Double bond introduced

Additional double bonds introduced into existing mo-nounsamrated fatty acids are always separated from each other by a methylene group (methylene interrupted) except in bacteria. Since animals have a desaturase, they... 

The carotenoid isomerase (CRTISO) was the first isomerase associated with the desaturation steps and named at a time when Z-ISO was unknown to exist ise.ws.ieo.iei (and reviewed in references ). In vitro analysis of substrate conversion " and transcript profiling in planta associated CRTISO with the desaturation steps. Isaacson demonstrated that CRTISO is specific for the 7,9 or 7,9- cis bond configuration and is not involved in the isomerization of the l5-l5-cis double bond to the trans conformation. As recently shown, Z-ISO is required for isomerization of the 15-15 cis double bond of phytoene produced in dark-grown tissues as well as in stressed photosynthetic tissues. Therefore, desaturation of phytoene to lycopene involves a two-step desaturation by PDS, followed l5-cis isomerization by Z-ISO, and then each pair of double bonds introduced by ZDS is followed by CRT-ISO-mediated isomerization of the resulting conjugated double bond pair. 

If a fatty acid already has a double bond in it, the scheme by which the fatty acid is oxidized depends on where the double bond ends up after several of the C-2 fragments have been removed by normal p oxidation. With a double bond already present, the enzyme that catalyzes the first step (insertion of the double bond at C-2) gets confused when there is already a double bond at C-2 or at C-3. The fact that the double bonds in unsaturated fatty acids are invariably cis also complicates life since the double bond introduced at C-2 by the desaturating enzyme of p oxidation is a trans double bond. 

Figure 5.4 Dependence of the MNDO heats of formations (AHf°) of the Q0H2 isomers on the number of [5,6] double bonds introduced by the hydrogen addition [30]. The heats of formation are average values of the different structures from Table 5.2 with the same number of [5,6] double bonds.

Diphenyl-1,3-butadiene. The excited-state behavior of this diene differs significantly from stilbene and is the subject of a review. Unlike tS in which the lowest vertical excited singlet state is the 1 B state and S2 is the 2 Ag state in solution, these two excited states lie very close to each other in all-trans-1,4-diphenyl-1,3-butadiene (DPB). The additional carbon-carbon double bond introduces a new conformational equilibrium involving the s-trans and s-cis rota-mers. Most spectroscopic studies in solution have concluded that the l B state is S. The DPB compound has a low quantum yield for photoisomerization, so the use of DPB in time-resolved spectroscopic studies on photoisomerization, especially those that monitor only fluorescence decay, needs to be considered cautiously and critically. 

It is a considerable challenge to isolate a pure adduct. One of the unique features of fullerene chemistry is the large number of products that sometimes result from addition of even one mole eqivalent of reagent. Owing to its relatively high abundance, most fully characterized complexes are for C60, but the behavior of higher fullerenes is broadly similar. The availability of only small amounts of higher fullerenes, coupled with the inequivalency of some of the double bonds, introduces additional complications. 

It is worth considering a bit the proiecting-group strategy that has here been employed. The double bond introduced at the beginning of the synthesis (3 —> 4 is not activated in the form of an aldehyde equivalent until the transformation of 7 into 8 This is an example of the place-holder concept 6 In this way alkenes or protected alcohols - which are simpler and can be protected in various ways - are introduced in the course of a synthesis as equivalents for sensitive carbonyl functions. 

The remarkable ability of rubber to stretch and then contract to its original shape is due to the irregular shapes of the polyaner chains causeo by the double bonds. These double bonds introduce bends anc kinks into the oolv-... 

The three-dimensional structures of the fatty acids in Figure 10.6 illustrate how Z double bonds introduce kinks in the long hydrocarbon chain, decreasing the ability of the fatty acid to pack well in a crystalline lattice. The latter the numher of Z double bonds, the more kinks in the hydrocarbon chain, and the lower the melting point. 

The transfer of methyl hydrogen is then carried out through a six-center transition state. The supplementary double bond introduced in the cycle does not participate in the observed transformations. ... 

The presence of a cis double bond introduces a kink that prevents packing of the fatty acid chains. Cis double bonds maintain fluidity. Trans fatty acids have no structural effect, relative to saturated fatty acids, and so they are rare. 

The quaternary salts of codeine and its isomers can be degraded to unsaturated bases to which the general term methylmorphimethine has hitherto been applied, but here the less cumbersome and more systematic term codeimethine [1] will be used, the term methylmorphimethine being reserved for the derivatives obtained from nuclear methylated morphines. The term dihydromethine will be applied to bases in which the double bond introduced by degradation has been saturated. In this way a convenient and unambiguous system of nomenclature is available. 

