Chain sensing

Recent communications from Bristol-Myers Squibb [14,15] describe peptidomimetic inhibitors (Figure 7) that were designed to bind thrombin with an N- to C-polypeptide chain sense opposite that of the substrate and form interactions similar to those made by the first three residues of hirudin ( 1, Thr2, Tyr3). In the x-ray crystal structure of BMS-183507 (K = 17.2 nM) with thrombin [15], the N terminus is facing the catalytic site while the methyl ester is... 

Fig. 17.—Three Possible Arrangements of Chitin Chains. (The chain sense is indicated by arrows. (A) is for chitin 1 (B) and (C) show two possible ways of deriving chitin 2 from chitin 1 by a solid-state transformation. (B) shows up and down chains sliding into each other (C) shows chains folding upon themselves. ")...

With greater maturity, the company aligns the supply chain market to market to become market driven. In this stage, the supply chain senses buy- and sell-side market shifts and orchestrates tradeoffs horizontally in a bidirectionally coordinated response. This represents less than 1 percent of companies interviewed. 

In the early distinction by Carrothers (1) between condensation and addition polymers, polyesters were uniformly considered as examples of the former. However it was soon appreciated that a large family of polymers could be made which were functionally "polyesters" but did not fall into the synthesis based definition of condensation polymers. This family of polymers are usually prepared by ionic routes, generally a ring opening polymerization, and are structurally different from the condensation polyesters by having a chain sense i.e. the crystalline chain conformation possesses an intrinsic dipole moment. Accordingly, a structurally based definition of polyesters in terms of "polar and non-polar" crystalline conformations will be used in this report. The non-polar molecules considered will be aromatic polyesters and the polar molecules will be polyalkanoates. 

The Phantom Network Model. The theory of James and Guth, which has subsequently been termed the phantom network theory, was first outlined in two papers (186,187), followed by a mathematically more rigorous treatment (188-190). More recent work has been carried out by Duiser and Staverman (191), Eichinger (192), Graessley (193,194), Flory (82), Pearson (195), and Kloczkowski and co-workers (196,197). The most important physical feature is the occurrence of junction fluctuations, which occur asymmetrically in an elongated network in such a manner that the network chains sense less of a deformation than that imposed macroscopically. As a result, the modulus predicted in this theory is substantially less than that predicted in the affine theory. 

A basic theme throughout this book is that the long-chain character of polymers is what makes them different from their low molecular weight counterparts. Although this notion was implied in several aspects of the discussion of the shear dependence of viscosity, it never emerged explicitly as a variable to be investi-tated. It makes sense to us intuitively that longer chains should experience higher resistance to flow. Our next task is to examine this expectation quantitatively, first from an empirical viewpoint and then in terms of a model for molecular motion. 

The three-step mechanism for free-radical polymerization represented by reactions (6.A)-(6.C) does not tell the whole story. Another type of free-radical reaction, called chain transfer, may also occur. This is unfortunate in the sense that it complicates the neat picture presented until now. On the other hand, this additional reaction can be turned into an asset in actual polymer practice. One of the consequences of chain transfer reactions is a lowering of the kinetic chain length and hence the molecular weight of the polymer without necessarily affecting the rate of polymerization. 

A second class of monolayers based on van der Waal s interactions within the monolayer and chemisorption (in contrast with physisorption in the case of LB films) on a soHd substrate are self-assembled monolayers (SAMs). SAMs are well-ordered layers, one molecule thick, that form spontaneously by the reaction of molecules, typically substituted-alkyl chains, with the surface of soHd materials (193—195). A wide variety of SAM-based supramolecular stmctures have been generated and used as functional components of materials systems in a wide range of technological appHcations ranging from nanoHthography (196,197) to chemical sensing (198—201). 

QuaHtative arguments deal primarily with the sense of ownership and security which result from individually owned generation systems. Additional complexity will arise from the aesthetic criteria specific to both individual homes and the surrounding community. Of course, the autonomy inherent in distributed rooftop arrays probably constitutes an institutional barrier to their acceptance by some utiHty companies, unless they are involved in the financing and/or marketing chain. 

The success of the O2 sensor has made the auto manufacturers, regulators, and environmentalists anxious to extend chemical sensing to a variety of tailpipe gases, notably CO, NO, and short-chain hydrocarbons. Considerable research and development is needed for these molecules to be monitored in the hostile exhaust system environment (36). 

Crystallinity. Generally, spider dragline and silkworm cocoon silks are considered semicrystalline materials having amorphous flexible chains reinforced by strong stiff crystals (3). The orb web fibers are composite materials (qv) in the sense that they are composed of crystalline regions immersed in less crystalline regions, which have estimates of 30—50% crystallinity (3,16). Eadier studies by x-ray diffraction analysis indicated 62—65% crystallinity in cocoon silk fibroin from the silkworm, 50—63% in wild-type silkworm cocoons, and lesser amounts in spider silk (17). 

OC-Methylstyrene. This compound is not a styrenic monomer in the strict sense. The methyl substitution on the side chain, rather than the aromatic ring, moderates its reactivity in polymerization. It is used as a specialty monomer in ABS resins, coatings, polyester resins, and hot-melt adhesives. As a copolymer in ABS and polystyrene, it increases the heat-distortion resistance of the product. In coatings and resins, it moderates reaction rates and improves clarity. Physical properties of a-methylstyrene [98-83-9] are shown in Table 12. 

Many substituted, ie, branched, fatty acids, particularly methacryUc, 2-ethylhexanoic, and ricinoleic acids, are commercially significant. Several substituted fatty acids exist naturally (Table 5). Fatty acids with a methyl group in the penultimate position are called iso acids, and those with a methyl group in the antepenultimate position are called anteiso acids (1) (see Carboxylic acids, branched-CHAIN acids). However, the term iso is often used in a broader sense to mean branched or mixtures of branched-chain industrial acids. 

The first example is the plasma-borne retinol-binding protein, RBP, which is a single polypeptide chain of 182 amino acid residues. This protein is responsible for transporting the lipid alcohol vitamin A (retinol) from its storage site in the liver to the various vitamin-A-dependent tissues. It is a disposable package in the sense that each RBP molecule transports only a single retinol molecule and is then degraded. 

Subunits VP2 and VP3 from different pentamers alternate around the threefold symmetry axes like subunits B and C in the plant viruses (Figure 16.12b). Since VP2 and VP3 are quite different polypeptide chains, they cannot be related to each other by strict symmetry, or even by quasi-symmetry in the original sense of the word. To a first approximation, however, they are related by a quasi-sixfold symmetry axis, since the folded structures of the cores of the subunits are very similar. 

In the lightly cross-linked polymers (e.g. the vulcanised rubbers) the main purpose of cross-linking is to prevent the material deforming indefinitely under load. The chains can no longer slide past each other, and flow, in the usual sense of the word, is not possible without rupture of covalent bonds. Between the crosslinks, however, the molecular segments remain flexible. Thus under appropriate conditions of temperature the polymer mass may be rubbery or it may be rigid. It may also be capable of ciystallisation in both the unstressed and the stressed state. 

---