Rigid helix

Recently, a very interesting example of solvatochromism was reported by Fujiki and co-workers.206 Poly(methyl-3,3,3-trifluoropropylsilylene), 87, synthesized via Wurtz coupling, showed solvatochromism as a result of weak, non-covalent intramolecular Si- -F-G interactions which rendered the conformation of the polysilane uniquely controllable by solvent choice and molecular weight. UV, shown in Figure 18, photoluminescence, NMR, and viscosity studies on the polymer indicated a 73 helical rod-like conformation at room temperature in non-coordinating solvents (e.g., toluene and decane), since the intramolecular interaction resulted in constraining the chain in a rigid helix. 

The general shape of a macromolecule is sometimes referred to as macroconformation—in contrast to microconformation, which concerns the succession of torsion angles about a limited number of consecutive bonds. The random coil, rigid rod, and rigid helix represent examples of macroconformations. The same term is used to specify the molecular shape in semicrystalline polymers, one of the most important and still open problems in the field of polymer morphology. 

Nucleic acid polymers are known to undergo transitions between single strands and double and triple helices. These transitions are readily detected by NMR because of the large differences in the line widths of a flexible random coil compared to a rigid helix. A mixture of adenosine, a mononucleoside, and poly U, a large single-stranded polymer, is known to form a triple helix at low temperatures.6 The down-... 

Relative importance of rigid-helix and side-chain motions were studied by Corbin et al. [14]. The contribution of two types of rigid-body motions were approximated. In the first set of calculations helices of deoxymyoglobin were treated as rigid units while in the second set side-chains were considered to be... 

In Figure 1, we sketch the persistence length as a function of s for several values of E. For small values of the cooperativity parameter it is too costly to have very many domain walls and, thus, the system tends to a situation of a completely random coil or a rigid helix depending on whether s is smaller or larger than unity. 

In order to simplify the algebra, let us specialize to the case of sharp helix-coil transitions, i.e., E 0 as is common for the synthetic polypeptides. (The general situation will be discussed in a later publication.) For this limit, a given chain is either a flexible coil or a rigid helix, i.e., V = 0 or N (Fig. 

Helical polymer stmctures can be classified into three categories. The first is a rigid helix having a stable existence at room temperature. In this case, one may expect to obtain an optically active polymer with an excess of a screw sense through the polymerization process using a chiral initiator or catalyst. This kind of polymerization has been called helix-sense-selective polymerization (lUPAG nomenclature asymmetric... 

Price et a/. have reported C Ti and NOE measurements on narrow fractions of poly(y-benzyl-L-glutamate) in the helix-supporting solvent dimethyl formamide. The strong molecular weight dependence of Tx and was satisfactorily interpreted on the basis of anisotropic rotational diffusion of a rigid helix. 

The reaction of chitosan and PtCLt was studied as a function of reactant ratio, time, and pH. In general product yields were approximately 50%, based on a dichelated product and percentage of platinum found. Structure A represents the preferred structure containing two labile Pt l sites. Structure B also contains two labile sites in the cis position, but represents a crosslink or infernally cyclized structure, both giving a product with decreased solubility. The amount of nonadjacent same-chain chelation should be low since the chitosan chain is a fairly rigid helix in solution, Umiting the close association of different parts of the same chitosan chain with itself. 

---