Sequencing fractionation

Sulphide S Layer sequence Fractional site occupancy between successive S layers Reference... 

Probability for the formation of the intramolecular bond in the meso sequence. Fraction of OH groups associated with intramolecular hydrogen bonds. Unannealed sample prepared from the aqueous solution. 

NMR spectroscopy allows testing whether in a particular polymerization the propagation follows the Bernoulli, Markov, or enantiomorphic statistical form. Attempts are usually made to fit data for dyads, triads, tetrads, and higher sequence fractions to the equation for the different models. Spectral intensities can be associated with theoretical expression involving reaction probability parameters. Theoretical intensities are compared with the observed ones. This is optimized to obtain the best-fit values of reaction probability parameters and fully characterize the structure of the macromolecule. The fitting of data can be carried out with the aid of computers. 

FIGURE 20.3 Dyad monomer sequence fraction versus the ATRIF mol fraction in the poly(AN-co-ATRIF) copolymers [77]. 

Length of aromatic sequence Fraction of terephthalic acid monomers in the sequences of... 

Finally, it is not the usual situation to determine the average numbers of each type of unit in a copolymer chain. Instead it is the mole fractions that are obtained and the sequence fractions, [ABA -I- (ABB-l-BBA)] and/or [BAB + i(BAA-l-AAB)] as opposed to sequence numbers. The sequence number now becomes the number of sequences per 100 repeat units. For the above model copolymer chain, the sequence fraction equals 0.265 and the sequence number is 26.5 per hundred repeat units. If either of the A units or B units existed only in a single triad, BAB or ABA, then the sequence fraction and the mole fraction would be the same. For a perfectly alternating copolymer,... 

For example, the many deepwater fields located in the Gulf of Mexico are of Tertiary age and are comprised of complex sand bodies which were deposited in a deepwater turbidite sequence. The BP Prudhoe Bay sandstone reservoir in Alaska is of Triassic/ Cretaceous age and was deposited by a large shallow water fluvial-alluvial fan delta system. The Saudi Arabian Ghawar limestone reservoir is of Jurassic age and was deposited in a warm, shallow marine sea. Although these reservoirs were deposited in very different depositional environments they all contain producible accumulations of hydrocarbons, though the fraction of recoverable oil varies. In fact, these three fields are some of the largest in the world, containing over 12 billion barrels of oil each ... 

Most reactions in cells are carried out by enzymes [1], In many instances the rates of enzyme-catalysed reactions are enhanced by a factor of a million. A significantly large fraction of all known enzymes are proteins which are made from twenty naturally occurring amino acids. The amino acids are linked by peptide bonds to fonn polypeptide chains. The primary sequence of a protein specifies the linear order in which the amino acids are linked. To carry out the catalytic activity the linear sequence has to fold to a well defined tliree-dimensional (3D) stmcture. In cells only a relatively small fraction of proteins require assistance from chaperones (helper proteins) [2]. Even in the complicated cellular environment most proteins fold spontaneously upon synthesis. The detennination of the 3D folded stmcture from the one-dimensional primary sequence is the most popular protein folding problem. 

The sequence space of proteins is extremely dense. The number of possible protein sequences is 20. It is clear that even by the fastest combinatorial procedure only a very small fraction of such sequences could have been synthesized. Of course, not all of these sequences will encode protein stmctures which for functional purjDoses are constrained to have certain characteristics. A natural question that arises is how do viable protein stmctures emerge from the vast sea of sequence space The two physical features of folded stmctures are (l)in general native proteins are compact but not maximally so. (2) The dense interior of proteins is largely made up of hydrophobic residues and the hydrophilic residues are better accommodated on the surface. These characteristics give the folded stmctures a lower free energy in comparison to all other confonnations. 

The probability of B reacting is rp and the fraction of these reactions with Af molecules is rpp. The probability of the entire sequence is... 

Suppose we define as pjj the probability that a unit of type i is followed in the polymer by a unit of type j, where both i and j can be either 1 or 2. Since an i unit must be followed by either an i or a j, the fraction of ij sequences out of all possible sequences defines p ... 

Since L equals the total number of Mi sequences of any length, the fraction of sequences of length y 1,0, , is given by... 

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. 

Figure 7.3 Fraction of nj sequences of the indicated length for copolymers prepared from equimolar feedstocks with rj r2 = 0.03 (shaded) and T1T2 = 0.30 (unshaded). [Data from C. Tosi, Fo/ym. 5ci. 5 451 (1968).]...

Table 7.5 Fraction of Mj Sequences of Length nj for Copolymers Prepared from Feedstocks of Different Composition for a System with rjr2 = 1...

The fraction of isotactic sequences increases as increases, as required by the definition of these quantities. 

Use the dyad and triad fractions in Table 7.9 to calculate the average lengths of isotactic and syndiotactic sequences for the polymers of Fig. 7.10. Comment on the results. 

The fraction of sequences of the length indicated below have been measured t for a copolymer system at different feed ratios ... 

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 ... 

Formaldehyde homopolymer is composed exclusively of repeating oxymethylene units and is described by the term poly oxymethylene (POM) [9002-81-7]. Commercially significant copolymers, for example [95327-43-8] have a minor fraction (typically less than 5 mol %) of alkyUdene or other units, derived from cycHc ethers or cycHc formals, distributed along the polymer chain. The occasional break in the oxymethylene sequences has significant ramifications for polymer stabilization. 

The incidence of these defects is best determined by high resolution F nmr (111,112) infrared (113) and laser mass spectrometry (114) are alternative methods. Typical commercial polymers show 3—6 mol % defect content. Polymerization methods have a particularly strong effect on the sequence of these defects. In contrast to suspension polymerized PVDF, emulsion polymerized PVDF forms a higher fraction of head-to-head defects that are not followed by tail-to-tail addition (115,116). Crystallinity and other properties of PVDF or copolymers of VDF are influenced by these defect stmctures (117). 

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