Optical activity cooperativity

Another significant cooperativity effect in preferential helical screw sense optically active copolymers is the majority rule phenomenon.18bl8q In this case, the screw sense of a helical main chain with unequal proportions of opposite chirality enantiopure chiral side groups is controlled by the enantiomeric excess only. Since this phenomenon was first reported from poly-a-olefins made of vinyl co-monomers bearing nonenantiopure chiral moieties by Green et al.8b and Pino et al.,16b this majority rule has been established in... 

It is considered that, if ideal, optically active poly(alkyl(aryl)silane) homopolymer and copolymer systems could be obtained which had stiffer main-chain structures with longer persistence lengths, it should be possible to clarify the relationship between the gabs value and the chiral molar composition. The magnitude of the chirality of the polyisocyanates allowed precise correlations with the cooperativity models.18q In the theory of the cooperative helical order in polyisocyanates, the polymers are characterized by the chiral order parameter M, which is the fraction of the main chain twisting in one helical sense minus the fraction of the main chain twisting in the opposing sense. This order parameter is equal to the optical activity normalized by the value for an entirely one-handed helical polymer. The theory predicts... 

Table 4.6 shows some experimental data on a-a dialkyl succinic acids. The most remarkable finding is that some of these molecules have a very large negative cooperativity, far beyond what could be explained by electrostatic theories. These molecules exist in two isomers—the meso and racemic forms. The latter exists in two optically active enantiomers that are mirror images of each other—only one of these is shown in Fig. 4.29. 

The difference in the ligands of each tertiary atom is due mainly to the different length of chain segments to which it is bonded. If we consider that, in the absence of cooperative factors, optical activity is a short-range phenomenon, the contribution to the rotatory power of most parts of the chain is zero or near to zero. One can thus foresee that the optical activity of a single enantiomer is very small and falls into the domain of cryptochirality. Only oligomers of low molecular weight can present measurable optical activity. 

The cooperativity of amplification, switching, and memory in synthetic helical polymers might thus be shared with ideas of a scenario for the biomolec-ular homochirality, autocatalytic mechanism in chiral chemical synthesis, and bifurcation equilibrium mechanisms in crystallization of chiral crystals. Indeed, amplification phenomena in several optical activity and helicity of synthetic polymers in isotropic solution appears to be common and are now established as sergeants and soldiers experiment and majority rules in polymer stereochemistry [17,18]. Any minute chiral forces caused by intramolecular and intermolecular systems can be detectable, when a proper model polymer system is chosen to elucidate the cooperativity of amplification, switching, and memory. 

Wilson G, Hecht L, Barron LD. Evidence for a new cooperative transition in native lysozyme from temperature-dependent Raman optical activity. J Phys Chem B 1997 101 694-698. 

The change in the optical activity of PGA, the dependence of bo vs. pH (Fig. 7, curve 4) is an indication of the helix-coil transition. These data show that the breakdown of the a-helix in PGA is accompanied by a threefold increase in the IMM of the chain occurring cooperatively at the same pH at which bo (the fraction of a-helical sequences of PGA) changes. 

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