above findings suggest that O₂, H₂O₂, and iron ions participate in the reaction. SOD possibly enhances the formation of the hydroxyl radical in reaction mixtures of photosensitizers that can produce O₂ ^?·.     

Analysis of degradation mechanism of disperse orange 25 in supercritical water oxidation using molecular dynamic simulations based on the reactive force field

4-[N-(2-cyanoethyl)-N-ethylamino]-4′-nitroazo-benzene (disperse orange 25, DO25) is one of the main components in dyeing wastewater. In this work, supercritical water oxidation (SCWO) process of DO25 has been investigated using the molecular dynamic simulations based on the reactive force field (ReaxFF). For the SCWO system, the effects of temperature, the molecular ratio of DO25, O₂ and H₂O as well as the reaction time have been analyzed. The simulated results showed that the aromatic rings in DO25 could be attacked by hydroxyl radical, oxygen molecule, and hydroxyl radical together with oxygen molecule, respectively, which caused the aromatic ring-opening reaction to happen mainly through three different pathways. The hydroxyl radicals were mainly from water clusters and H₂O₂ (which was produced from oxygen molecules reacting with water clusters). However, for the SCW system as comparison, the aromatic rings in DO25 could be attacked by hydroxyl radical only, and the OH radicals just come from water clusters. During the DO25 SCWO degradation process, we also found that N elements in one DO25 molecule were difficult to be converted into environmentally friendly N₂ molecules because of steric hindrance, but increasing the number of DO25 molecules could improve the possibility for the connection of N elements, thus promoting N element converting into N₂. Extending reaction time could also improve N elements in DO25 to transform into N₂ rather than carbonitride.

Open image in new window

Graphical Abstract The processes of making DO25 wastewater by SCWO into clean water

     

Isolation and Physiological Activity of Sclerin,a Metabolite of Sclerotinia Fungus

Sclerin, a colorless crystalline, C₁₃H₁₄O₄ compound melting at 123°C was at first isolated as a lipase formation stimulating constituent of Sclerotinia libertiana, from its own mycelial extract, and infrared spectrum showed the presence a hydroxyl group and a lactone ring. Sclerin was found to promote also the enzyme formation and growth of various plant seedlings such as those of castor bean-, mung bean-, and rice seedlings. In the growth of the sclerin-treated plant, promotion of root formation and increase of dry weight per unit shoot length were noticed, and the combined use of sclerin and gibberellin brought about a synergistic effect on the growth of rice seedlings. The relationship between sclerin and some other plant growth regulators in the enzyme formation of germinating seeds was also described.     

Ninefold coordination LaPO4: Pentagonal interpenetrating tetrahedral polyhedron

The crystal structure of LaPO₄ has been determined from three-dimensional single-crystal X-ray diffraction data. The respective residual indices R₁ and R₂ have been refined to 0.020 and 0.021, based on 625 unique reflections. Crystallization occurs in the monoclinic space group P2₁/n (No. 14) with a = 6.825(4), b = 7.057(2), c = 6.482(2) Å, and β = 103.21(4)°. Z = 4. The lanthanum metal atom is nine-coordinated to oxygen atoms forming a polyhedron best described as a pentagonal interpenetrating tetrahedron. The nine-coordinated La atoms are linked together by distorted tetrahedral phosphate groups. Important interatomic distances and angles are presented.     

Theoretical investigation of CdSe clusters: influence of solvent and ligand on nanocrystals

Based on experimental zinc blende and wurtzite models of CdSe nanocrystals, four clusters of CdSe, seven Se-Cd-ligand structures, and their characters are studied at DFT/B3LYP/Lanl2dz theoretical level. Cd₃Se₃, (Cd₃Se₃)₂ and (Cd₃Se₃)₃ clusters which have a ring with six atoms are similar to wurtzite structures, Cd₄Se₄ have resemblant conformation with zinc blende for they are all composed of tetrahedron. Calculated Raman spectra of Cd₃Se₃, Cd₄Se₄, (Cd₃Se₃)₂ and (Cd₃Se₃)₃ are about 175 cm⁻¹ which is consistent with the experimental result. Then, through investigation of Se-Cd-ligand clusters, we find that all Se-Cd-ligand structures have similar characters because main influence of ligands on nanocrystals comes from thiol. Finally, we testify that both solvent and ligand make absorption peaks shift to blue, compared with those in gas phase and without ligand. Under these conditions, calculated data of four clusters are almost identical with the absorption peaks of CdSe nanocrystals. Besides, we also prove that the absorption peaks of four clusters are the transitions from HOMO to LUMO or from d to p orbitals. And HOMO-LUMO gaps reduce in order of Cd₃Se₃, (Cd₃Se₃)₂ and (Cd₃Se₃)₃, which is induced by the quantum size effect.     

