Integration of biocatalytic conversions into chemical syntheses

The future application of enzymes in organic synthesis is in part dependent on excellent integration with the neighbouring chemical conversions. This paper outlines some of the key issues to be addressed to achieve optimal integration either by modification of preceding (or following) chemical steps, biocatalyst evolution or combinations of these approaches together with process engineering. Copyright © 2007 Society of Chemical Industry     

New chemical syntheses of cholest-4,6-dien-3-one

Steroidal dienones represent a significant class of compounds that are useful intermediates in the further functionalization of the steroid nucleus. Their chemical synthesis can be problematic owing to the lack of a simplified method of preparation and the occurrence of impurities that can be difficult to remove. We have endeavored to develop new methods of chemical synthesis of cholest-4,6-dien-3-one that would yield a product of high purity.     

Structure of monohydroperoxides formed by chlorophyll photo-sensitized oxidation of methyl linoleate

The structures of the hydroperoxides prepared by photochemical oxidation of methyl linoleate in the presence of chlorophyll were determined by gas liquid chromatography-mass spectrometry (GLC-MS) of the trimethylsilyl (TMS) derivatives of the hydroxyesters. The position of the double bonds were deduced via hydroxylation with OsO₄, followed by GLC-MS analysis of the TMS derivatives. Structures of the original hydroperoxides were elucidated from these data. One of 28 papers presented at the Symposium “Metal-Catalyzed Lipid Oxidation,” ISF-AOCS World Congress, Chicago, September 1970.     

Monitoring monohydroperoxides in docosahexaenoic acid using high-performance liquid chromatography

The oxidation of free DHA has been investigated with respect to monohydroperoxides and polyhydroperoxides, which were analyzed with a novel HPLC method. The temperature and physical system, i.e., bulk and liposome, were varied. We have also studied the effects of antioxidants such as α-tocopherol, ascorbic acid, and juice from sea buckthorn on DHA. The HPLC method, which was performed isocratically, eluted eight peaks, each containing one or two isomers of monohydroperoxy-DHA. This method showed that the double bond farthest from the carboxyl group was easily oxidized, as shown by the rapid increase in the amount of C20-monohydroperoxy-DHA, which always provided the largest contribution to the total amount of monohydroperoxides. The monohydroperoxy-DHA containing the hydroperoxy group located on the double bond nearest the carboxyl group also was shown to incease considerably during an increase in the total amount of monohydroperoxides. This demonstrates that the double bonds located nearest and farthest from the carboxyl group were the most prome to hydroperoxide formation. DHA was more stable when stored in liposomes than as bulk. Addition of α-tocopherol to the DHA-containing liposomes reduced the oxidation of these double bonds. The antioxidant effect of α-tocopherol was prolonged when combined with ascorbic acid, since α-tocopherol was regenerated.     

Simple chemical processes based on low molecular-mass alkanes as chemical feedstocks

The present state of new developments in direct catalytic conversion of low-molecular-mass alkanes (C₁–C₃) to petrochemical feedstocks and petrochemicals is reviewed. Special attention is given to the following reactions: methane to methanol and formaldehyde by partial oxidation as well as to C₂ hydrocarbons by oxidative coupling, ethane and propane to their olefins by oxidative dehydrogenation and to their oxygenates, i.e., acetic acid, acrylic acid and acrolein by partial oxidation. Specific research results are presented on the oxidative dehydrogenation of ethane and propane.     

Study of TAG ethanolysis to 2-MAG by immobilized Candida antarctica lipase and synthesis of symmetrically structured TAG

