CISS sequence

The overwhelming power of the 3D CISS sequence is its combination of high signal levels and extremely high spatial resolution (Fig. 2.10). CISS images yield the best detail available of the cisternal portions of cranial nerves. The sequence has inherent flow... 

Held P, Pellner C, Pellner P, Seitz J, Strutz J (1997) MRI of inner ear anatomy using 3D MP-RAGE and 3D CISS sequences. Br J Radiol 70 465-472... 

Fig. 1,14a,b. Patient examined for tinnitus. CISS sequence showed tubular structure with the same signal intensity as the sigmoid sinus. Again, after contrast injection diagnosis of a DVA is obvious... 

Fig. 5.16. a Giant ICA aneurysm inducing optic nerve compression in a 10-year-old boy with visual deficit on the right eye. b Brain stem aneurysm between origin of the superior cerebellar artery and posterior cerebral artery resulting in right sided oculomotor palsy. c,d Pcom aneurysm (c DSA, lateral view) in a 46-year-old-patient with oculomotor palsy note the close relationship of the aneurysm and the oculomotor nerve (arrow) but without visible contact (d, sagittal reconstruction of CISS sequence)... 

Grierson, C., Du, J.S., de Torres Zabala, M., Beggs, K., Smith, C., Holdsworth, M., and Bevan, M. (1994). Separate cis sequences and trans factors direct metabolic and developmental regulation of a potato tuber storage protein gene. Plant J. 5(6) 815-826. 

Chapters 7 to 12 deal with factors affecting conformational equilibria and complex formation. Angyal explains the effect of calcium chloride on the anomeric equilibria of certain sugars by formation of especially stable complexes. These involve an axial-equatorial-axial sequence of three hydroxyl groups on a six-membered ring or a cis-cis sequence on a five-membered ring. Lemieux and Brewer use model compounds to study solvation effects on the orientation of the hydroxymethyl... 

The NMR spectra of D-allose show that the signal of the anomeric proton of the a-furanose form also increases and moves downfield on the addition of metal ions. The spectrum of the /2-furanose form, like that of the /2-pyranose form, is not affected (Figure 2). It appears that a cis-cis sequence of three hydroxyl groups on a five-membered ring also constitutes an arrangement suitable for the formation of complexes with metal ions. Since the proportion of furanoses in the equilibrium solution of D-allose is small, the changes in their proportion cannot be accurately measured. Hence the equilibrium of 5-O-methyl-D-ribose was studied ... 

All of these examples illustrate the complexing ability of an ax-eq-ax sequence of oxygen atoms on a six-membered or of a cis-cis sequence on a five-membered ring. Attempts have been made to find other complexing sites on sugar molecules but with little success. The 0-3, 0-5, and 0-6 arrangement of hexofuranoses, which allows the ready formation of tri-dentate periodate (IS), and orthoformate esters (19), does not form stable complexes with metal ions. In solution the side-chain of 1,2-0-isopropylidene-a-D-glucofuranose (13) is in the extended zig-zag form... 

PMCS-4 with different ratios of trans- and cis-sequences in the backbone possess one and the same glass transition temperature, and the crystallinity degree is lower that for PDMS. While polymeric chain is enriched by trans-units, crystallinity increases [27], By methods of DSC, X-ray diffraction and thermooptical analysis, it have been detected that besides glass transition (Tg) and melting (Tmeit), polymers synthesized from monomers containing up to 95 - 100% of tram-isomers display a phase transition in the temperature range of 70 - 90°C, which is simulated as a transition from meso-morphous to isotropic state (Figure 2). 

The first criterion for a candidate tRNA to be used as a delivery vehicle for noncanonical amino acids is the ability to insert its appended amino acid site specifically into a target protein. An obvious route is redefining existing codons to encode the noncanonical residue. Rather than using a cis sequence element... 

Poly(7-MeNBD) prepared with 10b and 11a contains between 47% and 52% trans C=C units, which are atactic. It is reported that the cis sequences in these samples comprise more r than m dyads. A model has been proposed suggesting that the change in polymer tacticity from syndiotactic to atactic is due to switching from a mode of coordination of the previously formed C=C double bond (similar to structure E in Scheme 20.15) to a mode of noncoordination. 

If you are running an updated version (V 8.0) of PC Model, click on force fields mm3. Omit this step for older versions. Click on Analyze (or compute depending on the version of PCMODEL) to obtain a menu of options. Select minimize. The geometry changes can be seen on the screen and a sequence of numbers appears in the right panel of the CRT screen, ending in Hf, the enthalpy of formation. This is the PCMODEL-MM3 calculated value of for cis-2-... 

Cis-olefins or cis./rjns-dienes can be obtained from alkynes in similar reaction sequences. The alkyne is first hydroborated and then treated with alkaline iodine. If the other substituents on boron are alkyl groups, a cis-olefin is formed (G. Zweifel, 1967). If they are cir-alkenyls, a cis, trans-diene results. The reactions are thought to be iodine-assisted migrations of the cis-alkenyl group followed by (rans-deiodoboronation (G. Zweifel, 1968). Trans, trans-dienes are made from haloalkynes and alkynes. These compounds are added one after the other to thexylborane. The alkenyl(l-haloalkenyl)thexylboranes are converted with sodium methoxide into trans, trans-dienes (E. Negishi, 1973). The thexyl group does not migrate. 

