The modulation of membrane fluidity by hydrogenation processes. II. Homogeneous catalysis and model biomembranes

A homogeneous catalyst, chlorotris (triphenylphosphine) rhodium (I) has been incorporated into model biomembrane structures in the form of lipid bilayer dispersions in water. This enables the hydrogenation of the double bonds of the unsaturated lipids within the bilayers to be accomplished. To decide the optimum conditions for efficient hydrogenation the reaction conditions have been varied. The effect of catalyst concentration, hydrogen gas pressure and lipid composition (with and without cholesterol) have all been studied. The partition of the catalyst into the lipid medium was checked by rhodium analysis. The results show that an increase of catalyst concentration or an increase of hydrogen gas pressure leads to increasing rates of hydrogenation. Successful hydrogenation was accomplished with different types of lipid dispersions (mitochondrial, microsomal and erythrocyte lipids). A selectivity of the homogeneous hydrogenation process is indicated. The polyunsaturated fatty acyl residues are hydrogenated at an earlier stage and at a faster rate than the monoenoic acids. Furthermore, an increase in the proportion of cholesterol to lipid within the bilayer structures causes a progressive decrease in the rate of hydrogenation. The fluidity of the lipid bilayers can be altered to such an extent by the hydrogenation process that new sharp endotherms corresponding to the order-disorder transition of saturated lipids occur at temperatures as high as 319 K. Some potential uses of hydrogenation for the modulation of cell membrane fluidity are discussed as well as the design of new types of catalyst molecules.     

Complex hydrogenation/oxidation reactions of the water-soluble hydrogenation catalyst palladium di (sodium alizarinmonosulfonate) and details of homogeneous hydrogenation of lipids in isolated biomembranes and living cells.

Palladium di (sodium alizarinmonosulfonate) is a highly efficient catalyst for the hydrogenation of unsaturated fatty acids esterified in lipids of model or biological membranes, enabling the study of the relationship between function and the physical state of membranes. However, the catalyst shows a complex behavior in the action of molecular hydrogen and oxygen, giving rise to the formation of at least four products. Two of these are free radicals. Owing to this complexity, precise control of the reaction requires pretreatment of the catalyst. When partial hydrogenation of the palladium complex is followed by air oxidation, a catalyst solution is produced which is stable on air and maintains catalytic hydrogenation activity for several days. This form of the catalyst induces hydrogenation of unsaturated lipids with no induction period making a strict timing of the procedure possible. Of the several other factors affecting the outcome of membrane hydrogenations, one of the most important is the accessibility to the catalyst of particular membrane regions or lipid pools. Differences in accessibility may arise as a consequence of different local microviscosities or their change during hydrogenation, of the appearance of distinct liquid crystalline phases, and of strong protein-lipid interactions. Obviously, in case of whole-cell hydrogenations, the accessibility is influenced by the spatial separation of the organelles, as well.     

The modulation of membrane fluidity by hydrogenation processes. III. The hydrogenation of biomembranes of spinach chloroplasts and a study of the effect of this on photosynthetic electron transport.

A method is reported for the in situ modification of the lipids of isolated spinach chloroplast membranes. The technique is based on a direct hydrogenation of the lipid double bonds in the presence of the catalyst, chlorotris(triphenylphosphine)rhodium (I). The pattern of hydrogenation achieved suggests that the catalyst distributes amongst all of the membranes. The polyunsaturated lipids within the membranes are hydrogenated at a faster rate and at an earlier stage than are the monoenoic lipids. Whilst addition of the catalyst to the chloroplast causes an initial 10--20% decrease in Hill activity, saturation of up to 40% of the double bonds present can be accomplished without causing further significant alterations in photosynthetic electron transport processes or marked morphological changes of the chloroplast structure as observed in the electron microscope.     

Unusual amino acids. I: Asymmetric synthesis of furylalanine derivatives.

Nonproteinogenic amino acids are valuable active compounds from their pharmacological and biochemical effects and also as novel building blocks for peptides. The preparation of furylalanine derivatives by asymmetric hydrogenation is described. Amino-phosphine-phosphinite-rhodium complexes catalyzed the hydrogenation of the prochiral dehydroamino acid precursors in high rate and with enantioselectivities of 70-90% ee. Substrate-catalyst ratios up to 2,000 can be used depending on the catalyst applied. The procedure turns out to be suitable for larger scale preparations.     

