Diffusion molecular weight cutoff

Normally, the immobilization of heparinase to agarose catalyst particles is terminated after 4-5 h because greater than 85% of the initial heparinase is bound (49). Based on a cyanate ester stability study, the cyanate ester concentration drops to only 88% of its initial value. For modeling purposes, the cyanate ester concentration was assumed constant. In addition, because of its small size relative to the large molecular weight cutoff (1.5 x 106 daltons) of the catalyst particle, cyanogen bromide (MW 106) should diffuse rapidly into the particle and uniformly activate the matrix. 

In a few cases it has been demonstrated that there is a solute molecular weight cutoff or size (MWC) above which a solute will not diffiise into a solid. Therefore, bottlenecks in cell walls may control the rate of diffusion in solids. Solute molecules that camrot pass through these bottlenecks cannot enter or leave the solid (excepting those portions in which the cell walls are raptured). This suggests that if MWC is known, D, for a solute i can be estimated from die D, for a reference solute r and corresponding known values of Dt, and MW that is. 

Today, dialysis tubing for laboratory applications comes in a variety of dimensions and molecular-weight cutoffs (MWCO). In addition to tubing, dialysis membranes are also found in a wide range of different preformatted devices that significantly improve the performance and ease of use of dialysis. The MWCO typically refers to the smallest average molecular mass of a standard molecule that will not effectively diffuse across the membrane upon extended dialysis. Thus, a dialysis membrane with a lOK... 

Transport to and from encapsulated cells of nutrients, respiratory gases, and similar small molecules is not affected by pore sizes commonly found in encapsulation motifs. However, transport of desired proteins, synthesized by the cells, out of the encapsulation matrix can be Hmited by the pore size of the material. Large secreted proteins (e.g.,MW 660 kDa) will be blocked by the MW cutoffs needed to shield the implanted cells from antibodies, whereas the diffusion of MW = 28 kDa and smaller proteins wiU not. Consequently, the molecular weight cutoff of the encapsulation material must be carefully chosen when transport of a secreted natural compound such as insulin is desired. 

Ultrafiltration membrane manufacturers frequently characterize their membranes using the cutoff concept rather than pore size. The nominal molecular weight cutoff (MWCO) is a performance-related parameter, defined as the lower limit of a solute molecular weight (e.g., dextran) for which the rejection is 95-98% (Boerlage, 2001). As the MWCO decreases, the mean pore diameter for most UF membranes has been foimd to decrease (Kim et al., 1990). However, the MWCO may be sharp or diffuse, that is, thoe is a range of MWCO, and in reality MWCO is only a rough indication of the membrane s ability to remove a given compound as molecular shape, polarity, and interaction with the membrane affect rejection (Mulder, 1996). Moreover, membrane surface characteristics (e.g., surface porosity and pore size distribution) may influence the apparent size of particles retained. 

The formation of low-molecular weight products of the a-amylase-cat-alyzed starch hydrolysis can be assayed by using a glucoamylase-GOD electrode (Pfeiffer et al., 1980). The sensor is covered by a dialysis membrane with a cutoff of 15 kDa which prevents starch from reaching the enzymes. The cleavage products can easily diffuse into the bienzyme membrane where they are successively degraded to glucose by glu-coamylase. As only the (B-anomer is formed, the sensitivity of the method... 

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