Catalytic roles

The VFe protein also has the equivalent of P-cluster pairs which have similar properties to those found in the MoFe protein (159). No information is available on whether P-cluster pairs exist in the FeFe protein, but because of the relatively high sequence identity and the similar genetic basis of its biosynthesis, the occurrence seems highly likely. The catalytic role assigned to the P-cluster pair involves accepting electrons from the Fe protein for storage and future deUvery to the substrate via the FeMo-cofactor centers. As of this writing (ca early 1995), this role has yet to be proved. 

The catalytic cycle (Fig. 5) (20) is well estabUshed, although the details of the conversion of the intermediate CH COI and methanol into the product are not well understood the mechanism is not shown for this part of the cycle, but it probably involves rhodium in a catalytic role. The CH I works as a cocatalyst or promoter because it undergoes an oxidative addition with [Rh(CO)2l2]% and the resulting product has the CO ligand bonded cis to the CH ligand these two ligands are then poised for an insertion reaction. 

Most catalyst supports are simply nearly inert platforms that help stabilize the dispersion of the catalyticaHy active phase. Sometimes, however, the supports play a direct catalytic role, as exemplified by the alumina used in supported Pt and RePt catalysts for naphtha reforming. 

There are numerous stmctures that are similar to 2eofites, such as aluminophosphate molecular sieves, AlPOs, but these have not found catalytic apphcations. Zeofites can be modified by incorporation of cations in the crystalline lattice which are not exchangeable ions, but can play catalytic roles. For example, sificahte, which has the stmcture of ZSM-5 but without Al, incorpora ting Ti in the lattice is a commercial catalyst for oxidation of phenol with H2O2 to give diphenols the catalytic sites may be isolated Ti cations (85). 

Nutritional Requirements. The nutrient requirements of mammalian cells are many, varied, and complex. In addition to typical metaboHc requirements such as sugars, amino acids (qv), vitamins (qv), and minerals, cells also need growth factors and other proteins. Some of the proteins are not consumed, but play a catalytic role in the cell growth process. Historically, fetal calf semm of 1—20 vol % of the medium has been used as a rich source of all these complex protein requirements. However, the composition of semm varies from lot to lot, introducing significant variabiUty in manufacture of products from the mammalian cells. 

Until recently, the catalytic role of Asp ° in trypsin and the other serine proteases had been surmised on the basis of its proximity to His in structures obtained from X-ray diffraction studies, but it had never been demonstrated with certainty in physical or chemical studies. As can be seen in Figure 16.17, Asp ° is buried at the active site and is normally inaccessible to chemical modifying reagents. In 1987, however, Charles Craik, William Rutter, and their colleagues used site-directed mutagenesis (see Chapter 13) to prepare a mutant trypsin with an asparagine in place of Asp °. This mutant trypsin possessed a hydrolytic activity with ester substrates only 1/10,000 that of native trypsin, demonstrating that Asp ° is indeed essential for catalysis and that its ability to immobilize and orient His is crucial to the function of the catalytic triad. 

Craik, C. S., et al., 1987. The catalytic role of the active site aspartic acid in serine proteases. 237 909-919. 

These are important not only for their part in stimulating the development of bonding theory (for a fuller discussion, see p. 931) but also for their catalytic role in some important industrial processes. 

The use of a nichrome stirrer or a catalytic amount of nickel ion is recommended 1 for such reactions to minimize the accumulation of diazonium xanthate however, the catalytic role of nickel ion has not been explored with other diazonium salts. 

A plausible mechanism accounting for the catalytic role of nickel(n) chloride has been advanced (see Scheme 4).10 The process may be initiated by reduction of nickel(n) chloride to nickel(o) by two equivalents of chromium(n) chloride, followed by oxidative addition of the vinyl iodide (or related substrate) to give a vinyl nickel(n) reagent. The latter species may then undergo transmetala-tion with a chromium(m) salt leading to a vinyl chromium(m) reagent which then reacts with the aldehyde. The nickel(n) produced in the oxidative addition step reenters the catalytic cycle. 

Thus, in spite of its lack of reactivity, iodine reacts chemically with unsaturated compounds, whereby the silica gel of the TLC layer can sometimes be assigned a catalytic role [11, 12]. Irreversible oxidations and electrophilic substitution and addition reactions have been observed on the interaction of iodine with tertiary nitrogen compounds such reactions possibly depend on particular steric relationships or are favored by particular functional groups [13, 14]. 

