AM1* parameters for aluminum, silicon, titanium and zirconium

Our extension of the AM1 semiempirical molecular orbital technique, AM1*, has been parameterized for the elements Al, Si, Ti and Zr. The basis sets for all four metals contain a set of d-orbitals. Thus, AM1* parameters are now available for H, C, N, O and F (which use the original AM1 parameters), Al, Si, P, S, Cl, Ti, Mo and Zr. Special attention was paid to reproducing homolytic and heterolytic bond-dissociation energies correctly. Such bond-energy data help to avoid eccentricities in the parameterization caused by inaccurate experimental heats of formation. The performance and typical errors of AM1* for the newly parameterized elements are discussed. Generally, the new method performs less well than established techniques for heats of formation but considerably better for the heats of reaction. Electronic Supplementary Material Supplementary material is available for this article at     

外消旋薄荷醇对映选择性酶促酯化中酸酐与羧酸的比较研究

利用脂肪酶在有机溶剂中催化对映选择性酯化反应对外消旋薄荷醇进行了有效的光学拆分。对分别使用酸酐和相应的游离羧酸作酰基给体时的反应性能进行了比较。发现酸酐的反应性远高于对应的游离羧酸,但在酶的催化作用下酸酐易水解成为游离羧酸;在微水系统中使用过高浓度的酸酐会导致酶缺水而失活,同时会促进手性醇的非选择性酯化,从而降低产物的光学纯度。然而,在连续流加丙酸酐的半批式反应系统中,所有这些缺点均可有效地克服。与使用游离丙酸的批式反应系统相比,ｄｌ-薄荷醇的反应时间缩短了一半,酶的稳定性大幅度提高,而产物ｌ薄荷醇酯的光学纯度不相上下（＞９８％ｅ）。     

Lipases catalyse hydrolysis of fatty acid anhydrides

Regio-specific and non-regio-specific lipases from mammals and microorganisms catalyse the hydrolysis of short, medium and long-chain fatty acid anhydrides. All the lipases tested in the present study can catalyse the hydrolysis of pure fatty acid anhydrides more efficiently than that of glycerol tributyrate. Molecular turnovers more than four times higher than that measured using glycerol tributyrate were calculated. The presence of 0.5% (by mass) anhydride in a triacylglyceride can double the initial rate of proton release during enzymatic hydrolysis. This should be taken into account when testing the chain specificity of a lipase for various synthetic substrates. Lipase inhibition was found to be associated very often with anhydride hydrolysis. The inhibition rates depended on the anhydride and the origin of the lipase. Inhibition of lipase activity is probably due to the formation of a poorly reversible acyl-lipase complex which differs from the classical fully reversible acyl-lipase complex at the catalytic centre.     

Kinetics of gelation and thermal sensitivity of N-isobutyryl chitosan hydrogels

N-Acylation of chitosan with carboxylic anhydrides in dilute acetic acid/methanol has been a well documented strategy to selectively modify chitosan. Although this reaction is known to lead to irreversible gel formation, the kinetics and mechanism of this process have not so far been addressed. To this purpose, gel formation during the N-isobutyrylation of chitosan was investigated as a function of the reaction stoichiometry (R), chitosan concentration, and temperature by small deformation oscillatory rheology. Gel formation follows closely the chemical reaction and it proceeds predominantly under second-order kinetics as established from the dependence of critical gel time, t(gel), on R and concentration. The activation energy value derived from t(gel) vs 1/T data (E(a) = 68.29 +/- 1.80 kJ/mol) was almost identical to values reported for the chitosan N-acetylation reaction in previous studies. An excess isobutyric anhydride is suggested to be necessary for nucleation and hydrophobic association. The potential application of N-isobutyrylchitosan (NIBC) hydrogels in the design of thermally sensitive materials is also demonstrated.     

The preparation of amino bile acid derivatives

Bile acid derivatives have been prepared by reaction of their mixed anhydrides with diaminoethane. The new compounds have side chains modified by amide bond formation which extends the side chain by two carbon atoms and terminates in a primary amino group. Important properties of the new compounds are their solubility in acid, and their ability to act as nucleophiles in the carbodiimide reaction or mixed anhydride reaction. The derivatives have been used to prepare high molar ratio immunogens with bovine serum albumin and to prepare I labelled ligands for radioimmunoassay     

Acetic anhydride: an intermediate analogue in the acyl-exchange reaction of citramalate lyase.

