Diffusion coefficient of macromolecule

Tao T 1969 Time-dependent fluorescence depolarization and Brownian rotational diffusion coefficients of macromolecules Biopolymers 8 609-32... 

Loading capacities in size exclusion chromatography are very low because all separation occurs within the liquid volume of the column. The small diffusion coefficients of macromolecules also contribute to bandspreading when loads are increased. The mass loading capacities for ovalbumin (MW 45,000) on various sizes of columns can be seen in Table 10.5. The maximum volume that can be injected in size exclusion chromatography before bandspreading occurs is about 2% of the liquid column volume. The maximum injection volumes for columns of different dimensions can also be seen in Table 10.5. 

Figure 4.11 shows that ultrafiltration and pervaporation for the removal of organic solutes from water are both seriously affected by concentration polarization. In ultrafiltration, the low diffusion coefficient of macromolecules produces a concentration of retained solutes 70 times the bulk solution volume at the membrane surface. At these high concentrations, macromolecules precipitate, forming a gel layer at the membrane surface and reducing flux. The effect of this gel layer on ultrafiltration membrane performance is discussed in Chapter 6. 

Example 6.10 Prediction of diffusion coefficients of macromolecules Predict the diffusivity of human semm albumin at 293 K in water as a dilute solution and compare with the experimental data in Table 6.4. 

Quasi-elastic laser light scattering (also called intensity fluctuation spectroscopy, light-beating spectroscopy or photon correlation spectroscopy) is an accurate method to measure the translational diffusion coefficients of macromolecules. The diffusion coefficient is a parameter, that depends on the size and shape of the macromolecules and on the thermodynamic and hydrodynamic interaction between the macromolecules. 

One of the most serious obstacles in the phase equilibrium studies of polymer blends is the viscosity of the system. At temperahires limited by the degradation the self-diffusion coefficient of macromolecules is of the order of magnitude 10 to 10 m /s [Kausch and Tirrell, 1989]. As a result the phase separation is slow. To accelerate the process a low speed centrifuge, the centrifugal homogenizer (CH), with PICS has been used [Koningsveld et al, 1982]. In short, centrifugation within the immiscibility zone permits determination of binodal and critical points, while the use of PICS mode allows location of the spinodal. 

Medvedevskikh Yu. G. Diffusion Coefficient of Macromolecules into Solutions and Melts/r . G. Medvedevskikh, Conformation of Macromolecules. Thermodynamic and Kinetic Demonstrations — N. Y. Nova Sci. Publishing, 2007,107-123. 

The diffusion coefficients D of macromolecules in dilute solutions have values of 10 cm /s (Table 7-1). The diffusion coefficients of the proteins ribonuclease, hemoglobin, collagen, and myosin, as well as deoxyribonuclease, were measured in dilute aqueous salt solutions. They were transformed into pure water diffusion coefficients by assuming that any differences are caused by differing viscosities only, and not by changes in the asymmetry or solvation coefficients. The diffusion coefficients of macromolecules in the melt are much lower, being 10" -10 cm /s. 

FCS is a type of spectroscopy that is based on the measurement of fluorescence intensity and the analysis of its fluctuations, which result from diffusion of the observed fluorophore in the excitation volume or changes in the fluorescence quantum yield arising from chemical reactions. Measurements are usually made on only a few molecules at a time, of the order of 10 milliseconds. This is achieved by illuminating tiny sample volumes ( 1 femtoliter) by employing two-photon excitation microscopy (see section Two-Photon Confocal Microscopy above). FCS is used to measure translational diffusion coefficients of macromolecules, the number of fluorescent molecules under observation and the... 

Profiles in which this latter profile can be found are electrodialysis, per/aporation, gas separation, dialysis, diffusion dialysis, facilitated transport or carrier mediated transport and membrane contactors. The extent of the boundary layer resistance varies from process to process and even for a specific process it is quite a lot dependent on application. Table Vn.2 summarises the causes and consequences of concentration polarisation in various membrane processes. The effect of concentration polarisation is very severe in microfiltration and ultrafiltration both because the fluxes (J) are high and the mass transfer coefficients k (= EV8) are low as a result of the low diffusion coefficients of macromolecuiar solutes and of small particles, colloids and emulsions. Thus, the diffusion coefficients of macromolecules are of the order of lO ° to 10 m /s or less. The effect is less severe in reverse osmosis both because the flux is lower and the mass transfer coefficient is higher. The diffusion coefficients of low molecular weight solutes are roughly of the order of 10 m /s. In gas separation and pervaporation the effect of concentration polarisation is low or can be neglected. The flux is low and the mass transfer coefficient high in gas separation (the diffusion coefficients of gas molecules are of the... 

The hydrodynamic properties of molecule are strong related to its size and shape. Diffusion is of two main types — translational and rotattcmal. Out these, the latter is characteristic of the type of molecule. A large spherical mcdecule wdl have a stn rotary difl ision coefficient. An ellipsoid of revolution is characterised by two coefificients as it possesses two axes while a general ellipsoid molecule will have three coefficients, mie few each of the axes. These rotary diffusion coefficients of macromolecules various shapes can be determined by flow trirefringence see Chapter 9) and by electrical methods if the mcdecules are dipolmr In nature. 

Physicists and physical chemists developed the technique for three purposes curiosity about the statistical nature of light as a tool to study critical phenomena and as a way to probe the dynamics of polymer solutions." Early on it was used to measure the diffusion coefficient of macromolecules, from which a hydrodynamic size was calculated. A few industrial users tried it for submicron particle sizing, mostly to replace TEM measurements in QC applications. The approach proved rather awkward in those early days. 

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