The modulation of membrane fluidity by hydrogenation processes. III. The hydrogenation of biomembranes of spinach chloroplasts and a study of the effect of this on photosynthetic electron transport

A method is reported for the in situ modification of the lipids of isolated spinach chloroplast membranes. The technique is based on a direct hydrogenation of the lipid double bonds in the presence of the catalyst, chlorotris(triphenylphosphine)rhodium (I). The pattern of hydrogenation achieved suggests that the catalyst distributes amongst all of the membranes. The polyunsaturated lipids within the membranes are hydrogenated at a faster rate and at an earlier stage than are the monoenoic lipids.Whilst addition of the catalyst to the chloroplast causes an initial 10–20% decrease in Hill activity, saturation of up to 40% of the double bonds present can be accomplished without causing further significant alterations in photosynthetic electron transport processes or marked morphological changes of the chloroplast structure as observed in the electron microscope.     

Action of a homogeneous hydrogenation catalyst on living Tetrahymena mimbres cells

Various conditions were tested in an attempt to hydrogenate the unsaturated fatty acids of living Tetrahymena mimbres with the homogeneous catalyst palladium di-(sodium alizarine monosulfonate) without causing serious damage to the cells. Using a low (20 micrograms/ml) catalyst concentration in the external medium, hydrogenation of greater than 20% of surface membrane lipid double bonds were obtained, but hydrogenation of intracellular membranes was minimal. When exposed to H2, cells preincubated with inactive catalyst for several hours and visibly loaded with the catalyst lost viability as soon as hydrogenation exceeded trace levels. Material secreted by Tetrahymena into their medium effectively inhibited hydrogenation of added oleic acid, normally a good substrate. Mucus secreted by the cells, soluble proteins isolated from cell homogenates, bovine serum albumin, and cysteine were also inhibitory, but the inhibition could be overcome by employing higher catalyst concentrations. Although some enzymatic retroconversion of saturated lipids back to unsaturated lipids appeared to take place, the scale of the conversion was small, and further experimentation will be required to understand the mechanism involved. The selective hydrogenation of surface membranes achieved by these methods may be especially useful to those interested in fluidity effects on plasma membrane properties.     

New stereoselective catalysts in doxycycline synthesis

Formation of the doxycycline diastereoisomer along with doxycycline during hydrogenation of the methacycline exocyclic double bond lowered the yield of the main product. The use of tris-(triphenylphosphine)-rhodium chloride (Wilkinson catalyst) as a catalyst provided higher stereoselectivity of the hydrogenation and resulted in predominant production of doxycycline. Inclusion of some ligands such as hydrazine or others into the composition of the compound rhodium catalyst increased its activity. The stereoselectivity during hydrogenation of the methacycline exocyclic double bond can be explained by different rates of hydrogen attachment to the two enantiomeric products of the compound rhodium catalyst addition to the exocyclic double bond.     

Mapping of Heterogeneous Catalyst Degradation in Polymer Electrolyte Fuel Cells

Pt catalysts in polymer electrolyte fuel cells degrade heterogeneously as the catalyst particles are exposed to local variations throughout the catalyst layer during operation. State‐of‐the‐art analytical techniques for studying degradation of Pt catalysts do not possess fine spatial resolution to elucidate such non‐uniform degradation behavior at a large electrode level. A new methodology is developed to spatially resolve and quantify the heterogeneous Pt catalyst degradation over a large area (several cm²) of aged MEAs based on synchrotron X‐ray microdiffraction. PEFC single cells are aged using voltage cycling as an accelerated stress test and the degradation heterogeneity at a micrometer length scale is visualized by mapping Pt catalyst particle size after voltage cycling. It is demonstrated in detail that the Pt catalyst particle size growth is non‐uniform and follows the flow field geometry. The Pt particle size growth is greater in the area under the flow field land, while it is minimal in the area under the flow field channel. Additional non‐uniformity is observed with the Pt particle size increasing more rapidly at the air outlet area than the Pt particle size at the inlet area.     

Natural alkaloids and synthetic relatives as chiral templates of the Orito's reaction

The enantioselective hydrogenation of methyl or ethyl pyruvate over cinchona‐platinum catalyst system (Orito's reaction) is one of the most intensively studied heterogeneous catalytic asymmetric hydrogenation reactions. Studies aiming at systematic changes of the chiral template have played a crucial role in creating hypotheses for the mechanism of Orito's reaction. It is very important to clarify which structural unit of the alkaloid takes part in the enantiodifferentiation, and learn about the role of the different structural units of chiral templates. In this article, we made an attempt to describe the behavior of natural alkaloids, their synthetic derivatives, and analogues as chiral templates in the heterogeneous catalytic asymmetric hydrogenation of activated ketones. Chirality, 2010. © 2009 Wiley‐Liss, Inc.     

