Dimerization combined

At the lowest reaction temperatures, tetrahedra rather than P2 dimers may be produced, or as the gas cools, dimers combine as ... 

Fig. 62. A schematic drawing of the backbone of the prealbumin dimer, viewed down the 2-fold axis. Arrows represent p strands. Two of these dimers combine back-to-back to form the tetramer molecule.

Figure 8.10 Dependence of the Laplacian values, V pc (an), on the H- -H distances (A) calculated for dihydrogen-bonded complexes formed by the hydrides LIH, NaH, BeH2, and MgH2 with the proton donors HCN, HNC, and HCCH, including monomer-dimer and dimer-dimer combinations of proton donors and proton acceptors. (Reproduced with permission from ref. 17.)...

Analogous to heterotypical dimerization of transcription factors (see Chapter 1.2), heterologous dimerization is observed within different members of a receptor family. A certain growth hormone can thus bind to and activate different dimeric combinations of the members of a receptor family. Fig. 8.5. shows the possibilities for heterodimerization of receptors, using the PDGF receptor as an example (review Lemmon and Schlessinger, 1994). 

The enzyme consists of three main isozymes formed by the dimeric combination of two different protein chains. The two types of protein chains have been labelled E (for ethanol active) and S (for steroid active). About 90% of liver alcohol dehydrogenase (LADH)) is EE and the remaining 10% consists of ES and SS. SS is also ethanol active, although lower than ES and EE, and vice versa. Polymeric forms of the isozymes are also known. 

In Figure 3 we show a typical deflection pattern for one of the alkali halide-dimer combinations (RbCl). 

Horse liver alcohol dehydrogenase, HLADH, (also abbreviated as ADH or LADH) is the most extensively studied oxido-reductase. It plays a central role in ethanol metabolism and has been one of the main tools for understanding the mechanism of this process.15 it was crystaliized from horse liver in 1948 by Bonnichen and Wassen and is commercially available. Three isozymes EE, ES and SS are formed by dimeric combination of two different, E or S (E "ethanol-active" and S "steroid active"), protein chains. 16 The EE- isozyme of HLADH has been used in organic synthesis. 

Many species that violate the octet rule are quite reactive. Eor instance, compounds containing atoms with only four valence shell electrons (limitation type A above) or six valence shell electrons (limitation type B above) frequently react with other species that supply electron pairs. Compounds such as these that accept a share in a pair of electrons are called Lewis acids a Lewis base is a species that makes available a share in a pair of electrons. (This kind of behavior will be discussed in detail in Section 10-10.) Molecules with an odd number of electrons often dimerize (combine in pairs) to give products that do satisfy the octet rule. Examples are the dimerization of NO to form N2O2 (Section 24-15) and NO2 to form N2O4 (Section 24-15). Examples 7-5 through 7-9 illustrate some limitations and show how such Lewis formulas are constructed. 

Fig. 10.28. Formation of a cytokeratin filament. The central rod of the keratin monomer is principally a-helical structure. A specific acidic keratin monomer combines with a specific basic keratin monomer to form a heterodimer coil (a coiled coil structure). Two dimers combine in antiparallel fashion to form a tetramer, and the tetramers combine head-to-tail to form pro to filaments. Approximately eight protofilaments combine to form a filament. The filament is thicker than actin filaments (called thin filaments or micro filaments) and thinner than microtubules (thick tubes) and is therefore called an intermediate filament.

It is now clear that at the functional level, the heterotrimeric G-proteins behave as if they were a dimeric combination of an a-subunit and an inseparable Py pair. Py-Subunits have the following functions ... 

Costas et al. have reported spectroscopic evidence for an Fe Fe complex that can be considered a structural model for the putative Fe Fe (/x-0)2 core of methane monooxygenase intermediate The synthetic complex was prepared at —80 °C in CH2CI2 by decay of a mononuclear low-spin Fe peroxo precursor. The Mossbauer spectra showed that all iron in the sample is intermediate spin (5 = 1) Fe, but the data were compatible with either a mononuclear site or a weakly coupled ( J <5cm ) symmetric dimer. Combination of the Mossbauer technique with resonance Raman and EXAFS spectroscopies provided evidence for a bis-/x-oxo bridged diiron(IV) complex. The complex of Costas et al however, is not an electronic model for intermediate Q, as the latter contains high-spin Fe sites. 

OH stretch of alcohol groups OH stretch 1 carboxylic acid group overtones and > hydrogen bonded dimer combinations J... 

Step polymerization indicates a mechanism of growth where monomers combine with each other to form dimers, the dimers combine with each other or other monomer units to form tetramers or trimers, respectively, the process continuing until polymer is formed. While each coupling step in a step polymerization is often accompanied by the elimination of a small molecule (e.g., water), making it a condensation polymerization, this is not always the case (e.g., isocyanates and alcohols reacting to make polyurethanes). Furthermore, not all polymerizations in which a condensate is formed follow a stepwise mechanism. 

Fig. 3.8. The role of cAMP as an intermediate to environmental stimulus (hormone secretion) and the onset of physiological response. Adrenaline and GTP bind to a specific membrane bound regulatory subunit of adenyl cyclase. This elicits a conformational change such as to activate the catalytic subunit and stimulate cAMP synthesis. cAMP activates a protein kinase by causing its dissociation into an active catalytic subunit (R = regulatory subunit C = catalytic subunit). This in turn catalyzes the phosphorylation of two serine residues of an inactive phosphorylase b enzyme. Two dimers combine to form an active tetramer phosphorylase a. Phosphorylase catalyzes the breakdown of the sugar storage form glycogen (phosphorolysis) to glucose 1-...

According to Stogryn Hirschfelder (1959) there are two additional terms to be taken into account, one from expansion of the Eucken type correction for polyatomic molecules and a second one due to the actual formation of dimers. Combining all these contributions gives estimates for B j, which are included in Table 5.1. 

Four different dimeric and three different tetrameric structures have been described to date. In the most common dimer the back sheets of the lectin monomers associate to form a continuous 12-stranded p-sheet along the length of the dimer (Figure 2A). This dimer, called the canonical legume lectin dimer has been found in most legume lectins studied [26]. In some lectins these dimers combine to form tetramers of which there are two types. In the case of concanavalin A, the dimers interface at the central parts of their back sheets to form a structure as shown in Figure 2B. In the case of such lectins as PHA-L, the soybean agglutinin and the D. biflorus seed lectin, the dimers interface at the outermost strands of the 12-stranded P-sheets to... 

Kabelac and Hobza carried out a comprehensive study [69] for exploring the most stable A, C, G, T, and U mono- and dihydrate structures and their relative energies. Furthermore, these authors studied all kinds of dimeric combinations of the A, C, G, T bases in their hydrogen-bonded, stacked, and T-shape forms. The canonical forms for C, T, and U are prevalent in the gas phase and are maintained in a water environment. Rare G and second and third most stable A tautomers in the gas phase appear in low-energy hydrates. Increasing number of water molecules reduce the energy separation of the most stable hydrogen-bonded and stacked forms. Methylation leads to the preferential formation of the stacked dimeric forms in many cases. 

---