The cycle starts with reaction between the acetyl moiety of acetyl-CoA and the four-carbon dicarboxylic acid ox-aloacetate, forming a six-carbon tricarboxylic acid, citrate. In the subsequent reactions, two molecules of COj are released and oxaloacetate is regenerated (Figure 16-1). Only a small quantity of oxaloacetate is needed for the oxidation of a large quantity of acetyl-CoA oxaloacetate may be considered to play a catalytic role. 

Figure 16-1. Citric acid cycle, illustrating the catalytic role of oxaloacetate.

The scope of reactions involving hydrogen peroxide and PTC is large, and some idea of the versatility can be found from Table 4.2. A relatively new combined oxidation/phase transfer catalyst for alkene epoxidation is based on MeRe03 in conjunction with 4-substituted pyridines (e.g. 4-methoxy pyridine), the resulting complex accomplishing both catalytic roles. 

Scheme 8 Catalytic role of iron in cycloisomerization reactions [17]...

In line with these current developments, publishing a book dealing with the most recent achievements in this field is particularly timely. The volume is structured in chapters according to the type of metal complex. In every chapter, a brief introduction on the general chemical properties of the respective class of Fe-complexes will be given. Subsequently, representative examples for different catalytic transformations with a special emphasis on the various reaction manifolds will be presented. This structure implies that the reviews are not comprehensive but are meant to improve the understanding of the catalytic role a certain iron complex plays within the mechanism. 

The catalytic chemistry of M° depends on the elementary properties of M and on the structure and size of the M° nanoclusters ( quantum dots ) [2]. S may play a role as a reactivity enhancer of M°/S as a whole (co-catalytic role) and/or as a promoter of its catalytic chemoselectivity (promotional role) [3,4]. 

The highest autoinflammation temperatures are obtained when the container is made of glass. With a metai container not only are the values low but they also depend on the state of the metal surface and especially on the potential presence of oxides that play a catalytic role. A drop in temperature can then reach at least 50°C with substances that have AIT greater than 290°C. The effect of the material of the container varies hugely depending on the substance. Thus the AIT results are the same for toluene with glass and metal whereas with methyl acetate they are respectively 502 and 464°C. But is it due to the presence of oxides ... 

Presence of zeolites with 5 A, which were used to dry ethylene under pressure and catalysed the polymerisation. The installation was destroyed. The zeolites with 3 A do not play any catalytic role. 

A road haulier carrying epichlorhydrin and who had covered a distance of 500 km noticed that there was an unusual temperature rise in the tanker. It was probably due to the beginning of polymerisation in the presence of an impurity, which played a catalytic role. The temperature reached 115 C very quickly and later the safety valve opened releasing a large quantity of fumes. 

Usually the nitric acid/amine interaction is more dangerous when impurities that can play a catalytic role are present. This goes for metal oxides such as copper oxides, iron (III) oxide and divanadium pentoxide. Salts such as sodium or ammonium metavanadates, iron trichloride, alkaline chromates and dichromates, cyanoferrates and alkaline or nitrosopentacyanoferrates can also act as catalysts. 

There are a few similarities between this accident and the previous one. When dinitroanilines (all isomers) are submitted to the effect of hydrogen chloride in the presence of chlorine, which seems to play a catalytic role, they give rise to a very violent reaction after a period of induction that can be very long if the temperature is low. Again a large volume of gas is also released. 2,3-Dinitroaniline is the most reactive, 3,5-the less reactive. 

In the presence of an acid (sulphuric acid, acetic acid) or a metal (iron) that plays a catalytic role, acetaldehyde gives rise to polymerisation reactions that are often violent and cause the compound to overflow. An accident of this type has also been observed with anhydrous formol at -189°C. 

The CO-X bond breaking is the result of an electrophilic attack (on the carbonyl oxygen atom, hence the catalytic role of acids in these rupture reactions) or a nucleophilic one (on the carbonyl carbon atom whose positive property is due to the X electron-withdrawing property). The dangers of this type of reaction come from its speed and high exothermicity and/or instability of the products obtained in some cases. The accidents that are described below can make one believe that acid anhydrides in general and acetic anhydride in particular represent greater risks than acid chlorides since they constitute the accident factor of almost all accidents described. This is obviously related to their frequent use in synthesis rather than acid chlorides, that are rarely used. 

Gong X, Hu P, Raval R. 2003. The catalytic role of water in CO oxidation. J Chem Phys 119 6324-6334. 

SCHEME 4 Catalytic role of PADA in the extraction of Ni(pan)2. Copyright 2001 Marcel Dekker, Inc. 

The catalytic role of the liquid-liquid interface in the solvent extraction of metal ions described in this chapter and some important remarks on the interfacial phenomena reported in other studies are summarized ... 

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