Gels ultracentrifugation

Light scatteting and gel-permeation chromatography (qv) can be used to measure the weight-average mol wt, whereas ultracentrifugation can provide a measure of the -average mol wt. 

Techniques for study of higher orders of protein stmcture include x-ray crystallography, NMR spectroscopy, analytical ultracentrifugation, gel filtration, and gel electrophoresis. 

Physical separation methods can be based on equilibrium considerations, but the majority are not. Ordinary filtration is an example of a non-equilibrium, physical method and so is ordinary centrifugation— e.g.—the separation of a precipitate from the suspending liquid using an artificial gravity field. There are separation methods, which are called filtration which are not such as gel filtration. Ultracentrifugation in a salt gradient is a physical equilibrium method. 

Molecules that vary significantly in their size can be separated by ultrafiltration or dialysis, while molecules that are only slightly different in size can often be separated by gel permeation chromatography. Ultracentrifugal techniques, while apparently separating on the basis of size, are strictly speaking more influenced by the mass and density of the molecule and to a lesser extent by its shape. 

The humates present in soil are polyelectrolytes and bear some similarity to polyacrylic acid and polymethacrylic acid (49, 50). The molecular weight distribution for the humates is considerable fulvic acid fractions of 1,000 daltons have been isolated (51) while humic acid molecular weights obtained by gel chromatography are in the range 17,000 to 100,000 daltons according to the type of soil from which it was extracted (52). However, ultracentrifugation analysis indicates a molecular range of 2,000 to 1,500,000 daltons for humic acids (55). 

To determine the shape of ribosomal proteins in solution, ultracentrifugation, digital densimetry, viscosity, gel filtration, quasi-elastic light scattering, and small-angle X-ray or neutron scattering have all been used. With each technique it is possible to obtain a physical characteristic of the protein. Combining these techniques should allow the size and shape of the protein to be characterized quite well. However, the values determined in various laboratories for the same ribosomal proteins differ considerably. To help understand some of the reasons we will initially discuss each method briefly as it relates to proteins and then review the size and shape of the ribosomal proteins that have been so characterized. 

Potl inhibitors differ from other protease inhibitors, and from all other defense peptides mentioned thus far, in their relative lack of disulfide bonds. This means that the loop with the reactive site is not fixed, as it is in the Bowman-Birk inhibitors, yet they still form a stable fold, as shown in Figure 11. An interesting feature of some Potl inhibitors is their tendency to form stable, noncovalently bound oligomers. This has, for example, been shown for chymotrypsin inhibitor I from tomato. This peptide has a monomer weight of 8300 Da under dissociating sodium dodecyl sulfate (SDS) gel conditions. Gel filtration and ultracentrifugal analysis revealed a... 

Analyses of the reeonstituted eomplexes by quantitative agarose gel electrophoresis [404,405] and analytical ultracentrifugation [266,406] in the presence of MgCl2 showed that the arrays were able to fold in a way that is almost indistinguishable from complexes reconstituted with major histones (see Fig. 14A-B). Despite this, it was found that histone H2A ubiquitination affects the MgCl2 solubility of the reconstituted complexes (see Fig. 14C) suggesting that this modification may play a role in enhancing the intermolecular associations between chromatin fibers [221]. 

The production of the biobonded particle boards has been described recently in detail (1,31). Analytical results by gel permeation chromatography and ultracentrifugation have been presented in support of an in vivo and in viiro polymerization of lignin sulfonates. Besides polymerization, the lignin molecule was found to be modified by carboxylation, and this was shown by difference spectroscopy (32). The following is a short summary of pertinent results. 

Hulley SB, Cook SG, Wilson WS, Nichaman MZ, Hatch FT, Lindgren FT (1971) Quantitation of serum lipoproteins by electrophoresis on agarose gel standardization in lipoprotein concentration units (mg-100 ml) by comparison with analytical ultracentrifugation. J Lipid Res 12 420-433... 

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