1. Reactivation of deacetyl citramalate lyase by acetic anhydride proceeds through an enzyme-anhydride complex prior to actual acetylation. The reaction is inhibited by citramalate which is competitive with acetic anhydride. 2. A corresponding complex is an intermediate in the carboxymethylation of deacetyl enzyme by iodoacetate. However, the inhibition of this reaction by S-citramalate appears to be non-competitive with iodoacetate. 3. The results lead to the conclusion that acetic anhydride can be regarded as a structural analogue of citramalic acetic anhydride, the proposed intermediate in the acyl exchange reaction on citramalate lyase. 4. The formation of 6-citryl thiolester from the 1-thiolester via the cyclic citric anhydride provides a chemicla model for enzymic acyl exchange. 5. The data suggest that anhydrides are of general importance in acyl exchange reactions of thiolesters.     

Enzymatic resolution of 2-aryloxy-1-propanols via lipase-catalyzed enantioselective acylation using acid anhydrides as acyl donors

Pseudomonas sp. lipase-catalyzed enantioselective acylation procedure using acid anhydrides as acyl donors was exploited for the resolution of 2-aryloxy-1-propanols carrying different substituents on the benzene ring. These primary alcohols, which belong to primary alcohols with an oxygen atom at the stereocenter, were resolved generally with moderate to good enantioselectivity (E of up to 55) through the acylation with hexanoic anhydride in diisopropyl ether at 25 °C in a short reaction time. With the alcohol substrate, which gave a low enantioselectivity in the acylation at ordinary temperature, the selectivity proved to be enhanced by conducting the reaction at low temperature (−10 °C). By this acylation procedure employing the acid anhydride, enantiomerically pure (R)-2-phenoxy-1-propanol was prepared in a gram-scale reaction.     

Model studies of the precipitation of silica in the presence of aluminium; implications for biology and industry

The unique chemical affinity between the oxides of silicon and aluminium has been cited as a potential route for the amelioration of the detrimental effects of aluminium in the environment and in biological systems. A greater understanding of silicon-aluminium interactions may assist in this endeavour and also provide a means of overcoming silica fouling problems encountered by industry which are exacerbated by the presence of aluminium. It is also conceivable that this increased knowledge may demonstrate a positive use for aluminium in the processing of the silicon dioxide phase. In this study we report the effect of aluminium ions, derived from aluminium chloride, on silicic acid species obtained from potassium catecholato complexes of silicon at circumneutral pH at the molar ratios 1000Si:Al, 100Si:Al and 50Si:Al. Silica and low levels of aluminium-rich silica materials were formed with Si:Al ratios of about 3.5:1 comparable with the element ratios detected in senile plaques and aluminium-rich scale. A kinetic study showed that aluminium in the reaction medium slowed down the rate of formation of one of the silica species formed early in the condensation process, e.g. trimers, but increased the rate at which silicic acid was removed from sub 1 nm diameter particles. The materials precipitated in the presence of aluminium were composed of smaller particles and aggregates with smaller pores (Si100:Al and Si50:Al systems) or larger pores (Si1000:Al) compared to the control. The nature of the interactions responsible for these differences is discussed. The effects described here demonstrate the ability of silica and aluminium to interact under conditions such as those found in biological systems. That silica reacts with aluminium in the presence of catechol supports the protective role assigned to silicon.     

Study of the stability of aluminium trimeric clusters in aqueous solutions

Ab initio molecular dynamics (AIMD) based on density functional theory has been used to study small aluminium–oxygen complexes in water. Such Al–O clusters have been seen in several recent mass spectrometry studies. In this study, we have focused on trimeric Al–O clusters. The initial very compact trimeric Al–O structures opened up and formed linear Al–O chains. The typical Al–O coordination number in these chain structures was 5. We have performed long (up to 200 ps) AIMD simulations and these chain structures are stable on the nanosecond time scale. We have also studied the reactivity of the Al–O dimer and solvated Al. We found a formation path for the trimeric cluster, which has a action barrier (0.04 eV) and a reaction free energy of ? 0.55 eV. This suggests that the association of a dimer and a monomer Al–O species is fast and thermodynamically a very favourable process.     

