Parallel dimers

Subsequent analysis of the polypeptides contained in the three complexes excised from the mildly-denaturing deoxycholate gel confirm these observations. In this system, the oligomer contains three polypeptides in the 25-27 kDa range (Fig. 3). The monomer contains a 27 kDa polypeptide, the apoprotein of CP 29, and only a trace of a 25 kDa species. The dimer contains these polypeptides as well as a number of contaminating polypeptides of both higher and lower MW. It is interesting to note that in chilled thylakoids, the 31 kDa polypeptide is present in both the monomer and dimer, paralleling the distribution of the related CP 29 apoprotein (Fig. 3). 

Single-Stack Acceptor. Simple charge-transfer salts formed from the planar acceptor TCNQ have a stacked arrangement with the TCNQ units facing each other (intermolecular distances of ca 0.3 nm (- 3). Complex salts of TCNQ such as TEA(TCNQ)2 consist of stacks of parallel TCNQ molecules, with cation sites between the stacks (17). The interatomic distance between TCNQ units is not always uniform in these salts, and formation of TCNQ dimers (as in TEA(TCNQ)2) and trimers (as in Cs2(TCNQ)Q can lead to complex crystal stmctures for the chainlike salts. 

The two peptides form a symmetrical dimer stabilized by four hydrogen bonds (red dashes) and hydrophobic contacts. The two monomers form a four-stranded, anti-parallel pleated sheet. 

The reactions of A -steroids with nitrosyl fluoride parallel those of their A -isomers. Thus, 17 -acetoxyandrost-4-ene (37) is converted to the nitrimine (38), in 67 % yield and thence to the 4-ketone (39), which can be dehydrofluorinated to the A -4-ketone (40) with lithium bromide in di-methylformamide. In the corresponding 19-nor series the nitroso dimer is also formed. 

The accessibility of the +4 and +6 oxidation states for sulfur and, to a lesser extent, selenium gives rise to both acyclic and cyclic molecules that have no parallels in N-O chemistry. Thus there is an extensive chemistry of chalcogen diimides RN=E=NR (E = S, Se, Te) (Section 10.4). In the case of Te these unsaturated molecules form dimeric structures reflecting the increasing reluctance for the heavier chalcogens to form multiple bonds to nitrogen. The acyclic molecule N=Sp3,... 

FIGURE 10.41 (a) Gramicidin forms a double helix in organic solvents a helical dimer is the preferred strnctnre in lipid bilayers. The strnctnre is a head-to-head, left-handed helix, with the carboxy-termini of the two monomers at the ends of the strnctnre. (b) The hydrogen-bonding pattern resembles that of a parallel /3-sheet. 

The dithionite ion has a remarkable eclipsed stmcture of approximate C2v symmetry (Fig. 15.32b). The extraordinarily long S-S distance (239 pm) and the almost parallel SO2 planes (dihedral angle 30°) are other unusual features. Electron-spin-resonance studies have shown the presence of the S02 radical ion in solution ( 300ppm), suggesting the establishment of a monomer-dimer equilibrium 8204 ... 

The dimerization of skatole proceeds in an entirely analogous manner, cation (44) now being the electrophilic reagent. This is sufficiently reactive to effect substitution at the a-position of a neutral skatole molecule. Attack by the less hindered side of cation (44) will be favored, leading to the stereochemistry shown in structure (30). The failure of 2-methylindole to dimerize is paralleled by the failure of 2-methylpyrrole dimer to react with a further molecule of 2-methylpyrrole. The main reason is almost certainly again the reduction in the electrophilic character of the immonium carbon by... 

The ratio of the ionic liquid to the organic phase present in the reactor also plays an important role. A too high level of ionic liquid results in much longer decantation time and causes lower dimer selectivity. To combine efficient decantation and a reasonable size for the settler in the process design, it has been proposed that the separation of the two phases be performed in two distinct settling zones arranged in parallel [38]. 

Table 9.2b). Increasing also is the list of phenotypes associated with these dimerization processes. With the emergence of receptor dimers as possible therapeutic targets have come parallel ideas with dimerized ligands (see Section 9.5). 

Figure 4-11. INDQ/SCI-caleulalcd evolution of the transition energies (upper pan) and related intensities (bottom pan) of the lowest two optical transitions of a cofacial dimer formed by two stilbenc molecules separated by 4 A as a function of the dihedral angle between the long molecular axes, when rotating one molecule around the stacking axis and keeping the molecular planes parallel (case IV of Figure 4-10). Open squares (dosed circles) correspond to the S(J - S2 (S0 — S, > transition.

Figure 4-10. Sketch of Hie operations applied ui a colacial dimer formed by two slilbene molecules separated by 4 A when investigating the role of positional disorder. The modilicalions are induced by (I) the translation of one molecule along the chain-axis direction (II) the translation of one molecule along the in-planc transverse axis (III) the rotation of one slilbene unit around its long axis and (IV) the rotation of one slilbene molecule around the slacking axis while keeping the parallelism between the molecular planes.

Cholinesterase. Figure 1 Shown are the seven subgroups of ChEs Molecular forms Top, in green monomeric, dimeric and tetrameric AChE-S forms (G1, G2, G4 and PRiMA). Middle, in green ColQtailed AChE-S forms (A4, A8 and A12). Parallel forms exist for BChE. Down other AChE splicing variants (AChE-R, AChE-E, N-AChE). 

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