Dimerization crossed

RG-II has been isolated from potato cell walls (Ishii etal, 1999), but the detailed structure of its B side chain has not been determined. However, the monosaccharides that may be confined to RG-II have all been detected in potato cell walls (Stevenson et al., 1988 O Neill et al., 2004). In cell walls, including those of potatoes, RG-II occurs mostly as a dimer cross-linked by 1 2... 

Stability of the Cross-Linked Tropomyosin Dimer Cross-Link Effect on the Cooperatlvity of the Ordering Process and on the Maximum in the Helix Probability Profile" Mattice, W. L. Skolnick, J. Macromolecules 1982, IB, 1088. 

There are other processes besides the production of piperidine that occur during cathodic transformations of pyridine. Depending on conditions, dimers, crossed hydrocoupling products, or polymers can be formed these other products can also have a partially or fully reduced ring. 

Plants contain signiFcant concentrations of polysaccharides of which the potentially negatively charged oxygen functions can bind cations electrostatically or chelate them via polyhydroxy groups [89]. Particular attention was attracted by a structurally complex pectic polysaccharide rhamnogalacturonan-II (RG-II) [90]. This ubiquitous component of primary plant cell walls forms dimers cross-linked by 1 2 borate diol esters (dRG-II) that were found to complex in vitro sped be divalent cations and the majority of Ba, Pb, Sr, and rare earth elements (REEs) in fruit and vegetables [45, 91]. 

Intrinsic tyrosine kinases are covalently incorporated in the intracellular domains of some receptors, such as epidermal growdh factor receptor and the insulin receptor. When such receptor tyrosine kinases dimerize, cross-phosphorylation occurs. The phosphorylated tyrosines in activated receptor tyrosine kinases serve as docking sites for SH2 domains present in numerous signaling proteins and permit further propagation of the signal. A prominent component of such pathways is the small GTPase Ras. The Ras protein, like the G subunit, cycles between an inactive form bound to GDP... 

The design of such systems is based on the incorporation of photoaaive dye (chromophore) molecules into macromolecules of LC polymers, and these dye molecules undergo reversible or irreversible chemical transformations under the aaion of light, such as -Z (trans-cis) isomerization, cydization, dimerization, cross-linking, and polymerization reactions. 

Figure 9.46 shows an example of a fluorescence excitation spectmm of hydrogen bonded dimers of x-tetrazine (1,2,4,5-tetraazabenzene). The pressure of x-tetrazine seeded into helium carrier gas at 4 atm pressure was about 0.001 atm. Expansion was through a 100 pm diameter nozzle. A high-resolution (0.005 cm ) dye laser crossed the supersonic jet 5 mm downstream from the nozzle. 

Fig. 9. A de novo designed P sheet protein, betabellin, formed by the dimerization of two identical four-stranded -sheets and a disulfide linking the two sheets. This model is for betabeUins 9 and later progenies the earher betabeUins contained a two-armed cross-linker connecting the sheets (51).

The degradation of fibrin by plasmin is more complex because the fibrin molecule is a cross-linked polymer. Fragments released include D—D dimer or D2 (two cross-linked D fragments) with a molecular weight of 160,000 daltons, D2E, which is beHeved to be the principally released fragment in vivo, and... 

This procedure is representative of a new general method for the preparation of noncyclic acyloins by thiazol ium-catalyzed dimerization of aldehydes in the presence of weak bases (Table I). The advantages of this method over the classical reductive coupling of esters or the modern variation in which the intermediate enediolate is trapped by silylation, are the simplicity of the procedure, the inexpensive materials used, and the purity of the products obtained. For volatile aldehydes such as acetaldehyde and propionaldehyde the reaction Is conducted without solvent in a small, heated autoclave. With the exception of furoin the preparation of benzoins from aromatic aldehydes is best carried out with a different thiazolium catalyst bearing an N-methyl or N-ethyl substituent, instead of the N-benzyl group. Benzoins have usually been prepared by cyanide-catalyzed condensation of aromatic and heterocyclic aldehydes.Unsymnetrical acyloins may be obtained by thiazol1um-catalyzed cross-condensation of two different aldehydes. -1 The thiazolium ion-catalyzed cyclization of 1,5-dialdehydes to cyclic acyloins has been reported. 

The leucine zipper DNA-binding proteins, described in Chapter 10, are examples of globular proteins that use coiled coils to form both homo- and heterodimers. A variety of fibrous proteins also have heptad repeats in their sequences and use coiled coils to form oligomers, mainly dimers and trimers. Among these are myosin, fibrinogen, actin cross-linking proteins such as spectrin and dystrophin as well as the intermediate filament proteins keratin, vimentin, desmin, and neurofilament proteins. 

The structure of the dimer can be derived simply by evaluation of the cross signals in the HH COSY plot. The cycloalkene protons form two AB systems with such small shift differences that the cross signals lie within the contours of the diagonal signals. 

An intensely colored by-product of the photolysis reaction of methyl-2-azidobenzoate has been identified as the first known derivative of 3,3 -diazaheptafulvalene 70 (94LA1165). Its molecular mass was established by elemental analysis and mass spectroscopy as that of a formal nitrene dimer, whereas and NMR studies demonstrated the twofold symmetry as well as the existence of a cross-conjugated 14 7r-electron system in 70. Involving l-azido-2,3-dimethoxy-5,6-dimethoxycarbonylbenzene in thermal decomposition reactions, the azaheptafulvalene 71 could be isolated and characterized spectroscopically and by means of X-ray diffraction. Tliis unusual fulvalene can be regarded as a vinylogous derivative of azafulvalenes (96JHC1333) (Scheme 28). 

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