Molecular uniformity

The molecular uniformity of constituting components of a nb/lcb glucan fraction of potato starch was investigated with Sepharose CL 2B (Fig. 16.16) as well as with Sephacryl S-1000 (Fig. 16.17). Therefore, each of the subsequently eluted 3-ml fractions was analyzed on their potential to form inclusion complexes with iodine, a sensitive test for the presence of nb/lcb glucans. Results are shown in Fig. 16.17 in terms of branching index, the ratio of extinction of pure iodine solution and of nb/lcb glucan/iodine complex the higher the index, the more pronounced the nb/lcb characteristics. 

The fundamental difficulty is that polymeric substances cannot be obtained in a structurally and molecularly uniform state, unlike low-molecular-weight compounds. Thus, macromolecular materials of the same analytical composition may differ not only in their structure and configuration (see Sect. 1.2) but also in molecular size and molecular weight distribution they are polydisperse, i.e., they consist of mixtures of molecules of different size. Hence, it is understandable that the expression identical is not, in practice, applicable to macromolecules. Up to the present time, there is no possibility of preparing macromolecules of absolutely uniform structure and size. It follows, therefore, that physical measurements on polymers can only yield average values. The afore-... 

The idea that antibodies also can stabilize the transition state of a reaction, that they feature a sterically or electronically complementary active center to the ratedetermining transition state just like enzymes, has existed since 1969. This concept of catalytic antibodies could be investigated only after the advent of monoclonal antibodies. The capability to raise molecularly uniform antibodies instead of polyclonal sera... 

Nonionic detergents might well be characterized as being heterogeneous material, particularly if the hydrophilic moiety is based on the addition of ethylene oxide11). Molecular uniformity is, therefore, not easy to obtain with these compounds. Due to the possibility of a delicate variation of the so-called hydrophile-lipophile balance212, these systems prompted considerable industrial interest. 

All living cells contain ribosomal RNAs, and it seems that these nucleic acids have changed very little in the course of evolution because their structures are similar in all organisms. Despite this enormous molecular uniformity, however, all species are slightly different in their ribosomal RNAs, and in 1977 Carl Woese showed that these little differences give us precious information on the very first stages of cellular evolution. 

In all cases of chirality within the polymer chain, however, only very small to nondetectable optical rotations can be expected, even for the unrealistic assumption of a molecularly uniform and optically pure sample of enantiomers since, especially in high molecular weight polymers,... 

This equation shows that the critical temperature TJ) is also dependent on the molecular weight of the polymer [82]. Thus, for a UCST diagram this critical parameter moves toward higher temperatures as the chain length increases (Fig. 25.5). This dependence is particularly useful for the fractionation of most synthetic polymers, which are seldom molecularly uniform [83]. 

The resulting poly(acrylic acids) are molecularly uniform. Nothing is known about their configuration—which is fixed precisely in biopolymers by the matrix mechanism. In addition, among the synthetic polymers, only unipolymers, have so far been constructed in such a way with a definite sequence such as exists in biopolymers. 

It is interesting and useful to extend these results to a three-phase system of a solid in contact with a liquid and a vapour (again, of negligible density). The solid is of different chemical constitution from the liquid, and quite insoluble in it. We assume moreover, that it is a perfectly rigid molecularly uniform array of density pa. The intermolecular potential between two molecules of the species in the liquid is denoted Un, between two of those in the solid U22, and that between a molecule of each U 2- By the argument of the last section, the force per unit area between a slab of liquid and one of solid at separation I is... 

Figure 2 Schematic phase diagram for solutions of a molecularly uniform polymer. Polymer-lean phase (sol) A, stable B, metastable and C, unstable, segregation of a gel phase. Polymer-rich phase (gel) D, stable E, metastable and F, unstable, segregation of a sol phase. Reproduced from Derham, K. W. Goldsbro, J. Gordon, M. PureAppl. Chem. 1974, 38, 97. International Union of Pure and Applied Chemistry.

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