Regiospecific ethanolysis of homogenous TAG with immobilized Candida antarctica lipase (Novozym 435) was studied using trioleoylglycerol (TO) as a model substrate. Optimization of the reactant weight ratio revealed that the 2-MAG reaction yield increased when a larger amount of ethanol was used. These results suggested that Novozym 435 showed strict regiospecificity in an excess amount of ethanol. The process optimization (reaction temperature and reactant molar ratio) and a study of lipase specificity for various substrates were performed. Under the optimized conditions (ethanol/TO molar ratio=77∶1 and 25°C), 2-monooleoylglycerol (2-MO) was obtained in more than 98% content among glycerides of the reaction mixture and approximately 88% reaction yield in 4 h. The above reaction conditions were applied for ethanolysis of tridocosahexaenoylglycerol, trieicosapentaenoylglycerol, triarachidonoylglycerol, tri-α-linolenoylglycerol, and trilinoleoylglycerol. Reaction yields ranging from 71.9 to 93.7% were obtained in short reaction times (2.5 to 8 h). Purified (>98%) 2-MO and 2-monodocosahexaenoylglycerol (2-MD) were reesterified with caprylic acid by immobilized Rhizomucor miehei lipase (Lipozyme IM) to afford symmetrical structured TAG. At a stoichiometric ratio of 2-MAG/caprylic acid, 25°C and 2–5 mm Hg vacuum, the glyceride composition of the esterification mixture was approximately 95% 1,3-dicapryloyl-2-oleoylglycerol (COC) at 4 h, and 96% 1,3-dicapryloyl-2-docosahexaenoylglycerol (CDC) at 4 h, and 96% 1,3-dicapryloyl-2-docosahexaenoylglycerol (CDC) at 8 h. The regioisomeric purity of both COC and CDC was 100%.     

Conducting polyaniline composite: From syntheses in waterborne systems to chemical sensor devices

The development of a conducting composite from its one-step synthesis in aqueous dispersed medium to the characterization of its chemical sensor performance is detailed. Composite films of polyaniline and polyacrylate were processed from the crude dispersion with no need for post-formulation. The synthesis has been optimized to be as simple, cheap and transferable as possible. The composite composition is tunable, i.e. the nature of the dopant was varied and its effect on kinetic parameters was observed. Ab initio calculations have been performed on a series of six ionic systems to gain a deeper insight into the contribution of both the stability of the anilinium salts in the composite polymerization. The conductivities of all composites were recorded from room temperature to 150 °C and the thermal stability versus dopant highlighted. The conducting films were then studied as active layers in a chemical sensor. The results showed that these composites, easily synthesized and processed, are ammonia sensitive and exhibit a fast response when exposed to air pollution.     

Numerical analyses of a microwave plasma chemical vapor deposition reactor for thick diamond syntheses

We have carried out simulations of microwave plasmas inside a reactor for thick diamond syntheses. In a model reactor used in the calculation, a diamond substrate with finite thickness and area is taken into account. Distributions of electric field, density of microwave power absorbed by the plasma, temperatures and flow field of gas have been studied not only in a bulk region inside a reactor but also a local region around the substrate surface. Numerically predicted distributions of (1) microwave power density, (2) temperature on the top surface of the substrate, and (3) gas flow around the substrate imply that the adopted arrangement of the substrate is not desirable for continuous growth of large diamond crystals.     

Design,syntheses and applications of chemical precursors to advanced ceramic materials in nanostructured forms

Our recent work directed at the syntheses, characterizations and applications of new organodecaborane molecular and polymeric precursors to technologically important nonoxide ceramic materials, such as boron carbide, is presented. When used in conjunction with currently available methods for nanofabrication, these new precursors enable the formation of ceramic nanostructures, including nanofibers, nanocylinders and nanoporous materials.     

基于简单碰撞理论化学反应动力学数值模拟

寻找合适和有效的数值模拟方法,对于越来越复杂的化学反应动力学研究有着重要的意义.以简单碰撞理论(SCT)模型为出发点,可以得到一种准确有效的化学动力学方程的数值求解方法.基于SCT的化学反应动力学数值模拟方法编程方便简单,具有较高的精度,与化学动力学过程机理相似,能体现化学动力学的过程.使用该方法对n级简单反应、连续反...     

