Repetitive fractionation

Figure 2. Schematic illustration of a C t curve. After dissociating a eukaryotic genome, the highly repetitive sequences reanneal very quickly and are represented in the left-most hump. Sequences of moderate repetition are represented in the next hump, while "unique" sequences reanneal slowest and are in the final fraction on the right. The actual proportion of the genome in each fraction varies considerably, but, with some exceptions, the repetitive fraction is seldom greater than 50%.

A few DNA-DNA hybridization experiments have been performed with the repetitive fraction as defined by C t curves (Caccone, unpublished ... 

The bimolecular couphng approach was the first to generate a high-molecular-weight cychc polymer to initiate studies on their physical properties, although on frequent occasions it was necessary to remove linear impurities via repetitive fractionation. While the traditional bimolecular approach has inherent disadvantages related to the requirement for exact stoichiometries and susceptibility to intermolecular side reactions, if measures are taken to overcome... 

In Ciona intestinalis (the sea squirt), which has a genome size comparable to that of Drosophila melanogaster, 30 percent of the DNA renatures with a greater than is expected on the basis of genome size assuming each nucleotide sequence is present only once (Laird, 1971) i.e., 30 percent of the DNA apparently contains repeated sequences. The information that would allow estimates of the average repetition frequency is not available. In the sea urchin the repetitive fraction, which comprises 20 percent of the total genome, consists of sequences repeated 5,000 times (Britten and Kohne, 1968 Britten, 1969). 

Prep HPLC for E = 10-14, repetitive fractionation on Shandon Hyperprep HS silica, 4,6 X 250 mm for higher homologs repetitive separation first on Merck silica, 8 x 30 cm, then on Shandon Hyperprep HS silica,... 

The classical methods used to separate the lanthanides from aqueous solutions depended on (i) differences in basicity, the less-basic hydroxides of the heavy lanthanides precipitating before those of the lighter ones on gradual addition of alkali (ii) differences in solubility of salts such as oxalates, double sulfates, and double nitrates and (iii) conversion, if possible, to an oxidation state other than -1-3, e g. Ce(IV), Eu(II). This latter process provided the cleanest method but was only occasionally applicable. Methods (i) and (ii) required much repetition to be effective, and fractional recrystallizations were sometimes repeated thousands of times. (In 1911 the American C. James performed 15 000 recrystallizations in order to obtain pure thulium bromate). 

Complex ( + )-(FeS)-3 is obtained by extraction of the diastereomeric mixture with 50 mL of pentane in portions at r.t. The specific rotation of the solid product obtained from the first fraction is [a]jJ6 +50 (o = 0.001. benzene). The rotation of the remaining mixture becomes more negative as the (+)-isomer is removed. After concentration of the combined fractions, the resulting material, enriched in the ( + )-isomcr, is further purified by extraction with 10 mL of pentane in portions. Repetition of the pentane extraction procedure affords ( + )-(FeS)-3 yield-. 750 mg (30%) mp 120"C (dee) [a] +72 (c = 0.001, benzene). The residue, enriched in (—)-(FeR)-3 is extracted for 1 h with boiling pentane. After separation from the extraction liquor, a yellow-orange solid is obtained yield I -35 g (50%) mp 120°C (dec) — 120... 

The second analytical method uses a combustion system (O Neil et al. 1994) in place of reaction with BrF,. This method was used for the crocodiles because they were represented by very thin caps of enamel. The enamel was powdered and sieved (20 mg), pretreated in NaOCl to oxidize organic material and then precipitated as silver phosphate. Approximately 10-20 mg of silver phosphate were mixed with powdered graphite in quartz tubes, evacuated and sealed. Combustion at 1,200°C was followed by rapid cooling in water which prevents isotopic fractionation between the CO2 produced and the residual solid in the tube. Analyses of separate aliquots from the same sample typically showed precisions of 0.1%o to 0.4%o with 2 to 4 repetitive analyses even though yields are on the order of 25%. 

If NMR measurements are intended, the eluting betalain fractions should be very carefully concentrated under vacuum by repetitive addition of purified water or directly lyophihzed before frozen storage. - For other purposes, such as for model studies, betalain stabihzation may be achieved by continuous pH control and addition of aqueous ammonia prior to concentration. ... 

The negative sign was introduced in the definition to avoid repetitive use of the sign which would have otherwise been inevitable, as most solutions handled have fractional values for the hydrogen ion concentration. 

Indeed, a bDNA assay for diagnosis of African trypanosomiasis was developed and compared with buffy coat microscopy for detection of T brucei in human blood samples (Harris etal., 1996). Two repetitive DNA sequences found only in the T. brucei complex, a 177-bp satellite repeat and the ribosomal mobile element, were selected as targets in the bDNA assay. The assay used the standard bDNA components capture probes, target probes, amplifier molecules, and alkaline phosphatase-labeled probes. Various blood fractions and sample preparation methods were examined. Ultimately, buffy coat samples resulted in the highest sensitivity. Although typanosomes do not infect leukocytes, they cosediment with them. 

Proton NMR and deuteron NMR spectra of soluble fractions and spent solvent mixtures were obtained by using a JE0L FX60Q FT NMR Spectrometer. A flip angle of 45° was used which corresponds to 14 ms for and 75 ms for 2H. The pulse repetition times were 6.0 and 9.0 s, respectively. Chloroform-d was used as the NMR solvent, and chloroform was used as the 2H NMR solvent. 

In several runs it was noted that, despite the acid extraction, small amounts of N.N-did-butylhydroxylamine crystallized in the cooler parts of the fractionating column head. In such cases, repetition of the acid extraction procedure is required before fractionation. 

Figure 3.8. Crystal structure of CsCl. The positions of the centres of the atoms in the unit cell are shown in (a). In (b) the same cell is described by means of its characteristic sections taken at the height 0, A, and 1 of the third axis. In (c) a projection of the cell on its square basis is presented the values of the third (fractional) coordinate are indicated. In (d) the shortest interatomic distances are shown dCs-ci = a)3/2 = 411.3 X 0.866025. = 356.2. In (e) the subsequent group of interatomic distances (d = a = 411.3) involving six atoms in the adjacent cells is presented. A group of eight cells is represented in (f) to suggest that the actual structure of CsCl corresponds to a three-dimensional infinite repetition of unit cells and to show that the coordination around the white atoms is similar to that around the black ones shown in (d). The unit cell of the CsCl structure is shown as a packed spheres model in (g).

Size exclusion chromatography (SEC), also known as gel permeation chromatography (GPC), was used for the separation and fractionation of macromolecules on an analytical and preparative scale [17]. The separation occurs predominantly by the hydrodynamic volume of the macromolecules in solution, however, in some cases the polarity of the molecules can also influence the retention times. Like HPLC, the SEC technique is generally very reproducible with regard to its elution times (typically < 1 h) and hence can be used for automated synthesis. But because the cost for an automated SEC system is high, it must be considered as a serial separation technique. In addition, larger scale separations > 100 mg, usually require repetitive injection of small aliquots. 

Hint row 8 of this design and row 8 of the saturated fractional factorial design in Table 14.7 suggest that the reflection or foldover must be carried out first. Repetitions of switching one row with another, and switching one column with another, will eventually yield the desired result. Retain the identities of the rows and columns. 

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