Dextran beads

Quench the reaction by the addition of 0.1 ml of glycerol per milliliter of reaction solution. Alternatively, the reaction may be stopped by immediate gel filtration on a Sephadex G-25 column. The dextran beads of the chromatography support will react with sodium periodate to quench excess reagent. To quench the reaction with cellular samples, wash the cells with buffer to remove remaining traces of periodate. 

Gel filtration chromatography (also known as size or molecular exclusion chromatography) separates molecules based on their ability to penetrate into the pores or channels in agarose or dextran beads. As a mixture of molecules in a fluid permeate through the beads of gel the volume available for diffusion is determined by their diameter and the size of the channels in the gel beads. The... 

Swell the dry beads made of cross-linked dextran, polyacrylamide or gelatin in PBS-A or Hepes buffered saline as described by the manufacturer. The cross-linked dextran beads require 2-3 h to swell at room temperature. Avoid stirring with a simple bar magnet as this may grind the beads. Addition of non-ionic detergent (e.g. Tween 80) to 0.1% may help initial wetting of the microcarrier. 

The solid phase to which the antibody is bound can be either suspended in solution, on particles such as cellulose, agarose, or dextran beads or can be attached to the surface of a test tube or a microtiter plate. The stationary solid phases eliminate the centrifugation step that is necessary with the suspended beads although plastic solid phases have a limited capacity of binding proteins. 

Other examples for the successful employment of co-cultures are the interaction between muscle and nerve cells (Shahar et aL, 1985) and the co-cultivation of vascular endothelial and smooth-muscle cells using microcarrier techniques (Davies Kerr, 1982) co-cultivation of cerebellar granule cells, cerebral cortical neurons and cortical astrocytes on collagen-coated dextran beads (CytodexS) and the production and release of specific neurotransmitters and enzyme synthesis resembling in vivo interactions between neurons and astrocytes (Westergaard et aL, 1991). 

It has been shown that 0.5 pm fluorescent dextran beads may penetrate the stratum corneum within 60 min under flexing, whereas 1pm beads could not penetrate [95]. Small-sized nanomaterials, such as fullerene, metal nanoparticles and quantum dots, were able to penetrate the intact skin [83, 96-99], with small-sized quantum dots (15 nm) penetrating deeper (observed in the epidermal region) than the larger (40 nm) quantum dots (observed only in the stratum corneum) during the same period (8h) [98]. Aside from the size of the quantum dots, their shape also seems have a role in skin penetrance, as spherical quantum dots were seen to penetrate the epidermal barrier more easily than eUipsoids [98]. 

Sephadex. Other carbohydrate matrices such as Sephadex (based on dextran) have more uniform particle sizes. Their advantages over the celluloses include faster and more reproducible flow rates and they can be used directly without removal of fines . Sephadex, which can also be obtained in a variety of ion-exchange forms (see Table 15) consists of beads of a cross-linked dextran gel which swells in water and aqueous salt solutions. The smaller the bead size, the higher the resolution that is possible but the slower the flow rate. Typical applications of Sephadex gels are the fractionation of mixtures of polypeptides, proteins, nucleic acids, polysaccharides and for desalting solutions. 

Sephadex type Grade Dry bead diameter (/urn) Fractionation range peptides and proteins (g/mol) Fractionation range dextrans (g/mol) Swelling factor (ml/g dry Sephadex) Maximum operating pressure" (cm H,0) Permeability Ko Maximum linear velocity" (cm/hr) Swelling time (h) ... 

The performance of several Sephacryl gel combinations is illustrated by results achieved for glucans from different types of starch granules. The applied Sephacryl gels of Pharmacia Biotech (15) are cross-linked copolymers of allyl dextran and N,N -methylene bisacrylamide. The hydrophilic matrix minimizes nonspecific adsorption and thus guarantees maximum recovery. Depending on the pore size of the beads, ranging between 25 and 75 im in diameter, aqueous dissolved biopolymers up to particle diameters of 400 nm can be handled. 

At end, it is important to mention that calcium pectate gel beads were compared with calcium alginates gel beads for all entrapment uses [65, 66] in this work, the authors determined the pore size of the beads by size exclusion chromatography using dextran standards and other solutes. 

Chromatographic resolution is also dependent on column efficiency (i). Column efficiency is directly dependent on the nature of the support matrix and how well that support is packed in its column. Available chromatographic supports are based on dextran, agarose, polystyrene, acrylic, cellulose, silica gel and a variety of other polymers. Althou cellulosic supports are manufactured in both microcrystalline and leaded forms, most supports are beaded. Newer supports may use hybrid bead construction where the base support is coated with a second materid (e.g., dextran or silica coated with agarose). 

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