Autoxidative dimerization of methyl linolenate and its monohydroperoxides,hydroperoxy epidioxides and dihydroperoxides

The structures of dimers and oligomers produced by autoxidation of methyl linolenate and its purified oxidation products were investigated to obtain a better understanding of the mechanism of oxidative deterioration of unsaturated lipids. The dimers were separated by gel permeation chromatography, characterized by molecular weight determinations before and after sodium borohydride reduction, and analyzed by ultraviolet, infrared,¹H NMR and fast atom bombardment mass spectrometry. Autoxidation of methyl linolenate at 40 C to peroxide value of 1062 produced 6.8% dimers mainly derived from hydroperoxides, hydroperoxy epidioxides and dihydroperoxides. These dimers were 88% peroxide-linked (C-O-O-C) and 12% ether- (C-O-C) and/or carbon-linked (C-C). Autoxidation of methyl linolenate monohydroperoxides at 40 C produced dimers that were 72% peroxide- and 28% ether/carbon-linked. Thermal decomposition of linolenate hydroperoxides at 150 C gave dimers that were 100% ether/carbon-linked, and catalytic decomposition with ferric chloride-ascorbic acid at room temperature gave dimers with 43% peroxide and 57% ether/carbon linkages. Autoxidation of linolenate hydroperoxy epidioxides at 40 C produced dimers containing hydroperoxy epidioxides, dihydroperoxides and monohydroperoxides joined with peroxide and ether/carbon linkages. Under the same conditions, autoxidation of linolenate dihydroperoxides produced dimers containing dihydroperoxides and hydroperoxy epidioxides joined with peroxide and ether/carbon linkages. These dimers contribute to oxidative and flavor deterioration of polyunsaturated fats in the same way as the hydroperoxide precursors by further decomposition to produce volatile compounds. Visiting scientist from Tohoku University, Sendai, Japan.     

Effect of DAG on milk fat TAG crystallization

The effect of milk fat and standard DAG on the crystallization behavior of milk fat TAG (MF-TAG) was investigated. When milk fat DAG were added to MF-TAG at the 0.1 wt% level, crystallization was delayed. Racemic purity was shown to be an important factor in the ability of DAG to influence TAG crystallization. Only sn-1,2 isomers of blends of MF-TAG with 0.1 wt% of the racemic mixtures of dipalmitin and diolein increased the activation free energy barrier to MF-TAG nucleation (ΔG _c ) and delayed the subsequent crystallization process by increasing the crystallization induction time (τ_SFC) determined from solid fat content-time measurements. Although crystallization kinetics were affected, the properties of the resulting network structures remained unchanged upon addition of milk fat minor components at the 0.1 wt% level     

Effect of FA composition on epoxidation kinetics of TAG

The rate constants of epoxidation were determined by reacting formic acid with a number of oils, pure TAG, and pure FAME. These results showed that FA composition had a significant effect on the value of the rate constant. In TAG, the double bonds of oleic acid and linoleic acid were equally reactive, and the double bonds of linolenic acid were approximately three times more reactive than oleic and linoleic acids. For FAME, the rate constants of epoxidation increased as the level of unsaturation increased. Furthermore, the rate constants of epoxidation for the FAME were higher than their respective TAG. We conclude that steric and electronic effects caused FA with different levels of unsaturation to have different reactivities. These results were used to derive a model that predicts the epoxidation kinetics of oils from their FA composition. The predictions of the model closely match the experimentally determined rate constants.     

Stereospecific analysis of TAG from sunflower seed oil

Stereospecific analysis of TAG from a sunflower seed oil of Tunisian origin was performed. The TAG were first fractionated according to chain length and degree of unsaturation by RP-HPLC. The four major diacid- and triacid-TAG fractions were palmitoyldilinoleoyl-glycerol, dioleoyllinoleoylglycerol, oleoyldilinoleoylglycerol, and palmitoyloleoyl-linoleoyl-glycerol, amounting to 7.2, 16.6, 29.5, and 12 mol%, respectively. The TAG of the four fractions were individually submitted to stereospecific analysis, using a Grignard-based partial deacylation, separation of sn-1,2(2,3)-DAG from sn-1,3-DAG by boric acid-impregnated silica gel TLC plates, conversion of the sn-1,2(2,3)-DAG to their 3,5-dinitrophenylurethane (DNPU) derivatives, fractionation of DNPU derivatives by RP-HPLC, resolution of the DNPU-DAG by HPLC on a chiral column, transmethylation of each sn-DNPU-DAG fraction, and analysis of the resulting FAME by GC. The data obtained were used to determine the triacyl-sn-glycerol composition of the main TAG of the oil. Fifteen triacyl-sn-glycerols were identified and quantified, representing, along with the monoacid-TAG, trilinoleoylglycerol and trioleoylglycerol, more than 90% of the total oil TAG. The two major triacyl-sn-glycerols were trilinoleoyl-glycerol and 1-linoleoyl-2-linoleoyl-3-oleoyl-glycerol (18.6 and 18.5% of the total, respectively). Results clearly identified linoleic acid as the major FA at the sn-2 position, whereas oleic and palmitic acids were the major FA at the sn-3 position. The sn-1 position was occupied to nearly the same extent by linoleic and oleic acids, and to a greater extent by palmitic acid, which was practically absent at the sn-2 position.     

