Turnover number—

Example 51 Add. 1 mole of 3-(3,4-methylenedioxyphenyl) propylene,. 25 mole of methyl nitrite., 5L of methanol, 36g of water,. 00025 mole of trimethylamine, and. 0005 mole of palladium chloride as a catalyst to a flask. Stir magnetically for 1.5 hours at 25C. The conversion of the starting material was 92%, the yield of MDP-2-P was 83% and the Pd Turnover Number was 166. 

The decarbonylation-dehydration of the fatty acid 887 catalyzed by PdCl2(Ph3P)2 fO.Ol mol%) was carried out by heating its mixture with acetic-anhydride at 250 C to afford the terminal alkene 888 with high selectivity and high catalyst turnover number (12 370). The reaction may proceed by the oxidative addition of Pd to the mixed anhydride[755]. 

Km for an enzymatic reaction are of significant interest in the study of cellular chemistry. From equation 13.19 we see that Vmax provides a means for determining the rate constant 2- For enzymes that follow the mechanism shown in reaction 13.15, 2 is equivalent to the enzyme s turnover number, kcat- The turnover number is the maximum number of substrate molecules converted to product by a single active site on the enzyme, per unit time. Thus, the turnover number provides a direct indication of the catalytic efficiency of an enzyme s active site. The Michaelis constant, Km, is significant because it provides an estimate of the substrate s intracellular concentration. 

Carbamate Insecticides. These are stmcturaUy optimi2ed derivatives of the unique plant alkaloid physostigmine [57-47-6] a cholinergic dmg isolated in 1864 from Phjsostigma venenosum (see Alkaloids) (17,24,35—39). The carbamates maybe considered synthetic derivatives of the synaptic neurotransmitter acetylcholine, with very low turnover numbers. The A/,A/-dimethylcarbamates of heterocycHc enols (36) and the Ai-methylcarbamates of a variety of substituted phenols (35) with a wide range of insecticidal activity were described in 1954 (35). The latter are the most widely used carbamate insecticides, and the A/-methylcatbamates of oximes have subsequentiy been found to be effective systemic insecticides. 

High enantioselectivities and regioselectivities have been obtained using both mono- and 1,2-disubstituted prochinal olefins employing chiral phosphine phosphite (33,34) modified rhodium catalysts. For example, i7j -2-butene ia the presence of rhodium and (12) (33) gave (3)-2-meth5ibutanal ia an optical yield of 82% at a turnover number of 9.84. ... 

The data most frequentiy collected and reported in catalyst performance evaluations are activity or turnover number, selectivity to the desired product(s), overall yield, catalyst life, and the identities and yields of by-products produced. These data are used to further catalyst or process development research efforts, to monitor catalyst manufacture, and to provide quaUty assurance information to catalyst users. 

Enzyme and substrate first reversibly combine to give an enzyme-substrate (ES) complex. Chemical processes then occur in a second step with a rate constant called kcat, or the turnover number, which is the maximum number of substrate molecules converted to product per active site of the enzyme per unit time. The kcat is, therefore, a rate constant that refers to the properties and reactions of the ES complex. For simple reactions kcat is the rate constant for the chemical conversion of the ES complex to free enzyme and products. 

The results of experiments in which the mutation was made were, however, a complete surprise. The Asp 189-Lys mutant was totally inactive with both Asp and Glu substrates. It was, as expected, also inactive toward Lys and Arg substrates. The mutant was, however, catalytically active with Phe and Tyr substrates, with the same low turnover number as wild-type trypsin. On the other hand, it showed a more than 5000-fold increase in kcat/f m with Leu substrates over wild type. The three-dimensional structure of this interesting mutant has not yet been determined, but the structure of a related mutant Asp 189-His shows the histidine side chain in an unexpected position, buried inside the protein. 

Some G proteins are slow GTP hydrolases with turnover numbers around two per minute, others such as Ras are only marginally catalytic. Kinetic experiments in solution have shown that in both cases the most likely mechanism... 

The turnover number of an enzyme, is a measure of its maximal catalytic activity, is defined as the number of substrate molecules converted into product per enzyme molecule per unit time when the enzyme is saturated with substrate. The turnover number is also referred to as the molecular activity of the enzyme. For the simple Michaelis-Menten reaction (14.9) under conditions of initial velocity measurements, Provided the concentration of... 

The term represents the kinetic efficiency of the enzyme. Table 14.4 lists turnover numbers for some representative enzymes. Catalase has the highest turnover number known each molecule of this enzyme can degrade 40 million molecules of HgOg in one second At the other end of the scale, lysozyme requires 2 seconds to cleave a glycosidic bond in its glycan substrate. 

Szent-Gyorgyi further showed that the viscosity of an actomyosin solution was lowered by the addition of ATP, indicating that ATP decreases myosin s affinity for actin. Kinetic studies demonstrated that myosin ATPase activity was increased substantially by actin. (For this reason, Szent-Gyorgyi gave the name actin to the thin filament protein.) The ATPase turnover number of pure myosin is 0.05/sec. In the presence of actin, however, the turnover number increases to about 10/sec, a number more like that of intact muscle fibers. 

The turnover number of methylmalonyl-CoA epimerase is 100 sec and thus the enzyme enhances the reaction rate by a factor of 10. ... 