The amelioration of aluminium toxicity by silicon in higher plants: Solution chemistry or an in planta mechanism?

Aluminium (Al) toxicity is a very important factor limiting the growth of plants on acidic soils. Recently, a number of workers have shown that, under certain conditions, silicon (Si) can ameliorate the toxic effects of A1 in hydroponic culture. The mechanism of the amelioration is unclear, but three suggestions have been put forward: Si‐induced increase in solution pH during the preparation of hydroponic solutions; reduced availability of Al due to the formation of hydroxyaluminosilicate (HAS) species in those solutions during plant growth; or in planta detoxification. It is now known that it is possible to make up Al and Si solutions in an order in which pH is lowered prior to Al addition; in these cases amelioration has still been observed. Amelioration has also been noted in experiments where HAS formation is minimal. These observations would suggest that, at least under some circumstances, there is an in planta component to the amelioration phenomenon. Several microanalytical investigations have noted codeposition of Al and Si in root cell walls. We propose a model in which root cell walls are the main internal sites of aluminosilicate (AS) and/or HAS formation and of Al detoxification. Factors promoting AS/HAS formation in this compartment include: high apoplastic pH; the presence of organic substances (e.g. malate); and the presence of suitable local concentrations of reactive forms of Al and Si, on or within the surfaces of the wall matrix. All these are likely to be important in the amelioration of Al toxicity.     

Dicarboxylic acid anhydrides as dissociating agents of protein-containing structures

Dissociation of protein-containing structures by modification of protein amino groups with dicarboxylic acid anhydrides is a mild procedure which, in some cases, offers advantages over treatment with alternative dissociating agents, such as urea, guanidine hydrochloride, detergents, high ionic strength, and extremes of pH: In addition to dissociating multimeric proteins and protein aggregates, dicarboxylic acid anhydrides are effective dissociating agents for membrane-bound proteins and nucleoprotein particles. With most dicarboxylic acid anhydrides reviewed, the introduced reagent residues can be eliminated under moderate acid conditions, which allows the purification of unmodified individual components, and the use of disassembly-reconstitution systems valuable for investigating the structural and functional roles played by the individual components of complex particles:Each reagent can be suitable for a particular purpose, depending on the required specificity of the modification and stability of the modified groups: The stability of the acylated amino groups ranges from the very stable succinylated amino groups to the very labile acylation obtained with dimethylmaleic anhydride: Between these extremes, the stability of the modified amino groups decreases stepwise in the following order: maleic, exo-cis-3,6-endoxo-⁴-tetrahydrophthalic, citraconic, and 3,4,5,6-tetrahydrophthalic anhydride. With respect to the selectivity of the produced modification, little or no modification of hydroxyamino acid and cysteine residues has been observed with dimethylmaleic, exo-cis-3,6-endoxo-⁴-tetrahydrophthalic, and 3,4,5,6-tetrahydrophthalic anhydrides: With the other reagents, the extent of modification of hydroxyamino acid residues increases in the order citraconic, maleic and succinic anhydride: Citraconic and maleic anhydrides can produce irreversible modification of cysteine residues, the reactivity of sulfhydryl groups being higher with maleic anhydride:     

The chemical composition of plant litter of black locust (Robinia pseudoacacia L.) and its ecological role in sandy ecosystems

Robinia pseudoacacia is a North American species and in Poland it is currently invasive in character. It is used to recultivate sand excavations and others, most often in order to make the process of plant and soil succession more advanced. It has been observed that in places were R. pseudoacacia dominated in plantations, the herbaceous vegetation under the trees is poor and sometimes other vascular plants are not appearing at all. Plants usually overgrow the space out of the canopy shade. The positive influence of R. pseudoacacia on a habitat is primarily connected with the chemical composition of plant litter, as well as with the biology of the species. Chemical composition of R. pseudoacacia litter has been researched. The greatest accumulation of elements has been observed in the following parts: green leaves (Ca > K > Mg > P > Si > Na > Fe > Zn > Al > Mn) and leaf litter (Ca > K > Mg > Si > Fe > P > Na > Al > Zn > Mn). Similar regularities are observed in the remaining litter of R. pseudoacacia. It must be emphasized that nitrogen occurs in similar quantities in particular samples and it varies from 1.01 to 2.65%. The plant litter reaction (pH) vary from acid to weakly acid. In a short period of time under the canopy of R. pseudoacacia a 10 cm organic and humus horizont (O/A) has developed.     