Metal cluster topology. 1. Osmium carbonyl clusters

Important theoretical approaches to metal cluster bonding including the Wade-Mingos skeletal electron pair method, the Teo topological electron count, the King-Rouvray graph theory derived method, and Lauher's extended Hückel calculations are shown to agree in their apparent skeletal electron counts for the most prevalent metal cluster polyhedra including the tetrahedron, the trigonal bipyramid (both ordinary and elongated), square pyramid, octahedron, bicapped tetrahedron, pentagonal bipyramid, and capped octahedron. The graph theory derived method is used to treat osmium carbonyl clusters containing from five to eleven osmium atoms. In this connection most osmium carbonyl clusters can be classified into the following types: (1) Clusters exhibiting edge- localized bonding containing multiple tetrahedral chambers (e.g., Os₅(CO)₁₆, Os₆(CO)₁₈, H₂Os₇(CO)₂₀ and HOs₈(CO)₂₂⁻); (2) Capped octahedral clusters derived from osmium carbonyl fragments of the type Os_6+p(CO)_19+2p (p = 0, 1, 2, and 4) (e.g., Os₆- (CO)₁₈²⁻, Os₇(CO)₂₁, Os₈(CO)₂₂²⁻, and H₄Os₁₀- (CO)₂₄²⁻). Other more unusual osmium carbonyl clusters such as the planar Os₆(CO)₁₇ [P(OCH₃)₃]₄, the Os₉ cluster [Os₉(CO)₂₁C₃H₂R]⁻, and the Os₁₁ cluster Os₁₁C(CO)₂₇²⁻ can also be treated satisfactorily by these methods. The importance of the number of ligands around isoelectronic Os_n systems in determining the cluster polyhedron is illustrated by the different cluster polyhedra found for each member of the following isoelectronic pairs: HOs₆- (CO)₁₈⁻/H₂Os₆(CO)₁₈. Os₇(CO)₂₁/H₂Os₇(CO)₂₀, Os₈(CO)₂₂²⁻/HOs₈(CO)₂₂⁻. The tendency for osmium carbonyl clusters frequently to form polyhedra exhibiting edge-localized rather than globally delocalized bonding relates to the facility for osmium carbonyl vertices to contribute more than three internal orbitals to the cluster bonding. In this way Wade's well-known analogy between boron hydride clusters and metal clusters, which assumes exactly three internal orbitals for each vertex atom, is frequently no longer followed in the case of osmium carbonyl clusters.     

Docosahexaenoic acid disrupts in vitro amyloid β1‐40 fibrillation and concomitantly inhibits amyloid levels in cerebral cortex of Alzheimer’s disease model rats

We have previously reported that dietary docosahexaenoic acid (DHA) improves and/or protects against impairment of cognition ability in amyloid beta_1‐40 (Aβ_1‐40)‐infused Alzheimer’s disease (AD)‐model rats. Here, after the administration of DHA to AD model rats for 12 weeks, the levels of Aβ_1‐40, cholesterol and the composition of fatty acids were investigated in the Triton X100‐insoluble membrane fractions of their cerebral cortex. The effects of DHA on the in vitro formation and kinetics of fibrillation of Aβ_1‐40 were also investigated by thioflavin T fluorescence spectroscopy, transmission electron microscopy and fluorescence microscopy. Dietary DHA significantly decreased the levels of Aβ_1‐40, cholesterol and saturated fatty acids in the detergent insoluble membrane fractions of AD rats. The formation of Aβ fibrils was also attenuated by their incubation with DHA, as demonstrated by the decreased intensity of thioflavin T‐derived fluorescence and by electron micrography. DHA treatment also decreased the intensity of thioflavin fluorescence in preformed‐fibril Aβ peptides, demonstrating the anti‐amyloidogenic effects of DHA. We then investigated the effects of DHA on the levels of oligomeric amyloid that is generated during its in vitro transformation from monomers to fibrils, by an anti‐oligomer‐specific antibody and non‐reducing Tris‐Glycine gradient (4–20%) gel electrophoresis. DHA concentration‐dependently reduced the levels of oligomeric amyloid species, suggesting that dietary DHA‐induced suppression of in vivo Aβ_1‐40 aggregation occurs through the inhibitory effect of DHA on oligomeric amyloid species.     

