Standard sedimentation coefficient

These absorption data are collected and analyzed to determine the standard sedimentation coefficient ( 20,w) of the samples. The S20,w values of both the unfolded and the folded samples are plotted as a function of RNA concentration (in absorption units). If s2o,w is constant or decreases linearly with concentration, the sample is behaving as monodisperse particles. In such cases, linear fits to the data are extrapolated to zero RNA concentration to determine the s20 w, which indicates the sample s hydro-dynamic behavior at infinite dilution. 

Since sedimentation rate studies may be performed using a wide variety of solvent-solute systems, the measured value of the sedimentation coefficient which is affected by temperature, solution viscosity and density, is often corrected to a value that would be obtained in a medium with a density and viscosity of water at 20°C, and expressed as the standard sedimentation coefficient or S20, to. For many macromolecules including nucleic acids and proteins the sedimentation coefficient usually decreases in value with increase in the concentration of solute, this effect becoming more severe with increase both in molecular weight and the degree of extension of the molecule. Hence s2o, to is usually measured at several concentrations and extrapolated to infinite dilution to obtain a value of s2o, to at... 

Standard Sedimentation Coefficient A synonym for reduced sedimentation coefficient. See Sedimentation. 

Tables. Molecular size and average standard sedimentation coefficient S2o,w for PM2 Hae III uniformly sized NaDNA fragments [77K1].

As with the diffusion coefficient, sedimentation coefficients are frequently corrected for concentration dependence and reduced to standard conditions ... 

Fig. 10.2B). At this stage, you may also convert the apparent sedimentation coefficient 5 to the standard apparent sedimentation coefficient 5 0 w ... 

Figure 4. Sedimentation Velocity Analysis of ZDD. A, Primary data collected at 1 mg/ml (10 scans). B, Apparent sedimentation coefficient distribution function, g(s ) versus s. The error bars represent the standard error of the mean. The solid line is the fit to equation 4. Apparent s, D, and Ms,D values were calculated as described.

Utilizing the buoyant densities listed in Table II and the frictional ratio of 1.11, values for the sedimentation coefficient have been calculated at two different solvent densities the standard S value routinely used to characterize human serum lipoproteins is defined as value of the flotation coefficient in Svedberg units (the negative sedimentation coefficient X 10 sec) in an aqueous NaCl solvent with a density of 1.063 g/ml and a viscosity of 1.021 centipoise (the viscosity of a 1.063 g/ml sodium chloride solution at 26°C). These values are listed in Table II. The S value is very sensitive to small variations in lipoprotein density because the solvent density is close to the lipoprotein density. To compare the particle sizes or molecular weights, values of the sedimentation coefficient (5) in a solvent with a density of 1.20 g/ml and the viscosity of KBr at 25°C are preferred, and the computed values are listed in Table II. 

Ribosomes contain several different RNA molecules, three in prokaryotic ribosomes and four in eukaryotic ribosomes. For historical reasons, each class is characterized by its sedimentation coefficient, which represents a typical size. For prokaryotes, the three Escherichia coli rRNA molecules are used as size standards they have sedimentation coefficients of 5S, 16S, and 23S. The E. coli rRNA molecules have been sequenced and contain 120, 1541, and 2904 nucleotides, respectively. The sizes of the prokaryotic rRNA molecules vary very little from one species of bacterium to another. 

Analytical Ultracentrifugation. Sedimentation coefficients were determined with an ultracentrifuge (Beckman L8-70M) fitted with a schlieren analytical attachment. Photographic images of schlieren (refractive index gradient) profiles were analyzed with a profile projector (Nikon, model VI0) using standard procedures (13). 

Figure 39-8 shows several sedimentation coefficients corrected to standard conditions. 

The above sedimentation coefficient is valid for the conditions at which it was obtained. However, to make sedimentation coefficients of value for identification purposes, they must be converted to a set of standard conditions. The reference solution is water at 20 °C (j2o.w) and the variables requiring correction are the viscosity (7) the density (p) and the partial specific volume (v), which is the volume change occurring when a particle is placed in an excess of solvent. Equation 39-11 below shows how it is corrected. 

Sedimentation The settling of suspended particles or droplets due to gravity or an applied centrifugal field. The sedimentation rate (or velocity) divided by acceleration is termed the sedimentation coefficient. The sedimentation coefficient extrapolated to zero concentration of sedimenting species is termed the limiting sedimentation coefficient. The sedimentation coefficient reduced to standard temperature and solvent is termed the reduced sedimentation coefficient. If the sedimentation coefficient is... 

To facilitate comparison with other biological polymers, the experimental value of the sedimentation coefficient is usually corrected to a standard basis corresponding to a reference solvent having the viscosity and density of water at 20°C. The equation for correction is... 

Since the viscosity of the solute is implicated in these equations, the sedimentation coefficient is not a priori a measurement of molecular weight it is also influenced by shape, flexibility, solvatation, etc., of the solute. Therefore, standards of known molecular weight are generally run with the sample (or in a parallel experiment) to calibrate the measurements. 

This model requires an excess of sulfate over reducible carbon. Concentrations may be measured in solutions squeezed from sediment cores, diffusion coefficients are known from standard chemical data tables and sedimentation rates determined from 14C, 210Pb, or 230Th dating. Therefore, this model finds its best use in the recovery of the kinetics of organic matter decay. A discussion of this and similar equations and numerical applications may be found in Berner (1980). 

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