Poly radical, conformation

Short segments of poly(dG—dC) incorporated within plasmids, or citcular DNA, adopt the Z-conformation under negative superhehcal stress. This left-handed DNA may be important in genetic control. On the other hand, the stmctural alteration of the helix requited in a B-to-Z transition within a plasmid is radical, and would involve either a multistep mechanism or the complete melting and reformation of helix. The improbability of such transitions has led to questions concerning the feasibility of a biological role for Z-DNA. 

For instance, poly-p-phenylenes in their doped states manifest high electric conductivity (Shacklette et al. 1980). Banerjee et al. (2007) isolated the hexachloroantimonate of 4" -di(tert-butyl)-p-quaterphenyl cation-radical and studied its x-ray crystal structure. In this cation-radical, 0.8 part of spin density falls to the share of the two central phenyl rings, whereas the two terminal phenyl rings bear only 0.2 part of spin density. Consequently, there is some quinoidal stabilization of the cationic charge or polaron, which is responsible for the high conductivity. As it follows from the theoretical consideration by Bredas et al. (1982), the electronic structure of a lithium-doped quaterphenyl anion-radical also differs in a similar quinoidal distortion. With respect to conformational transition, this means less freedom for rotation of the rings in the ion-radicals of quaterphenyl. This effect was also observed for poly-p-phenylene cation-radical (Sun et al. 2007) and anion-radical of quaterphenyl p-quinone whose C—O bonds were screened by o,o-tert-hutyl groups (Nelsen et al. 2007). 

Mroczka N, Bernhard WA. (1995) Electron paramagnetic resonance investigation of X-irradiated poly(U), poly(A) and poly(A) poly(U) Influence of hydration, packing and conformation on radical yield at 4 K. Radiat Res 144 251-257. 

Keywords silyl radicals, poly silanes, back-folded conformation, dissociation energy of Si-Si bonds... 

Free-radical and anionic polymerizations of TAD-DOL—MA (30) proceed exclusively via a cyclization mechanism, and the obtained polymer seems to have a helical conformation with an excess helicity.92-94 The main chain structure of poly(TADDOL—MA) with cyclized units (poly-30) is different from that of all other polymethacrylates discussed here. Similar monomers have been synthesized and polymerized.95... 

A poly(7-oxabicyclo[2.2.1]hept-2-ene) derivative (polymerization using RUCI3), 138, and a poly(/V-phenylmaleimide) derivative (radical polymerization) bearing phenyl groups having long alkyl chains form a hexagonal columnar liquid crystalline phase.240 The polymers are proposed to take a helical conformation that may be stabilized by the intra- and intermo-lecular interaction of the side chains. 

How adsorption of the side chains to a flat substrate effects the backbone conformation has been observed in further microscopic detail for brush molecules with a methacrylate backbone and poly-(n-butyl acrylate) side chains. These poly(/v-butyl acrylate) brushes were prepared by living radical grafting from a multifunctional macromolecular initiator.38 The synthetic approach allowed observation of the same batch of molecules without (macroinitiator) and with poly(n-butyl acrylate) side chains (brush). 

Keywords acrylate- and methacrylate guanidines radical polymerization poly (meth) acrylate guanidines, conformational behavior, micro-heterogeneity... 

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