Buoyant mass

Selected entries from Methods in Enzymology [vol, page(s)] Association constant determination, 259, 444-445 buoyant mass determination, 259, 432-433, 438, 441, 443, 444 cell handling, 259, 436-437 centerpiece selection, 259, 433-434, 436 centrifuge operation, 259, 437-438 concentration distribution, 259, 431 equilibration time, estimation, 259, 438-439 molecular weight calculation, 259, 431-432, 444 nonlinear least-squares analysis of primary data, 259, 449-451 oligomerization state of proteins [determination, 259, 439-441, 443 heterogeneous association, 259, 447-448 reversibility of association, 259, 445-447] optical systems, 259, 434-435 protein denaturants, 259, 439-440 retroviral protease, analysis, 241, 123-124 sample preparation, 259, 435-436 second virial coefficient [determination, 259, 443, 448-449 nonideality contribution, 259, 448-449] sensitivity, 259, 427 stoichiometry of reaction, determination, 259, 444-445 terms and symbols, 259, 429-431 thermodynamic parameter determination, 259, 427, 443-444, 449-451. 

The sedimentation coefficient, s, depends both on the shape and the effective buoyant mass (Mb) of the solute particle... 

Combined with the definition of buoyant mass (Eq. (10.3)), this equation can be written as... 

Sedimentation of amyloplasts within the cell has been correlated with the capacity of the plant to perceive gravity. The buoyant mass of amyloplasts present in specialized cells in the center of the root cap and in the stem (depending on the plant species, in the endodermis, the bundle sheath, or in the parenchyma to the inside of the vascular bundle) would allow the amyloplasts to sediment inside the cell, where the cytosol would have a relatively low viscosity. This sedimentation would translate into a signal of an unknown nature, maybe through pressure onto a sensitive part of the cell or acting as a mechano transducer, etc. Whatever the nature of the signal, it eventually results in the asymmetry of the organ and its curvature. The isolation of starchless mutants of Arabidopsis thaliana and Nicotiana sylvestris has made... 

Sedimentation-FFF. Retention measurements give the effective particle mass m (buoyant mass). If the particle density is known, the particle mass m, particle volume Vp, and hydrodynamic diameter dH can be calculated [80,81]. Apart from the particle dimensions, the density can be determined as well [82] as the difference in the densities of the solute and the solvent, Ap, is linearly correlated to X. Fractionation can be used in regions where the solvent density is lower than the solute density (pps. The determination of particle density in a single experiment is possible by sedimentation-flotation focusing-FFF [72,73,83] analogous to density gradient ultracentrifugation. 

In this model, the buoyant mass is then the sum of the buoyant mass of the three components, assuming that these are independent of the mass of solvent occupied in the solvation shell. Thus, the mass of the adsorbed shell can be calculated if information about the mass and density of the core particle and the density of the macromolecule and solvent are known. Photon correlation spectroscopy, electron microscopy, flow FFF, or other sizing techniques can readily provide some independent information on the physical or hydrodynamic particle size, and pycnometry can be used to measure the densities of the colloidal suspension, polymer solution, and pure liquid. 

Further, it is possible to utilize different fields to yield the various FFF subtechniques. The two most common fields are centrifugal and fluid cross-flow, which give rise to the sedimentation and flow FFF subtechniques. Other fields currently in use include thermal, electrical, and magnetic fields. In the normal mode, it is possible to extract physical parameters from retention data. For example, sedimentation FFF using a centrifugal force gives information about the buoyant mass, and flow FFF gives information about the sample s diffusivity or hydrodynamic diameter. 

In gravity sedimentation the dominant field force is due to the gravitational acceleration acting on the buoyant mass of the particle so ... 

One big advantage of normal mode FFF is that certain particle properties such as the buoyant mass, diameter, density, concentration and thermal diffusion coefficients, and electrophoretic mobility can be calculated directly from the elution time or volume of the sample peaks. For example, with flow FFF (see below) the diffusion coefficient and hence equivalent spherical hydrodynamic diameter can be estimated. Other fields enable different properties to be obtained. 

Both Brownian and steric/hyperlayer FFF are possible, enabling particulate samples between 0.05 and 50 pm to be processed. Normal sedimentation FFF (SdFFF) elution depends directly on the buoyant mass of the particles and hence on a combination of the equivalent spherical diameter (ESD) and density. 

Submicrometer particles Particles below 1 pm in diameter can be separated using normal mode FFF using a variety of subtechniques, the most widely used being sedimentation and flow. FIFFF can be used down to Inm and yields the diffusion coefficient and hence the equivalent spherical hydrodynamic diameter distribution. SdFFF instruments usually operate up to 2500rpm, which can resolve silica particles down to 50nm. The quantity directly measured is the effective or buoyant mass, and particle size distributions can be generated if the density of the particles is known. 

Shape is not a huge concern in these measurements, although for accurate work it should be considered. Shape will influence the conversion of the diffusion coefficient (D) to the hydrodynamic diameter in FIFFF, although the discrepancy in D between spheres and ellipsoids of the same volume will seldom exceed 50%. For SdFFF the buoyant mass... 

An interesting extension of SdFFF measurements is the determination of the particle density. Since the elution time enables the buoyant mass to be estimated from the FFF equations, then if the size of the particles eluted at a given time is measured independently, for example, using electron microscopy or dynamic light scattering, then the density can be calculated. Alternatively if the elution times of a sample are determined in a series of carriers that have different densities, the particle density can be extracted. 

Sedimentation FFF (SdFFF) contains a channel attached to a spinning centrifuge. The driving force is the difference in effective (buoyant) mass ... 

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