Clearing dimers

Figure Bl.17.11. Reconstructed density of an a,p-tiibulin protein dimer as obtained from electron crystallography (Nogales etal 1997). Note the appearance of the p-sheets ((a), marked B) and the a-helices ((b), marked H) in the density. In particular the right-handed a-helix H6 is very clear. Pictures by courtesy of E Nogales and Academic Press.

Zirconium and other nonmultivalent metal ions can act as promoters in some manner not currently clearly defined. For zirconium, the effect may be related to changes in monomer—dimer equitibria of Co(II) complexes (2,204,205). 

Extrapolation from the known reactivity of cyclobutadiene would suggest that azetes should be highly reactive towards dimerization and as dienes and dienophiles in cycloaddition reactions and the presence of a polar C=N should impart additional reactivity towards attack by nucleophiles. Isolation of formal dimers of azetes has been claimed as evidence for the intermediacy of such species, but no clear reports of their interception in inter-molecular cycloaddition reactions or by nucleophiles have yet appeared. 

A gene encoding this sequence was synthesized and the corresponding protein, called Janus, was expressed, purified, and characterized. The atomic structure of this protein has not been determined at the time of writing but circular dichroic and NMR spectra show very clear differences from B1 and equally clear similarities to Rop. The protein is a dimer in solution like Rop and thermodynamic data indicate that it is a stably folded protein and not a molten globule fold like several other designed proteins. 

The a-s-a and sp-s-sp cuts of the density profiles (Figs. 9(c) and 9(d)) clearly demonstrate that for a highly dimerized fluid the nonassociatively adsorbed dimers have a tendency to orient perpendicularly or slightly tilted... 

It was observed occasionally that the DTDAF 51a can revert back to the stable carbene so that the dimerization appears to be reversible (97LAR365). Tills splitting reaction could not always be clearly reproduced and seems to be influenced by trace contaminants that are as yet unidentified. 

Indole itself forms a dimer or a trimer, depending on experimental conditions the dimer hydrochloride is formed in aprotic solvents with dry HCl, whereas aqueous media lead to dimer or trimer, or both. It was Schmitz-DuMont and his collaborators who beautifully cleared up the experimental confusion and discovered the simple fact that in aqueous acid the composition of the product is dictated by the relative solubilities of the dimer and trimer hydrochlorides/ -This, of course, established the very important point that there is an equilibrium in solution among indole, the dimer, the trimer, and their salts. It was furthermore demonstrated that the polymerization mechanism involves acid catalysis and that in dilute solution the rate of reaction is dependent on the concentration of acid. 

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