Design and synthesis of a stereodynamic catalyst with reversal of selectivity by enantioselective self‐inhibition

Chirality plays a pivotal role in an uncountable number of biological processes, and nature has developed intriguing mechanisms to maintain this state of enantiopurity. The strive for a deeper understanding of the different elements that constitute such self‐sustaining systems on a molecular level has sparked great interest in the studies of autoinductive and amplifying enantioselective reactions. The design of these reactions remains highly challenging; however, the development of generally applicable principles promises to have a considerable impact on research of catalyst design and other adjacent fields in the future. Here, we report the realization of an autoinductive, enantioselective self‐inhibiting hydrogenation reaction. Development of a stereodynamic catalyst with chiral sensing abilities allowed for a chiral reaction product to interact with the catalyst and change its selectivity in order to suppress its formation, which caused a reversal of selectivity over time.     

The hydrogenation of phospholipid-bound unsaturated fatty acids by a homogeneous, water-soluble, palladium catalyst

The hydrogenation of unsaturated phospholipids by palladium di(sodium alizarine monosulphonate) activated for 5 min under H2 proceeded rapidly at 20 degrees C and 1 atm. H2. Multibilayer liposomes of dioleoyl- and dilinolenoylphosphatidylcholine were hydrogenated at similar rates while dilinoleoyl- and 1-palmitoyl-2-oleoylphosphatidylcholine were hydrogenated at slightly slower rates. The reduction of polyunsaturated fatty acids gave rise to a variety of natural and unnatural positional cis and trans isomers which were largely reduced further to saturated fatty acids as the hydrogenation continued. Dioleoylphosphatidylethanolamine was attacked by the catalyst more slowly at 20 degrees C than was the equivalent phosphatidylcholine molecular species. Experiments conducted using mixtures of phosphatidylethanolamine and phosphatidylcholine in varying proportions also suggested that phospholipids are slightly more susceptible to catalytic hydrogenation in the bilayer phase than in the hexagonalII phase. Understanding the sequence of hydrogenation reactions involving these one and two component lipid preparations is useful in interpreting the action of the palladium catalyst on living cells under the same mild conditions.     

Hydrogenation of d-fructose and d-fructose/d-glucose mixtures

d-Fructose and d-fructose/d-glucose mixtures have been hydrogenated in water at 60–80° and 20–75 atm. of hydrogen with Ni, Cu, Ru, Rh, Pd, Os, Ir, and Pt severally as catalysts. The selectivity for the formation of d-mannitol from d-fructose as well as the selectivity for the hydrogenation of d-fructose in the presence of d-glucose with Cu/silica as the catalyst are substantially higher than those for the other catalysts. With Cu/silica as the catalyst, the hydrogenation of d-fructose is first order with respect to the amount of catalyst and the hydrogen pressure, whereas a shift from first- to zero-order kinetics occurs on going from low (<0.3m) to high (0.8m) concentrations of d-fructose. d-Fructose is preferentially hydrogenated via its furanose forms, presumably by attack of a copper hydride-like species at the anomeric carbon atom with inversion of configuration. Preferential adsorption of pyranose with respect to furanose forms occurs, whereas the furanose forms show a much higher reactivity. The mechanism proposed for the copper-catalysed hydrogenation reaction explains both the enhanced yield of d-mannitol from boric esters of d-fructose and the diastereoselectivity of the hydrogenation of seven other ketoses.     

肉桂高聚原花青素的氢化降解工艺研究

低聚原花青素具有显著的生物活性,高聚体由于空间位阻的影响,活性受到抑制。肉桂在41种食品中原花青素含量位居第一,但高聚体占50％以上。为了充分利用肉桂原花青素资源,采用氢化降解法将肉桂高聚原花青素降解为低聚体。以10％钯碳做催化剂,高压氢气参加反应,考察温度、压强、反应时间和催化剂用量对降解效果的影响。结果表明,平均聚合度为8．2的肉桂高聚原花青素,在100℃,压强3．5MPa,反应200min,催化剂用量0．2g/100mL的条件下,平均聚合度降为2．68。降解产物经HPLC分析表明,此工艺确实实现了肉桂高聚原花青素的降解。     

A highly enantioselective rhodium catalyst for the hydrogenation of aliphatic α-keto esters

The enantioselective hydrogenation of several α-keto acid derivatives with rhodium diphosphine catalysts has been investigated using a random screening approach. The neutral rhodium catalyst prepared in situ from bis(2,5-norbornadiene rhodium chloride) and NORPHOS has been found to be an excellent catalyst for preparing aliphatic α-hydroxy esters in high optical purities. The reaction parameters for the hydrogenation of ethyl 2-oxo-4-phenyl-butyrate, an intermediate for the ACE inhibitor Benazepril, were optimized and the best optical yields obtained were 96%.     