Associate Hydrations and Energies of Nucleotide Bases as Revealed by Low-Temperature Field Ionization Mass-Spectrometric Data

Abstract

The formation of water clusters, polyhydrates of nucleotide bases and their associates during simultaneous condensation of water and base molecules in vacuo onto a surface of a needle emitter cooled to 170 K was studied by field ionization mass spectrometry. It was found that different emitter temperatures are characterized by a specific distribution of intensities of cluster currents, depending on the number of water molecules in clusters. These distributions correlate with structural peculiarities and the relative energetics of formation of water clusters, polyhydrates of nucleotide bases and their associates at low temperature. The features observed in mass spectra for clusters m⁹Ade (H₂O)₅, m¹Ura (H₂O)₄ and m⁹Ade m¹Ura (H₂O)₂ are treated as a result of formation of energetically favorable structures stabilized by H-bonded bridges of water molecules.

The relative association constants and formation enthalpies of the noncomplementary pairs Ade Cyt, Gua Ura and the associates which model the aminoacid-base complexes m¹Ura Gin and m₂ ^1,3Thy Gin were determined from the temperature dependencies of the intensities of mass spectra peaks in the range 290–320 K.     

Analysis of Photosynthetic Antenna Function in a Mutant of Arabidopsis thaliana (L.) Lacking trans-Hexadecenoic Acid

Several lines of evidence support the proposal that the unusual chloroplast-specific lipid acyl group Δ3,trans-hexadecenoic acid (trans-C_16:1) stimulates the formation or maintenance of the oligomeric form of the light-harvesting chlorophyll a/b complex (LHCP). To assess the functional significance of this apparent association we have analyzed LHCP structure and function in a mutant of Arabidopsis thaliana (L.) which lacks trans-C_16:1 by electrophoretic analysis of the protein-chlorophyll complexes and by measurements of chlorophyll fluorescence under a variety of conditions. By these criteria the putative oligomeric form of LHCP appears to be slightly more labile to detergent-mediated dissociation in the mutant. The oligomeric PSI chlorophyll-protein complex, associated with PSI, was also more labile to detergent-mediated dissociation in the mutant, suggesting a previously unsuspected association of trans-C_16:1 with the PSI complex. However, no significant effect of the mutation on the efficiency of energy transfer from LHCP to the photochemical reaction centers was observed under any of the various conditions imposed. Also, the stability of the chlorophyll-protein complexes to temperature-induced dissociation was unaffected in the mutant. The role of trans-C_16:1 is very subtle or is only conditionally expressed.     

31P and 195Pt NMR studies on the clusters [Pt4(μ2-CO)5L4]. L = PEt3, PMe2Ph,PMePh2, PEt2But. The molecular structure of a monoclinic modification of [Pt4(μ2-CO)5(PMe2Ph)4].

Several clusters complexes of composition [Pt₄(μ₂-CO)₅L₄] have been synthesized and characterized, using ³¹P and ¹⁹⁵Pt NMR. L = PEt₃, PMe₂Ph, PMePh₂, PEt₂Bu^t. The molecular structure of a new monoclinic modification of the PMe₂Ph derivative has been determined: space group P2₁/n with a = 19.698(4), b = 10.9440(20), and c = 21.360(6) Å, β = 112.432(18)°, Z = 4. Using 4751 reflections measured at 290 ± 1 K on a four-circle diffractometer the structure has been refined to R = 0.0846. The molecule has no imposed symmetry, but the central Pt₄(CO)₅P₄ core has the approximate C₂v architecture established for the previously known orthorhombic modification. The Pt₄ unit is thus a highly distorted, edge-opened (3.3347 Å) tetrahedron, with five edge-bridging carbonyl and four terminal phosphine ligands. In contrast to the crystallographic results ³¹P and ¹⁹⁵Pt NMR spectra reveal equivalent ³¹P and ¹⁹⁵Pt spins, which can be interpreted in terms of a tetrahedral arrangement of platinum atoms. It is suggested that this equivalence arises from time-averaging of all possible isomeric edge-opened tetrahedra.     

