Sequence isomerism

NMR can also provide clear evidence of sequence isomerism in polymers. Take, for example, poly(vinylidene fluoride), or 

FIGURE 7-24 UC NMR spectrum of a branched polyethylene in the region from 10-40 ppm (redrawn 

FIGURE 7-25 Part of a PVDF chain showing a segment inserted backwards.  

If you recall the stuff you shonld have learned in Chapter 2, then we don t have to 

Think about it a TH, for example, must be followed by another TH or an HT. Similarly, it must be preceded by a TH or an HT. It then follows that (Equation 7-21)  

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PVDF is just one polymer where sequence isomerism has been obseved using NMR spectroscopy. The 13C NMR spectrum of polychloroprene, for example, has also been analyzed by one of your authors (in collaboration with his coworkers) and the olefinic region is shown in Figure 7-28. It proved possible to assigu lines to triad sequence isomers of the trans-1,4 units (TH and HT) and also triad sequences containing the cis-lA... 

Now add the possibility of sequence isomerization ( backwards addition ) and the resulting high resolution NMR spectrum becomes a nightmare, as the sheer number of lines, many of which are unresolved and overlap, overwhelm our ability to analyze the data. In fact, it can sometimes be an advantage to limit the amount of information by degrading resolution or recording the NMR spectra on instruments with less powerful magnetic fields. 

Figure 5.7. Illustration of sequence isomerization exhibited by polymer chains. Shown are (a) head-to-tail and (b) head-to-head/tail-to-tail sequencing.

HPLC fmol Separation, profiling, sequencing Isomeric resolution, databases for peak assignment, amenable to online MS detection Moderate to low throughput, standards or complementary methods (i.e., exoglycosidase or MS) needed for unambiguous characterization... 

Homopolymers are made from the same monomer, but may differ by their microstructure, degree of polymerization or architecture. Examples of different microstructure of homopolymers such as tacticity, structural or sequence isomerisms were described in Section 1.2. Throughout this book we demonstrate that the degree of polymerization N tor the number of backbone bonds n) of macromolecules is a major ... 

Whereas the stereosequence distribution in isoregic and aregic PVF is nearly ideal random (Bernoullian with p(m) — 0.5), the latter has a first-order Markov regiosequence distribution. Accordingly the monomer sequence isomerism in PVF cannot be described by a single parameter such as the % defect, and requires two reactivity ratios for complete specification. 

Proteolytic hydrolysis of peptide bonds results in fragmentation of the protein chain. It is well established that in dilute acids, aspartic acid (Asp) residues in proteins are hydrolyzed at a rate at least 100-fold faster than that of other peptide bonds because of the mechanism of the reaction. An additional hydrolysis reaction is the deamidation of the neutral residue of asparagine (Asn) and glutamine (Gin) side-chain linkages, forming the ionizable carboxylic acid residues aspartic acid (Asp) and glutamic acid (Glu) (Fig. 6.26a). This conversion may be considered primary sequence isomerization. 

In addition, some irregularities of head-to-tail addition, leading to defect structures, can occur during polymerization for several reasons in polymers of the vinyHdene class, sequence isomerism may occur. The transmission band at 677 cm"i points to the presence of head-to-head and tail-to-tail configurations (B. Hilczer, 1998). Such defects are produced during the polymerization process and reduce the dipole moment of the aU-trans conformation. 

McLaughlin and colleagues have derivatized aminopropyl silica particles with alkyl and aryl residues in order to obtain hydrophobic anion-exchange supports suitable for mixed mode chromatography. These supports were successfully used for separation of sequence isomeric oligodeoxy nucleotides and tRNA fragments (15). 

Silica-based mixed mode column packing materials are not yet commercially available. At present it seems as if mixed mode chromatography is useful only for very specialized problems, for example separation of sequence isomeric oligonucleotides. 

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