The acid-catalysed condensation of resorcinol, 2-methylresorcinol and pyrogallol with aldehydes (or their synthetic equivalents) containing hydroxy-, aUcoxy-, aryldiazo-, sulphonyl- and B(OH)2 groups, halogens and double bonds introduces additional functional groups which can be further modified. 

One molecule of oxygen accepts two pairs of electrons, one from palmitoyl-CoA and the other from NADPH or NADH. The electrons NAD(P)H are transported via cytochrome-bs reductase to cytochrome bs (microsomal electron transport Chapter 14). An enzyme-bound superoxide radical is responsible for the oxidation of acyl-CoA. Four desaturases specific for introducing cis double bonds at C9, Ca, C5, and C4, respectively, are known. If the substrate is saturated, the first double bond introduced is C9. With an unsaturated substrate, other double bonds are introduced between the carboxyl group and the double bond nearest the carboxyl group. Desaturation yields a divinylmethane arrangement of double bonds (—CH=CH—CH2—CH=CH—). Usually desaturation alternates with chain elongation. Desaturation is inhibited by fasting and diabetes. The oxidation of unsaturated fatty acids occurs in mitochondria. 

Oxidation Mechanism of CR. A chlorine substituent on the double bond Introduces considerable complexity In the degradation of trans-l,4-polychloroprene (CR). CR Is more resistant to oxidation than BR and IR, but Is subject to dehydrochlorlnatlon (32. 33, 34). 

The presence of a double bond introduces a bend in the moiecuie that prevents oieic acid moiecuies from packing together as tightiy as do stearic acid moiecuies. As a resuit, oieic acid—iike most piant iipids—is a iiquid at room temperature, whereas stearic acid—iike most animai iipids—is a soiid. ... 

The oxidation of unsaturated fatty acids such as oleic acid requires additional enzymes. They are needed because, unlike the trans double bonds introduced during /1-oxidation, the double bonds of most naturally occurring unsaturated fatty acids have a cis configuration. The enzyme enoyl-CoA isomerase converts the cis-/ , ydouble bond to a trans-a,fS double bond. The /3-oxidation of oleic acid is illustrated in Figure 12A. 

Hofmann degradation of dihydrotazettine methohydroxide provided the ester (CXLI) anticipated by analogy with the Hofmann product from tazettine (55, 139, 140). The double bond introduced by the reaction was neither conjugated with the aromatic ring nor contiguous to... 

A cis double bond Introduces a rigid kink In the otherwise flexible straight chain of a fatty acid (Figure 2-18). In general, the fatty acids In biological systems contain only cis double bonds. 

Polymers for use in 193 nm lithography are co-, ter-, and tetra-polymers of 1) methacrylates, 2) norbornenes, 3) norbornene-maleic anhydride, 4) nor-bornene-sulfur dioxide, and 5) vinyl ether-maleic anhydride (Fig. 39). While 1), 3), 4), and 5) are prepared by radical polymerization, all-norbornene polymers 2) are synthesized by transition-metal-mediated addition polymerization [166-168].Norbornenes (Fig.40) are sluggish to undergo radical [168,169] and cationic [170] polymerizations. Their ring-opening metathesis polymerization (ROMP, Fig. 40) [ 171 ] has never produced worthy resist polymers. The C=C double bonds introduced in the ROMP polymer backbone must be hydrogenated to reduce the 193 nm absorption and the ROMP polymers tend to have low Tg. However, the major problem for the ROMP polymers was their unacceptable swelling in aqueous base development. While polymethacrylate systems contain etch-resistant alicyclic structures in the ester side chain, norbornene-based systems carry the alicyclic unit in the backbone. Essentially all the 193 nm re-... 

This double bond introduces a stereoisomerism the unit may have two different forms ... 

Bixin, a bolaamphiphile with one methylester and one carboxylate end group, appears in the waxy cover of seed granules of the South American and African tree Bixa orellana. This lively colored coating of the seed proteins may protect them against attack by microorganisms. Why is an unsymmetrical bolaamphiphile needed, if the protective role can presumably also be played by P-carotene Why is a cw-double bond introduced at one end ... 

Fig. 10. Dependence of the MNDO heats of formations of the Q0H2 isomers on the number of [5,6]-double bonds introduced by hydrogen addition...

Fig. 11. Biosynthesis of polyunsaturated fatty acids from exogenous and endogenous precursors. The figures 6, 5 and 4, refer to the position of double bonds introduced by desaturases.

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