Sequence lengths

GenBank [32] is a text-numeric database of genetic sequences with more than 28 billion bases in 22 million sequences (January, 2003) from genetic research. The collection of all publidy available sequences is annotated with information such as sequence description, source organism, sequence length, or references. The database, estabhshed in 1967, is updated daily and produced by the National Center for Biotechnology Information (USA). 

EMBL (European Molecular Biology Laboratory) [33] is a nucleotide sequence database provided from the online host EBl. Release 73 (December, 2002) consists of over 20 million nucleotide sequences with more than 28 billion nucleotides. The information includes sequence name, species, sequence length, promoter, taxonomy, and nucleic acid sequence. 

Figure 7.3 also shows that the proportion of alternating Mi units decreases and the fraction of longer sequences increases as rir2 increases. The 50 mol % entry in Table 7.5 shows that the distribution of sequence lengths gets flatter and broader for rir2 = 1, the random case. 

Table 7.5 also shows that increasing the percentage of Mj in the monomer solution flattens and broadens the distribution of sequence lengths. Similar results are observed for lower values of T1T2, but the broadening is less pronounced when the tendency toward alternation is high. 

Equations (7.40) and (7.41) suggest a second method, in addition to the copolymer composition equation, for the experimental determination of reactivity ratios. If the average sequence length can be determined for a feedstock of known composition, then rj and r2 can be evaluated. We shall return to this possibility in the next section. In anticipation of applying this idea, let us review the assumptions and limitation to which Eqs. (7.40) and (7.41) are subject ... 

Item (2) requires that each event in the addition process be independent of all others. We have consistently assumed this throughout this chapter, beginning with the copolymer composition equation. Until now we have said nothing about testing this assumption. Consideration of copolymer sequence lengths offers this possibility. 

As noted in Sec. 7.7, these different wavelengths correspond to absorbance by sequences of different lengths. Compare the appropriate absorbance ratios with the theoretical sequence length ratios calculated above and comment briefly on the results. 

From appropriate ratios of these sequence lengths, what conclusions can be drawn concerning terminal versus penultimate control of addition The following are experimental tacticity fractions of polymers prepared from different monomers and with various catalysts. On the basis of Fig. 7.9, decide whether these preparations are adequately described (remember to make some allowance for experimental error) by a single parameter p or whether some other type of statistical description is required ... 

It is basically a fractionation process that depends not only on molecular size, but also on chemical composition, stereo-configuration, branching, and crosslinking. For multicomponent systems, fractionation with different ion polymolecularity, chemical heterogeneity and sequence length distribution, solubility or elution fractionation is of primary importance. Therefore, gel permeation chromatography or size exclusion chromatography is used as an important tool for the characterization of PBAs. 

Figure 3.4 Packing of amides in unit cell (a) a-parallel structure of even-even PA (b) /3-antiparallel structure of even-even PAs with equal methylene sequence length in amine and acid unit, as in PA-4,620 (c) antiparallel PA-6-type polymer.

Using proton NMR of solutions, the composition of polymers can be analyzed.47 Carbon-13 NMR spectroscopy is a useful tool for studying the sequence length of segments in copolymers and thereby determining the blockiness of the copolymer. With solid-state NMR, the mobility of chain segments can be studied and the crystallinity determined. 

Sequence length distributions are occasionally important. They measure the occurrences of structures like YXY, YXXY, and YXXXY in a random copolymer. These can be calculated from the reactivity ratios and the polymer composition. See, for example. Ham. ... 

Crystallization of the same sequence length—such that, independent of comonomer content, a minimal sequence length always exists that is capable of crystallization at room temperature but this fraction decreases as comonomer content increases. 

Calculation of the N.A.S.L. (Number Average Sequence Length) can be performed, according to the definition given by Randall, given the model probabilities. Generation of the coefficients used in the equation required for this option, however, has not been automated. 

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