The chemical composition of plant litter of black locust (Robinia pseudoacacia L.) and its ecological role in sandy ecosystems

Robinia pseudoacacia is a North American species and in Poland it is currently invasive in character. It is used to recultivate sand excavations and others, most often in order to make the process of plant and soil succession more advanced. It has been observed that in places were R. pseudoacacia dominated in plantations, the herbaceous vegetation under the trees is poor and sometimes other vascular plants are not appearing at all. Plants usually overgrow the space out of the canopy shade. The positive influence of R. pseudoacacia on a habitat is primarily connected with the chemical composition of plant litter, as well as with the biology of the species. Chemical composition of R. pseudoacacia litter has been researched. The greatest accumulation of elements has been observed in the following parts: green leaves (Ca > K > Mg > P > Si > Na > Fe > Zn > Al > Mn) and leaf litter (Ca > K > Mg > Si > Fe > P > Na > Al > Zn > Mn). Similar regularities are observed in the remaining litter of R. pseudoacacia. It must be emphasized that nitrogen occurs in similar quantities in particular samples and it varies from 1.01 to 2.65%. The plant litter reaction (pH) vary from acid to weakly acid. In a short period of time under the canopy of R. pseudoacacia a 10 cm organic and humus horizont (O/A) has developed.     

Models of Water-Assisted Hydrolyses of Methyl Formate, Formamide, and Urea from Combined DFT-SCRF Calculations

In this work we present theoretical studies of the hydrolytic reaction of methyl formate, formamide and urea with one water molecule. The studied systems contain two additional water molecules which can act as bifunctional acid-base catalysts. These water molecules catalyze proton transfers between the primary reacting species. Our models describe the concerted transfer of two protons in every reaction step. The calculations have been carried out with the Becke3LYP/6-31G* method. Unspecific solvation effects have been included by means of a polarizable continuum model. Substrate reactivity differences as well as preferences for different reaction pathways can be discussed with the aid of these molecular systems. The studied alternative mechanisms include the common addition-elimination mechanism via a tetrahedral intermediate, and a concerted S_N-like mechanism without a reaction intermediate. Our results suggest that the proved decreasing substrate reactivity in the order ester, amide, urea is caused by a rising resonance stabilization of the reaction centre, and not by a different positive partial charge of the carbonyl carbon. It is also concluded, that the probability of a concerted addition of a nucleophile and elimination of a leaving group without a tetrahedral intermediate rises in the order ester, amide, urea. The ordering of reactivity is not influenced by this behaviour.     

High-performance liquid chromatographic analysis of neutral glycosphingolipids as their per-O-benzoyl derivatives

Previous reports from this laboratory (1–4) described the perbenzoylation of neutral glycosphingolipids (GSL)¹ with benzoyl chloride in pyridine and analysis of the perbenzoylated derivatives by high performance liquid chromatography (hplc). A disadvantage of this procedure is that N-benzoylation occurs as well as the desired O-benzoylation. This does not permit recovery of the parent GSL after mild alkaline hydrolysis due to formation of a mixture of N-acylated and N-benzoylated GSLs(1). It has also been demonstrated that the benzoylation with benzoic anhydride in pyridine does not lead to the formation of N-benzoylated products. However, the anhydride reaction is sluggish and the benzoyl chloride method has been the preferred procedure.Gupta et al. (5) used N,N-dimethyl-4 amino pyridine (DMAP) as a catalyst in the acylation of phospholipids by the anhydrides of fatty acids. F. B. Jungalwala (private communication) has shown that this catalyst greatly accelerates the reaction of benzoic anhydride with sulfatides.In this communication we report the preparation and hplc analysis of per-O-benzoyl derivatives of GSLs by reaction with benzoic acid anhydride in the presence of DMAP as a catalyst. Reaction with these reagents avoids amide acylation, forms single products with satisfactory chromatographic properties and parent GSLs can be regenerated by mild alkaline hydrolysis.     