The preparation of cis 4 tert butylcyclohexanol from its trans stereoisomer was earned out by the following sequence of steps Write structural formulas including stereochemistry for com pounds A and B... 

When the sequence rules permit alternatives, preference for lower-numbered locants and for inclusion in the principal chain is allotted as follows in the order stated Z over E groups and cis over trans cyclic groups. If a choice is still not attained, then the lower-numbered locant for such a preferred... 

The physical properties of any polyisoprene depend not only on the microstmctural features but also on macro features such as molecular weight, crystallinity, linearity or branching of the polymer chains, and degree of cross-linking. For a polymer to be capable of crystallization, it must have long sequences where the stmcture is completely stereoregular. These stereoregular sequences must be linear stmctures composed exclusively of 1,4-, 1,2-, or 3,4-isoprene units. If the units are 1,4- then they must be either all cis or all trans. If 1,2- or 3,4- units are involved, they must be either syndiotactic or isotactic. In all cases, the monomer units must be linked in the head-to-tail manner (85). 

However, when either P(CgH )(CH2)2 or P(CgH )2(CH2) is used to form cis- or /n j -M(N2)2(PR3)4j M = Mo or W, respectively, followed by treatment with acid, ammonia yields of about 2 mol or 0.7 mol pet mole of complex for M = W and Mo, respectively, are produced (193,194). These and related data have been used to suggest a possible stepwise sequence for the reduction and protonation of N2 on a single molybdenum atom ia nitrogeaase (194). However, acidificatioa leads to complete destmctioa of the complex. Using both the stabilizing effect of the chelating phosphine triphos,... 

Other authors disagree with the results of this carbometalation sequence. They find that the initial product results stricdy from cis-carbometalation but with a trace of base, H is abstracted from the medium homolyticaHy with loss of stereochemistry (288). They show that M in the above stmcture must be Ti, not Al. If the reaction mixture is quenched with D2O containing NaOD, 95 mol % D is incorporated in the olefin. 

In a copolymer of 33% acrylonitrile, the most common composition for commercial products, the butadiene occurs in the approximate ratio of 90% trans, 8% vinyl, and 2% cis. At higher acrylonitrile content the cis configuration disappears, and at lower levels it increases to about 5% the vinyl configuration remains approximately constant (6,7). Since actual compositions of commercial nitrile mbbers are between 15 and 50% acrylonitrile, they also vary somewhat in sequence distribution and in the content of the three isomeric butadiene configurations. 

The trivial name bisnorisopenicillin has been coined for the 6-acylamino-3-thia-l-a2abicyclo[3.2.0]heptane-2-carboxylic acid (85) structural class. While not extensively investigated, an example of this class has been prepared as shown in Scheme 66 (78JMC413). The following features of this sequence are noteworthy (a) the exclusively cis cycloaddition to form intermediate (86) and (b) the use of the 2,4-dimethoxybenzyl amide blocking group. 

As a class the aliphatic polyalkenamers have low values due to a combination of low chain stiffness and low interchain attraction. The presence of double bonds has the effect of increasing the flexibility of adjacent single bonds (see Chapter 4) and overall this leads to a reduction in. Thus in the sequence from polydecenamer down to polypentenamer an increase in the double bond concentration leads to a lowering of Tg. On the other hand the Tg of polybutenamer, i.e. poly butadiene, is somewhat higher than that of polypentenamer, presumably because the proportion of stiff links, i.e. double bonds, becomes sufficiently high to override the flexibilising effect on adjacent chains. Consequently the polypentenamers have the lowest Tg values known for hydrocarbon polymers (cis- -114°C, trans- -97°C). 

This reaction sequence proceeds by cis addition of deuterium and the reduction products usually exhibit high isotopic purity. For example, 5a-cholest-2-ene (136), which is known to give a product of very unsatisfactory isotopic purity when deuterated with heterogeneous catalysts (see section V-A), gives 2<, 3 -d2-5a-cholestane (137) with better than 95% isotopic purity in homogeneous solution. ... 

This impressive reaction is catalyzed by stearoyl-CoA desaturase, a 53-kD enzyme containing a nonheme iron center. NADH and oxygen (Og) are required, as are two other proteins cytochrome 65 reductase (a 43-kD flavo-protein) and cytochrome 65 (16.7 kD). All three proteins are associated with the endoplasmic reticulum membrane. Cytochrome reductase transfers a pair of electrons from NADH through FAD to cytochrome (Figure 25.14). Oxidation of reduced cytochrome be, is coupled to reduction of nonheme Fe to Fe in the desaturase. The Fe accepts a pair of electrons (one at a time in a cycle) from cytochrome b and creates a cis double bond at the 9,10-posi-tion of the stearoyl-CoA substrate. Og is the terminal electron acceptor in this fatty acyl desaturation cycle. Note that two water molecules are made, which means that four electrons are transferred overall. Two of these come through the reaction sequence from NADH, and two come from the fatty acyl substrate that is being dehydrogenated. 

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