Anion backbone

While often somewhat ignored by coordination chemists, the interaction of Group I ions has recently received increased attention, particularly with regard to duplex DNA. To a large extent these ions are involved in charge neutralization of the anionic backbone of (deoxy)-ribonucleotide polymers. However, more specific effects have been... 

Figure 5-3. Electrocatalytic nucleic acids detection based on the reduction of FefCNjg mediated by RufNHj) . The Ru(III) complex is electrostatically bound to the anionic backbone of an immobilized DNA oligonucleotide and upon the hybridization of a complementary target sequence is present at a higher concentration at the electrode surface. This increases the surface concentration of the redox mediator and leads to an enhanced electrochemical signal tliat can be used to detect specific DNA sequences. The presence of Fe(CN)6 chemically regenerates electrochemically reduced Rufll), and thereby catalytically amplifies the signals. (Adapted from ref. 11).

When two nucleic acid strands have complementary nucleic acid sequence, they can undergo hybridization to form double-stranded duplex structures. DNA forms a double-stranded helix composed of two complementary helical polynucleotide chains, aligned antiparallel, which are coiled around a common axis. In helix form, the anionic backbone lies on the outside of the structure with the nucleobases in the core, perpendicular to the axis and separated by a distance of 3.4 A. This B-form helix (Figure 2a) has a right-handed coil that repeats itself every 34 A with a turn every... 

Since suitable conducting polymers with anionic backbones are not available, we have resorted to a more complex approach for cation binding. Following Martin s workio on terpolymer redox films of polyvinylferrocene, we produced a new type of composite polymer, of cationic poly-(N-methylpyrrole) (PMP" ) with anionic poly-styrenesulfonate (PSS ). Upon reduction of a film of this material, cations are taken up. Thus, the entangled polymeric anion is not flushed out by reduction of the pyrrole units. Instead, cations are incorporated to balance the sulfonate charges. This has been shown for a variety of cations including protonated dopamine. The scheme below shows how this polymer works to bind protonated dimethyldopamine (DH ) cathodically and to release it anodically. 

Balomenou I, Bokias G (2005) Water-soluble complexes between cationic surfactants and comb-type copolymers consisting of an anionic backbone and hydrophilic nonionic poly(N, N-dimethylacrylamide) side chains. Langmuir 21 9038-9043. doi 10.1021/la0503505... 

The most effective and widely used dispersants are low molecular weight anionic polymers. Dispersion technology has advanced to the point at which polymers are designed for specific classes of foulants or for a broad spectmm of materials. Acrylate-based polymers are widely used as dispersants. They have advanced from simple homopolymers of acryflc acid to more advanced copolymers and terpolymers. The performance characteristics of the acrylate polymers are a function of their molecular weight and stmcture, along with the types of monomeric units incorporated into the polymer backbone. 

Internal surfactants, i.e., surfactants that are incorporated into the backbone of the polymer, are commonly used in PUD s. These surfactants can be augmented by external surfactants, especially anionic and nonionic surfactants, which are commonly used in emulsion polymerization. Great attention should be paid to the amount and type of surfactant used to stabilize urethane dispersions. Internal or external surfactants for one-component PUD s are usually added at the minimum levels needed to get good stability of the dispersion. Additional amounts beyond this minimum can cause problems with the end use of the PUD adhesive. At best, additional surfactant can cause moisture sensitivity problems with the PUD adhesive, due to the hydrophilic nature of the surfactant. Problems can be caused by excess (or the wrong type of) surfactants in the interphase region of the adhesive, affecting the ability to bond. 

The anionic moiety can substitute chlorines in PVC by an Sn mechanism [Eq. (31)]. The reaction can also take place by an Sn mechanism. This would involve the formation of a cationic center on the polymer backbone... 

Very few graft copolymers based on poly(arylene ether)s have been synthesized, probably because of their chemical inertness. Klapper et al. reported grafting the polystyrene or polyisoprene onto the poly(ether ether ketone ketone) (PEEKK) by anionic deactivation.229 The carbonyl groups on tire backbone can be attacked by the polystyrene monoanion or polyisoprene anion (Mn about 3000). Due to the steric hindrance only about 30% of tire carbonyl groups can be reacted. 

Two types of well defined branched polymers are acessible anionically star-shaped polymers and comb-like polymers87 88). Such macromolecules are used to investigate the effect of branching on the properties, 4n solution as well as in the the bulk. Starshaped macromolecules contain a known number of identical chains which are linked at one end to a central nodule. The size of the latter should be small with respect to the overall molecular dimensions. Comb-like polymers comprise a linear backbone of given length fitted with a known number of randomly distributed branches of well defined size. They are similar to graft copolymers, except that backbone and branches are of identical chemical nature and do not exhibit repulsions. 

Anionic grafting methods (vide infra) can be applied to the synthesis of comb-shaped polymers. As an example, a polystyrene backbone is partially chloromethylated (under mild conditions) and used as an electrophilic deactivator for a living polystyrene 89). The grafting onto process yields well defined species that have been characterized accurately. The branches are distributed randomly along the backbone 90). 

However, upon dissolution, an epimerization of the anions can occur in the presence of acidic counter-ions. This is particularly true for 16a-16d [39]. The nature of the solvent (MeOH, CHCI3) plays a crucial role on the kinetics of epimerization and the position of the resulting equilibrium. For anions made with a 2R, 3R) tartaric backbone, a A configuration is always preferred in MeOH the selectivity, obtained after a slow equilibration, being independent of the nature of the ester alkyl chain (diastereomeric ratio (d.r.) 3 1). However, in chloroform, the A diastereomer is rapidly obtained and the selectivity is best if the ester side chain is sterically demanding (d.r. 2 1 to 9 1 from 16a to 16d) (Scheme 16). 

Ligand 92 was readily prepared by reaction of (+)-pinocarvone with 1-phenacylpyridinium iodide. The authors similarly prepared corresponding 5,6-dihydro-1,10-phenanthrolines derived from (+)-pinocarvone and a tetrahydroquinolone (structure 93, [127]) and obtained up to 81% in the palladium-catalyzed test reaction. Chelucci et al. [ 128] reported the synthesis of chiral Ci-symmetric 1,10-phenanthrolines incorporated in asteroid backbone. Structure 94 derived from 5o -cholestan-4-one in Scheme 51, allowed very high yield and up to 96% ee using BSA and tetrabutylammonium fluoride to generate the malonate anion. 

---