Efficient kinetic resolution of dl-menthol by lipase-catalyzed enantioselective esterification with acid anhydride in fed-batch reactor

Acid anhydrides were used as highly reactive and non-water-producing acyl donors for hydrolase-catalyzed enantioselective esterification. Efficient kinetic resolution of dl-menthol has been achieved via lipase-catalyzed enantioselective esterification in cyclohexane when propionic anhydride as an acyl donor was continuously fed into a reactor containing dl-menthol and Candida cylindracea lipase OF 360, while a high concentration of the acid anhydride in a batch reaction system with a dehydrated organic solvent did not facilitate the reaction, because water necessary for the enzyme function was consumed by the competing hydrolysis of the anhydride catalyzed by the same enzyme. The efficiency of this fed-batch reaction system using acid anhydride was higher and the enzyme stability in repeated use was much better than those of conventional batch and fed-batch reaction systems using propionic acid as an acyl donor. The optical purity (more than 98% e.e.) of the l-menthyl ester produced in the fed-batch system using the anhydride was comparable to that in the system using the corresponding acid. *** DIRECT SUPPORT *** AG903062 00002     

Conversion of O species by cellobiose dehydrogenase (cellobiose oxidase) and glucose oxidase – a comparison

Cellobiose dehydrogenase from Phanerochaete chrysosporium produces HO by electron transfer between cellobiose and O with a lower yield than the 1:1:1 molar ratio displayed by Aspergillus niger. glucose oxidase in the similar reaction between glucose and O. The discrepancy could best be explained if both a FentonÕs reaction and the spontaneous reactivity of the oxygen species formed were taken into account.     

The chain effect. Part III. Induction of secondary structure in copolymerizations

Studies of the properties of copolymers of N-carboxy sarcosine anhydride with other N-carboxy α-amino acid anhydrides have shown that the secondary structure may be a function of the type of initiator used. In particular, when polysarcosine is the initiator, and the “chain effect” becomes possible, blocklike character appears in the copolymer. This is the result of selective and rapid polymerization of N-unsubstituted N-carboxy α-amino acid anhydrides by the chain-effect mechanism. In suitable circumstances, therefore, the latter may be used to induce order into copolymerizations of this type.     

Tissue distribution of S-adenosylmethionine and S-adenosylhomocysteine in the rat. Effect of age, sex and methionine administration on the metabolism of S-adenosylmethionine, S-adenosylhomocysteine and polyamines

In the presence of ATP and Mg2+, ATP sulphurylase from Saccharomyces cerevisiae catalysed the conversion of selenate into a compound with the electrophoretic and acid-lability properties of adenosine 5'-sulphatophosphate. Structural characterization, involving extensive purification of adenosine 5'-selenophosphate, proved impossible. However, we showed ATP-, Mg2+- and ATP sulphurylase-dependent, and inorganic pyrophosphatase-stimulated, production of elemental selenium from selenate in the presence of GSH (reduced glutathione). Since selenate was not reduced by GSH, this reaction proved that ATP sulphurylase had formed an active selenate. The enzyme catalysed formation of elemental selenium had the same kinetics and GSH-dependency as the non-enzymic reduction of selenite to elemental selenium by GSH. In the presence of inorganic pyrophosphatase, 2 mol of Pi was released for each mol of 'active selenate' formed. This was shown by a spectrophotometric assay for elemental selenium. The observed reactivity with thiols and the instability of the enzymic product were those predicted for selenium anhydrides. By analogy with the chemistry of sulphur, the product of the thiolytic cleavage of a selenium anhydride would be converted into selenite. The selenite would then be reduced by the thiol to elemental selenium. We conclude that ATP sulphurylase can catalyse the formation of adenosine 5'-selenophosphate. The anhydride can be reduced by thiols in a manner similar to the reduction of selenite. These results probably explain the ability of mammals, lacking a sulphate reductase system, to incorporate selenium from selenate into seleno-amino acids.