Interaction of copper(II) and nickel(II) with a bis-amide ligand functionalized with pyridine moieties: Thermodynamic and spectroscopic studies in aqueous solution

We synthesized a new bis-amide ligand derived from the l(+)-tartaric acid. We then determined its protonation constants and the stability constants of the copper(II) and nickel(II) chelates by potentiometry as well as ESI-MS and UV-Vis spectroscopy. We found that both metal ions are able to induce the deprotonation and the coordination of an amide nitrogen donor atom. In the case of copper complexes, the data show the formation of two major species: Cu₂(L₂H₋₃)⁺ and Cu₂(LH₋₄). EPR and XAS experiments led us to precise the relative structure of these compounds. In Cu₂(L₂H₋₃)⁺, each metal center is coordinated by pyridinic and amidic nitrogen atoms of one ligand and by nitrogen and oxygen atoms from pyridine and hydroxyl moieties from the other one. In Cu₂(LH₋₄), the copper centers are coordinated by pyridinic and amidic nitrogen atoms, as well as a deprotonated hydroxyl group of the ligand. In this latter complex, the lower value of the Cu-Cu distance determined from EXAFS experiments and compared to the one of the solid species likely involve the formation of an exogeneous hydroxyl bridge between the two copper centers. With Ni(II) ions, the only one major species is the mononuclear Ni(LH₋₂) complex, in which Ni(II) is held in an octahedral environment with the metal center chelated by the two pyridinic and the two amidic nitrogen atoms, and two oxygen atoms from water molecules.     

Study on the reaction of 2,3-dihydroxybenzoic acid with molybdenum in aqueous solution. I. Synthesis and characterization of the oligomeric complexes formed

Dimeric or oligomeric oxo-complexes of Mo(VI) with 2,3-dihydroxybenzoic acid were prepared in aqueous solutions in the presence or not of K₂S₂O₅ (acting as a reducing agent) in various conditions. The complexes were found to contain the cis-(Mo₂O₅)²⁺ core and the ligands in the catecholate, semiquinonate or mixed valence oxidation form, depending on the reaction conditions and especially on the presence or not of the reductant. The isolated complexes in the presence or absence of reductant and the oxidation products in solution in the presence of air were studied via elemental, thermogravimetric and electrochemical analysis, Infrared, Raman, NMR and ESR spectroscopies and Electrospray Mass Spectra. The general molecular formula for the complexes is {[(PPh₄)₂(Mo₂O₅L₂X₂] · xH₂O)}_n, where the coordinated ligand’s L oxidation form varies and X involves coordinated water or hydroxyl group depending on the ligand oxidation state.     

Hemiacetal stabilization in a chymotrypsin inhibitor complex and the reactivity of the hydroxyl group of the catalytic serine residue of chymotrypsin

The aldehyde inhibitor Z-Ala-Ala-Phe-CHO has been synthesized and shown by ¹³C-NMR to react with the active site serine hydroxyl group of alpha-chymotrypsin to form two diastereomeric hemiacetals. For both hemiacetals oxyanion formation occurs with a pK_a value of ~ 7 showing that chymotrypsin reduces the oxyanion pK_a values by ~ 5.6 pK_a units and stabilizes the oxyanions of both diastereoisomers by ~ 32 kJ mol^− 1. As pH has only a small effect on binding we conclude that oxyanion formation does not have a significant effect on binding the aldehyde inhibitor. By comparing the binding of Z-Ala-Ala-Phe-CHO with that of Z-Ala-Ala-Phe-H we estimate that the aldehyde group increases binding ~ 100 fold. At pH 7.2 the effective molarity of the active site serine hydroxy group is ~ 6000 which is ~ 7 × less effective than with the corresponding glyoxal inhibitor. Using ¹H-NMR we have shown that at both 4 and 25 °C the histidine pK_a is ~ 7.3 in free chymotrypsin and it is raised to ~ 8 when Z-Ala-Ala-Phe-CHO is bound. We conclude that oxyanion formation only has a minor role in raising the histidine pK_a and that the aldehyde hydrogen must be replaced by a larger group to raise the histidine pK_a > 10 and give stereospecific formation of tetrahedral intermediates. The results show that a large increase in the pK_a of the active site histidine is not needed for the active site serine hydroxyl group to have an effective molarity of 6000.