Beckmann rearrangements of several ketoximes were performed in room-temperature ionic liquids based on l,3-dialkylimida2olium or alkylpyridinium salts containing phosphorus compounds (such as PCI5) by Deng and Peng [59] (Scheme 5.1-31, BP = 1-butylpyridinium). Turnover numbers of up to 6.6 were observed, but the authors did not mention whether the ionic liquid could be reused. 

After ten consecutive runs the overall turnover number reaches up to 3500 mol 1-octene converted per mol Rh-catalyst. In agreement with these recycling experiments, no Rh could be detected in the product layer by AAS or ICP, indicating leaching of less then 0.07 %. In all experiments, very good selectivities for the linear aldehyde were obtained, thus proving that the attachment of the guanidinium moiety onto the xanthene backbone had not influenced its known positive effect on... 

Two interesting points that can be made from these values are the turnover number which is the maximum rate of transport per carrier, and the similarity of magnitude of ks and kMs. The turnover number, f, is given by the expression... 

The overall rate constant for conversion of the E S complex to products E + P is called the turnover number because it represents the number of substrate molecules the enzyme turns over into product per unit time. A value of about 103 per second is typical. 

Turnover number (Section 26.10) The number of substrate molecules acted on by an enzyme per unit time. 

Protein Data Bank and, 1048-1049 rate acceleration of, 1041 specificity of. 1041 substrate of, 1041 turnover number of, 1041 X-ray crystal structures of,... 

Quantum yield and luciferase activity The quantum yield of coelenterazine in the luminescence reaction catalyzed by Oplophorus luciferase was 0.34 when measured in 15 mM Tris-HCl buffer, pH 8.3, containing 0.05 M NaCl at 22°C (Shimomura et al., 1978). The specific activity of pure luciferase in the presence of a large excess of coelenterazine (0.9pg/ml) in the same buffer at 23°C was 1.75 x 1015 photons s 1 mg-1 (Shimomura et al., 1978). Based on these data and the molecular weight of luciferase (106,000), the turnover number of luciferase is calculated at 55/min. 

Renilla luciferase. The luciferase of Renilla reniformis has been purified and characterized by Karkhanis and Cormier (1971) and Matthews et al. (1977a). The purified luciferase has a molecular weight of 35,000, and catalyzes the luminescence reaction of coelenterazine. The luciferase-catalyzed luminescence is optimum at pH 7.4, at a temperature of 32°C, and in the presence of 0.5 M salt (such as NaCl or KC1). The luciferase has a specific activity of 1.8 x 1015 photons s"1mg"1, and a turnover number of 111/min. The luminescence spectrum shows a maximum at 480 nm. The absorbance A28O of a 0.1% luciferase solution is 2.1. The luciferase has a tendency to self-aggregate, forming higher molecular weight species of lower luminescence activities. 

Palladium(II) complexes provide convenient access into this class of catalysts. Some examples of complexes which have been found to be successful catalysts are shown in Scheme 11. They were able to get reasonable turnover numbers in the Heck reaction of aryl bromides and even aryl chlorides [22,190-195]. Mechanistic studies concentrated on the Heck reaction [195] or separated steps like the oxidative addition and reductive elimination [196-199]. Computational studies by DFT calculations indicated that the mechanism for NHC complexes is most likely the same as that for phosphine ligands [169], but also in this case there is a need for more data before a definitive answer can be given on the mechanism. 

Although dirhodium(II) carboxamidates are less reactive toward diazo decomposition than are dirhodium carboxylates, and this has limited their uses with diazomalonates and phenyldiazoacetates, the azetidinone-ligated catalysts 11 cause rapid diazo decomposition, and this methodology has been used for the synthesis of the cyclopropane-NMDA receptor antagonist milnacipran (17) and its analogs (Eq. 2) [10,58]. In the case of R=Me the turnover number with Rh2(45-MEAZ)4 was 10,000 with a stereochemical outcome of 95% ee. 

In summary, the order of reactivity for the most commonly used ruthenium-based metathesis catalysts was found to be 56d>56c>9=7. This order of reactivity is based on IR thermography [39], determination of relative rate constants for the test reaction 58—>59 (Eq. 8) [40], and determination of turnover numbers for the self metathesis of methyl-10-undecenoate [43]. 

True surface area, origination of method for obtaining (Leikis), 46 Turnover numbers, for minority carriers, 494... 

When a small fraction of irreversible mediator side reactions cause a rapid decrease of catalytic activity in the homogeneous case in a modified electrode this would be disastrous since there is no bulk supply of catalyst. Thus, higher turnover numbers are generally required than in the homogeneous case... 

Theories neglect that catalysts usually have limited turnover numbers due to destructive side reactions. This may not be so obvious in analytical experiments but it has severe consequences for large scale applications. A simple calculation can illustrate this problem if a redox polymer with a monomer molecular weight of 400 Da and a density of 1 g cm " is considered with all redox centers addressable from the electrode and accessible to the substrate with a turnover number of 1000, then, to react 1 nunol of substrate at a 1 cm electrode surface, at least 5 pmol of active catalyst centers corresponding to 2 mg of polymer, or a dry film thickness of 20 pm are required. This is 20 times more than the calculated optimum film thickness for rather favorable conditions... 

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