Positional distribution of CLA in TAG of lamb tissues

The content and positional distribution of CLA in TAG fractions of lamb tissues was examined with either preformed CLA or the linoleic acid precursor of CLA in the diet as experimental treatments. The CLA content of phospholipid (PL) from these tissues was also examined. Thirteen lambs were randomized to the following dietary treatments: (i) control diet (no supplement); (ii) CLA supplementation (0.33 g d⁻¹ for 21 d prior to weaning) to milk-replacer of pre-ruminating lambs, or (iii) feeding linoleic acid-rich oil (6% safflower oil on a dry matter basis) to weaned ruminating lambs. At slaughter, tissue samples were procured from diaphragm, rib muscle, and subcutaneous (SC) adipose tissue. Safflower oil supplementation in the diet resulted in an increase in CLA content of the TAG from diaphragm, rib muscle, and SC adipose tissue by about threefold (P<0.05) on a mol% basis. CLA was localized to the sn-1/3 positions of TAG. Animals that received pre-formed CLA, however, had increased proportions of CLA at the sn-2 position of TAG from SC adipose tissue, suggesting that there were tissue-specific dietary effects and possible age-related effects on the mode of FA incorporation into TAG. Safflower oil supplementation in the diet had no effect on the CLA content of PL from diaphragm, rib muscle, and SC adipose tissue, suggesting that CLA was preferentially incorporated into the TAG of these tissues.     

一种简便化学发光—化学需氧量测定方法

对以前提出的化学发光—化学需氧量测定新方法进行了较为深入的研究,利用溴化银胶体溶液的性质并以其作为样品进样,简化了以前实验过程中的过滤步骤,为进一步仪器化和实用化打下坚实的基础。本方法线性范围为0～600mg/L,r2=0.9989;回收率在100±10%范围内;RSD≤5%(n=6)。用于实际样品测定,结果满意。     

Rapid MS method for analysis of cocoa butter TAG

Ammonia negative ion CI-MS was applied to analyze the M.W. distribution and regioisomeric structure of TAG in cocoa butter and in cocoa butter equivalents. The M.W. distribution results obtained for a reference cocoa butter were consistent with corresponding results obtained in an intercomparison study by chromatographic methods. Minor but statistically significant differences were observed when proportions of the three major M.W. species (52∶1, 54∶1, and 50∶1; acyl carbon number/number of double bonds) in a mixture of nine cocoa butters and in mixtures containing 10 or 20% (w/w) of specific cocoa butter equivalents were compared. Tandem MS was used to determine the regioisomeric structure of the three major TAG M.W. species in cocoa butter and in cocoa butter equivalents. The regioisomeric structure in cocoa butter and in all the equivalents analyzed were nearly identical, oleic acid being located primarily in the sn-2 position. These results cannot be exploited in detecting added foreign fats in this case. However, the present study shows that useful TAG composition data, which may be used to detect foreign fats in cocoa butter by applying chemometric data evaluation, can be obtained by MS in a significantly shorter time compared to chromatographic methods.     

小球藻产三酰甘油过程的代谢途径分析

以基因工程强化微藻三酰甘油(TAG)积累是实现微藻生物柴油应用化的途径之一。为进行有效的基因工程操作,研究分析了微藻代谢网络,以找出其中对TAG合成重要的反应。构建了异养及自养条件下小球藻产TAG过程核心代谢网络。采用改进CASOP(Computational approach for strain optimization aiming at high productivity)法分析网络中各反应的重要度。结果表明,主要的关键反应包括脂肪酸和TAG的合成反应以及提供NADPH的反应,特别是将NADH转化为NADPH的反应。此外,分析结果也部分解释了环境压力对TAG积累的作用。