Enantioselective hydrogenation in ionic liquids: Recyclability of the [Rh(COD)(DIPAMP)]BF4 catalyst in [bmim][BF4]

Asymmetric hydrogenation of (Z)-α-acetamidocinnamic acid and methyl-(Z)-α-acetamidocinnamate by [Rh(COD)(DIPAMP)][BF₄] catalyst was studied in ionic liquid/isopropanol two-phase catalytic system. In this system 97–100% conversion was achieved and the enantioselectivity values were over 90%. Application of [bmim][BF₄] ionic liquid made it possible to recycle the catalyst in consecutive cycles. After four cycles, neither significant conversion nor enantioselectivity decrease was observed.     

Synthetic studies of polyene macrolide antibiotics. 3. Hydrogenation of methyl-6-deoxy-2,3-O-isopropylidene-4-methylene- alpha-L-lyxo-hexopyranoside

Experimental conditions of hydrogenation of the title compound are found (excess of Raney catalyst in benzene solution, 5 degrees C, 2h) to afford methyl 4,6-dideoxy-2,3-O-isopropylidene-4-C-methyl-alpha-L-mannopyranoside, the key intermediate in the synthesis of C33-C38 fragment of amphotericin B, with the yield as high as 57%.     

The combined hydrolysis and hydrogenation of inulin catalyzed by bifunctional Ru/C

A one-pot process for hydrolysis and hydrogenation of inulin to D-mannitol and D-glucitol over a bifunctional Ru/C catalyst was developed. The hydrolysis is catalyzed by the carbon support, onto which acidity was introduced by pre-oxidation. The effect of different carbon treatments on the hydrolysis of inulin was studied. Oxidation with ammonium peroxydisulfate resulted in a carbon with the highest hydrolysis activity. On this carbon, long chain inulin is hydrolyzed faster than inulin rich in short chains. The application of high pressure (up to 100 bar) increased the hydrolysis rate substantially. The combined process was successfully conducted with a Ru-catalyst supported on this oxidized carbon.     

Heterogeneous amino acid transport rate changes in an E. coli unsaturated fatty acid auxotroph.

Amino acid transport rates in an $\text{E. coli}$ unsaturated fatty acid auxotroph were non-uniformly affected by enrichment of membrane lipids in various unsaturated fatty acids. Proline and threonine transport rates were depressed much more than lysine and asparagine rates by trans unsaturated acids. Myristoleate and linolenate enrichment also produced non-uniform but lesser rate reductions. Although changes in the relative numoer of effective transport catalysts could account for these findings, comparisons of proline and lysine transport rates over a broad temperature range indicated that non-uniform alterations in transport catalyst reaction rates account at least partly for the activity changes associated with membrane lipid alterations.     

Increased thermal stability of pigment-protein complexes of pea thylakoids following catalytic hydrogenation of membrane lipids

The membrane lipids of pea thylakoids were hydrogenated in situ using the homogeneous catalyst palladiumdi-(sodium alizazine monosulphonate). Following hydrogenation, particle-free patches corresponding to phase-separated gel-phase lipids were observed in the fracture-faces of thylakoid membranes. Freeze-fracture studies on samples of hydrogenated thylakoids incubated at elevated temperatures indicated that hydrogenation reduces the tendency of the heated membranes to destack and vesiculate at higher temperatures. Measurements of chlorophyll a fluorescence emission and the thermal properties of hydrogenated thylakoids suggest that the hydrogenation process also leads to an increase in the thermal stability of pigment-protein complexes of the Photosystem II light-harvesting apparatus.     

Efficient conversion of d-glucose into d-sorbitol over MCM-41 supported Ru catalyst prepared by a formaldehyde reduction process

Ru/MCM-41 catalyst prepared by an impregnation-formaldehyde reduction method showed higher catalytic activity and sorbitol selectivity than other catalysts in the hydrogenation of glucose. SEM and XRD indicated the partial surface properties of Ru/MCM-41. Moreover, Ru dispersion and Ru surface area of Ru/MCM-41 were determined by pulse chemisorption, and the result further proved that Ru/MCM-41 had higher catalytic activity. A catalyst recycling experiment demonstrated that Ru/MCM-41 was a better catalyst and it could be reused three or four times. A speculated mechanism was proposed to illustrate the detailed process of d-glucose hydrogenation to produce sorbitol.